Victorian Boombox Mark IV

[J. Wilhelm]

I've recently discovered that my job is in peril again. There are very strong indications that my boss for 2 years is about to divest himself of the only production plant he has in the city. He will probably reduce his business back to a neighboring city north of Austin, and close this one plus the remaining two shops in the city.

The good news is that I'm somewhat prepared. I had been planning for some time to change jobs and use a bit of saved money to have a one month vacation in summer, because commuting took so many hours of my week, that I could not even look for another job while I worked - I know! In HR circles it's known as a "Hamster Wheel." Workers can't afford to leave the job because they don't earn enough, and they don't have time to look for a new job. The only solution is to borrow money or depend on others to survive while you get a new job.

Alas, things happened and I never took a break in summer. The implication being that my saved money is now a life raft while I find a new income.  If all goes well, and assuming I can work at my current job until the end of October, I should have enough money to cover living expenses until the end of January.

But I need to figure a way to pay rent and bills at the begging of February. The obvious choice is another job, but I'm also thinking about selling something online for Allhallowtide and Yuletide, while I look for another job. I know in advance I'll not get a job in December unless it's a temporary job, for obvious reasons. Meaning I have exactly zero time to waste and I need a fast project to start selling this month if possible. A few hundred dollars in profit for January helps me a lot.

I was thinking about a Mark IV Boombox, in the style of the Mark II: a small, portable device, but using the lessons learned during the Mark III project, including all the hardware I discovered during the project.

Victorian Boombox Mk. II
http://brassgoggles.co.uk/forum/index.php/topic,50307.msg997535.html

I'm not planning for a high performance device. It will not be a Transmission Line/ Inverted Horn project. I don't have the time and it would be too big, too expensive and wouldn't be able to sell at any significant frequency, because the projected price tag would be in the $1000 - $2000 range, which realistically you'll sell only once in a year.. I'm thinking a small device in the $150 - $350 range, about the same size of the Mark II maybe a bit larger, but less cumbersome, with Bluetooth and possibly FM tuner, and using Bass Reflex enclosures incorporating the "Harmonic Energy Cascade™" system I learned from the Sony bookshelf speakers I took apart during the Mark III project.

Victorian Boombox Mk. III
http://brassgoggles.co.uk/forum/index.php/topic,50846.msg1001879.html

I'm not sure if I could do the Ocarina™ double resonant frequency system (resonance at 30 Hz and 60 Hz for a 120 Hz driver), because that approaches the same size as the hybrid Bass Reflex+Inverted Horn system the Mark III had (it's basically the same length, although the pipes could be very narrow). No separate subwoofer unit will be used as in the Mark II either, although that was a very cool setup; instead, I aim to use an inexpensive battery powered class D amplifier paired to full range speakers with built in subwoofer capability, very similar to the small 4 inch drivers I used as subwoofers in the Mark III. Maybe pair with sealed soft dome tweeters, for bright sound. This is all hardware readily available online.

The system will look a lot more like the existing barrel-style Bluetooth boomboxes you find elsewhere, at a similar price, but will be bigger... because Steampunk (though I will not necessarily concentrate on a Steampunk style, you've seen what I can do outside of "Brown"). The electronic modules for those systems are readily available from the place I frequented online for  the Mark III, ready to hook up. The 4 inch full range drivers are the same brand as the sub/woofers I used for the Mark III, and come in the 40 W power range; they're also designed for that specific application, so all the innards for a Bluetooth box are commercially available to me. I'm just adding the Steam and the Energy Cascade system. I may have to shop around or get creative if the innards are too expensive, though. That's what eBay is for.

Brainstorming.
As usual, finding acceptable "horns" will be extremely difficult.

Whew! Well, here we go!

[J. Wilhelm]

The first cost plus sizing exercise excludes a design based on PVC pipes. The pipes are too expensive and narrow, even at 4 inches diameter, and the length of the pipe bass reflex enclosures would rival that of the horns for the Mk. III. That's unfortunate, but on the other hand I already have experience building boxes from the Mk. III project.

So I ran some quick calculations for square cross section enclosures, and a 15 x 15 cm cross section will comfortably allow the use of a 30 cm tall bookshelf styled box. That's a very practical size for me because I can use precut wood boards. A short 10 cm x ~3 cm diameter pipe would give you a resonance of around 80 Hz, easily adjustable by the length of the pipes. I haven't decided whether to use two ports or one. Generally two ports allows for less "whooshing" noise coming from the Helmholtz port, but it increases the resonance frequency.

The best and cheapest drivers are the "house brand" Dayton Audio. They come with full specs sheet and are relatively new technologically speaking compared to paper cone drivers. The full range driver at only 4 inches in diameter already rivals the performance of the larger more traditional paper cone drivers Sony used for the donor speakers in the Mk. III build. A tweeter is optional if the Dayton driver has a cutoff at 16 kHz, but I'd like to match the Sony performance if I can.

Wooden components for the Mk III. Center beam + the two end boxes, overall Length ~ 120 cm ~ 4 ft.
Mk. IV is proposed to be half as long, as if made only by the end enclosures glued back to back,
for a total length equal to that of the center beam.





Proposed Drivers for Mk. IV. 40 W RMS full range 80-16kHz + 20 W tweeter, 3 kHz - 20 kHz (optional)

[J. Wilhelm]

Are you thinking what I'm thinking?

I just need it for decorative purposes. It'll serve no acoustic function, and I'm thinking of building several variants of the Mk IV, so not all will be Steampunk.

I found this novelty snack bowl at my supermarket. Its made from melamine, which is hard to cut but it's seems to be the right diameter. It's fairly thick and heavy for it's size, but I bet it's nearly indestructible.

[J. Wilhelm]

So three days ago I started looking at components and costs. I think the drivers I show above are as good as it's going to get. Surprisingly there are many options for the electronics. The trick is in choosing which board you want to get. Bluetooth is pretty much mandatory. In terms of power, you can get some boards at 50 Watts per channel, but it's not convenient to seek less power. Oddly enough 100 Watt per channel boards are cheaper and more convenient. Otherwise you're stuck with 15 Watt boards which wouldn't be useful, because that's a maximum rating depending on the power supply voltage which ideally should be around 35 Volts to avoid clipping of the sound waves. That's a difficult voltage to get with a portable power supply so most of the time,the boards are powered with 25 Volts or so DC computer transformers. This means a 100 W board will give you considerably less power without clipping.

The board I chose for initial sizing was a simple 2 channel board with Bluetooth and 100 W per channel. It has a useless micro SD card reader for mp3 files, a volume control knob and a mini phono style jack for stereo input. No bass and treble control, and that's generally not an option on these boards, otherwise you need a passive or active tone control board in front of this board.

About as basic as it gets, but the 100 W channels should have no problem powering the drivers. For $20, that's not too bad. The buying public seems to like the board. It's based on an automotive stereo amplifier, TDA 7498, and it's got good reviews. With a total harmonic distortion of about 10 times that of 1980s Era Class B or AB Hi Fi amplifiers, it won't satisfy my scientific or audiophile standards, but this is meant for the public. If they seem satisfied with a Class D amplifier, then that's all I need to hear. It needs to be able to run on batteries and Class D amplifiers have a 90% energy efficiency, as opposed to 50% for a nice Class AB Hi Fi

Then the shop where I found this offers the necessary components to power the board:

All in all, I'm looking at a bill just shy of $150.  And there lies the quandary. $150 is not a bad cost, but that's already the price of a low end stereo Bluetooth boombox (I'm not taking about mono Bluetooth speaker which can cost less than half that amount).  This sets the minimum price at around $350. The price of a high end Bluetooth boombox. Which implies the customer will want more than a volume knob. There are other modules which give you a remote control, display and even FM radio (a cool addition for the old school crowd), but they're in the $40 to $60 range, and are built around lesser amplifier chips. Typically 50 W per channel..  if I raise my cost say by $20, I'll want to have a good amp chip with good reviews.

I don't think FM radio is required if the sound source is your phone. My own smartphone has FM radio built in! An alternative is to look at a passive tone control board, not as a gimmick, but to justify the price of the boombox... What I fear is that if I don't "raise my game" people will not buy.

So this is the decision I'm going to make this weekend. The items shown above are in my shopping cart. I just haven't pressed the "buy" button. There's no chance for Halloween sales though. There's no time left. But Christmas shopping is very much possible now.

[J. Wilhelm]

I'm getting closer on selecting components and a final design specification sheet for the first prototype. This time I've decided to take a very different approach to design, owing that I have full data spec sheets for the drivers, expressed in terms of Small/Thiele parameters. This is a more standard design approach in the audio industry and it departs markedly from working with First Principles (equations of mass, momentum, energy and entropy) to generate an acoustic design, like I did in the Mk. III project

The best way to explain Small/Thiele parameters ia that's these are characteristics of a speaker driver that define an idealized (theoretical) frequency response of a speaker. The parameters were designed to allow speaker designers to pick standard items off the shelf to expect a certain performance from a given type of speaker (ie suspension type speaker or bass reflex speaker type speaker).  This is an inverse design approach where a certain performance for a driver is set as a goal and you "reverse engineer" to find which physical properties the driver and box should have. Then speaker builders will match those Small/Thiele parameters to a compatible box to obtain something that approaches the idealized Small/Thiele performance.

Speaker spec sheets. Top to bottom: The (sub)woofers used for Mk III. The new tentative woofers, and new tweeters.

More specifically, the Small/Thiele parameters define a mechanical acoustic configuration that is equivalent to an electric Low Pass filter, thus enabling the designer to adjust the bass performance of a driver. If you have woofers, twitters and mid range speakers, then these "mechanical low pass filters" will allow you to determine the crossover frequency, in other words, choose the frequency target to separate the signal between a low frequency driver and a higher frequency driver. The method does not, however, guarantee performance, because there are many other factors which are particular to the driver's performance (it will not fine tune your speakers).

Idealized performance of (sub)woofers I used in Mk. III inside a bigger than ideal vented box (convenient size)


Idealized performance of (sub)woofers I used in Mk. III inside a smaller box still larger than ideal.


Idealized performance of new woofers in a convenient size vented box.

https://en.m.wikipedia.org/wiki/Thiele/Small_parameters

Thiele/Small parameters are named after A. Neville Thiele of the Australian Broadcasting Commission, and Richard H. Small of the University of Sydney, who pioneered this line of analysis for loudspeakers. A common use of Thiele/Small parameters is in designing PA system and hi-fi speaker enclosures; the TSP calculations indicate to the speaker design professionals how large a speaker cabinet will need to be and how large and long the bass reflex port (if it is used) should be.

The advantage for me, is that I can wisely choose components based on this idealized performance. It's the equivalent of a "Sizing Procedure" in Aircraft Design.

I've been mulling over whether to use the subwoofers I used on Mk. III or use the type I chose in my previous posts. Both are the same size, use the same size magnets and have the same power rating. The reason is obvious; the Mk. III subwoofers give me a lot more bass, especially if I oversize the box from the ideal specified by the Small Thiele parameters. But there's a price to pay for that. The (sub)woofers already have a fairly non-flat frequency response. The profile looks a bit like a curve (first picture on this post), rolling off after only 300 Hz, and it has a steep rolloff at 1kHz.

Worse for my decision process, the store where I'm purchasing actually sells a Bluetooth kit made for the same subwoofers I used for Mk III and the tweeters shown above. With good reviews. But that box relies heavily on the tweeters to produce midrange sound. With deficient sound above 300 Hz, the woofers can't give clear human voice, so it's up to the tweeters, which have an unusually low range cutoff between 1 and 3 kHz, to reproduce human voice. Me thinks this is the public liking that 1980s big bass big tweeter sound with no midrange.

If you constrain the (sub)woofer TCP115-8 and the full range PC105-4 to their ideal Small-Thiele recommended boxes, the only advantage the (sub)woofer has over the full range is about is about 5 Hz lower cutoff frequency (from 59 Hz to 54 Hz), practically nothing, but the full range gives you a very flat frequency response past 10 kHz, which in my mind would be more desirable, and you can always build a crossover to trim back the range of the tweeters which also have a very flat frequency response (The crossover would be a first order low pass filter, at those frequencies just a single buffering the tweeter capacitor). In my mind that'd be very close to the Sony bass reflex speakers I used as donors for Mk. III.

I'll make a decision this weekend *sweats*

It'd be nice for someone to ship me all the boards already precut as in the kit above, but you need to start thinking about having to carry big boxes through the mail (though shipping is free within US territory). I think I prefer to build my own box... Unless they offer to "substitute ingredients" to my specs.

[J. Wilhelm]

Alright. So I've crossed the Rubicon and thus cast the dice. It's not the decision you expected. I've decided to buy the kit instead of the individual parts. It's a downgrade from 100W per channel to 50 W and involves the TCP 115 subwoofers.

Why?? It's got nothing to do with the specs and everything to do with convenience at a time I don't have much time to spare. The kit simply solves too many of my problems at once while giving me a system that is very close to my original idea. Money is not that much of an issue. Basically I go from a roughly $185 cost for a 2x100W full range (PC 105) powered system without wood enclosure to a 2x$50W subwoofer (PC 105)+tweeter driven system with all cabinets and hardware included for $20 less. It's all about the peripherals.

I ran more careful calculations for the size of the enclosures using the pine boards I can buy at my local hardware store. The idea is to make as few cuts on wood as possible. This is a system I developed for the Mk. III boombox to good effect. The only downside is that you're constricted to very elongated boxes with a roughly 15 x 15 cm cross section. As it turns out the volume required for the ideal Small Thiele parameters is *very* sensitive to the cross section. Going from a square cross section to a rectangular cross section differing by less than 2 cm will produce enough of a difference to throw off the performance of the drivers. So the cure for that is to adjust the size (length) of the enclosure, resulting in very long boxes.

It turned out that using the pine boards I used for the Mk. III, you get two very different enclosure lengths depending on which woofers you're using. The Small Thiele parameters for the full range (PC 105) drivers require that use 3 ft. long boards (91.5 to fit the stereo pair of drivers), a 50% increase over the size needed if I stick with the full range TCP 115 (sub) woofers I used for the Mk. III. It's got a lot to do with the fact that the woofers are softer and the cone can travel twice as far as the full range speakers. The subwoofers oddly enough are happier with smaller boxes. I used the pine boards below for comparison:

Top: the Small Thiele calculations indicate a 3ft long (91.5 cm) enclosure using 5.5 inch wide boards,
to make a near ideal box for the PC 105 full range drivers.
Bottom: the 2 ft long leftover center horn box of the first Mk III prototype, 61 cm long,
actually is ideal for the TCP 115 subwoofers according to the Small Thiele calculations.

And the size of the kit enclosure is even smaller. Very compact actually.The reason is that the particle board box they use box has 50 % more depth (it's more"spherical"), so the system is more compact. I saw however, that the 2 ft long boards I used for the center ("core") horns in Mk. III are perfectly sized to accommodate the two TCP 115 sub(woofers) according to their ideal Small-Thiele box, and that gave me the idea that I could build the kit first, test it, photograph it, and for fairly little money change components (internals, drivers), and fit them in a box of my own design ($16 extra in pine boards). That is, as of now have two different configurations for the same components. For a little more money more I can upgrade the electronics and even change the speakers to add full range drivers and a box with mono subwoofer... So I could in the future improve the performance of the enclosure by scaling, swapping and adding components. For example, I could have a 50+50 W system, a 100 + 100W system, and if I upgrade the amplifier board, a 50+50W + 100W subwoofer system by adding a central box. In theory, I could have two, three or more different models of boomboxes based on the same components before I even sell the first boombox!

The kit's amplifier+ Bluetooth board.


Another board sold by the same manufacturer.

Now, having multiple models based on common components is a very effective way of developing a line of products for minimal cost. This can be done by sticking with all-Dayton Audio brand components, because they're interchangeable. The other reason to go this route is knowing everything you need is arriving at the same time. No need for a large inventory. No need to wait additional weeks for parts. This allows on-demand manufacture with minimal travel.

The frequency profile may or may not be flat like a board. All I have is laymen's word of mouth with good ratings, and the shop doesn't provide performance data spec sheets for the finished system. They do provide, however, a nice touch: an old-school passive crossover circuit of hefty proportions, owing to the fact they're making those tweeters perform as low as possible on the frequency spectrum to cover the subwoofer slack. The tweeters do have a nearly flawless flat frequency response down to human voice range... Lower crossover frequency translates to bigger capacitor and inductor values, as I showed with the King Kong sized crossover I didn't actually build for the Mk III subwoofers. Instead of using an op-amp crossover like I did, they did the old fashioned design work to tune the speakers with coils. You don't see that often with modern PC sound systems or Bluetooth boxes. That's unusual. I guess I'll just have to measure the performance after I build the kit.

http://youtube.com/watch?v=NDGF-qujBHw

[SeVeNeVeS]

I will admit most of this is right over my head with all the graph charts and specification details.......

BUT I am watching with interest, you have built some lovely boom box stuff in the past so........ Bring it on good sir! (even though I have no clue of what you talk about)

Me Luddite, me hit things with hammer........ etc etc

[J. Wilhelm]

I will admit most of this is right over my head with all the graph charts and specification details.......

BUT I am watching with interest, you have built some lovely boom box stuff in the past so........ Bring it on good sir! (even though I have no clue of what you talk about)

Me Luddite, me hit things with hammer........ etc etc

I appreciate your words Seven! Well, the good news is that as far as acoustic theory is concerned, you've seen the bulk of it for now.  The Small Thiele method is basically a sizing method only. Beyond this it's just testing to see how it sounds. I'm more interested in first impressions for this project, because this is meant for the public and not really my (at times somewhat unreasonable) expectations.

I just got the kit in less than a week. There's a major limitations on the kit, and that's the available shipping methods due to the lithium ion batteries and regulations regarding air cargo. Basically the kit with batteries can only be shipped over ground. For me this means I get the kit via ground transportation, and my shipping destinations are limited, probably to North America (Canada, US, Mexico). If I want to ship elsewhere it'll be without batteries. So this is an issue I have to explore, since I'm used to selling all over the world. All over the English speaking world was a common market. Kazakhstan and Japan were the two furthest locations I have sold to, and Rovaniemi (Finland) was the northernmost location. Some locations like Brazil and Russia are impossible to ship to, because taxes are too high...

Look at what "The Great Pumpkin" brought me. I guess my pumpkin patch was "sincere"
(ref. Peanuts cartoon strip).The kit arrived yesterday.

Ok. So this is what I got. There's an awful lot of peripheral hardware I got which is bigger and heavier than I'd anticipated. Which is good, because this is hardware I had not even thought about, and I realize that buying these peaces peacemeal or at local shops wouldn't be possible on my work schedule. The quality of that hardware is high. The other thing I hadn't given a thought to was the crossovers. I'm not going to call them massive, because they're not that big, but the coils are compact and heavy. Ceramic resistors, large gage magnet wire around an iron core and another coil around an air core. Definitely old school. Wonderful looking hardware that makes you want to display it. The amplifier is very small but heavy as well, it's the new generation of power amplifiers - Class D with around 90% thermodynamic efficiency (which is nuts), but with a total harmonic distortion about 10 times higher than old school type B and AB amplifiers running at 50% efficiency. I guess this is what the future looks like.

One of two crossovers for the speaker enclosures in the cabinet.


The amplifier module with built in Bluetooth.


Back panel plate with heavy duty switch, LED light fixtures and auxiliary phono input jack.

Every component comes with associated wire harnesses. All of them heavy duty looking. The amplifier comes with on off switch witch I'm not sure I'll get to use, because the back panel already comes with an automotive style on off switch and LED light assemblies.

One oddity of this kit is that it came with two identical front panels. I don't known why, but I'm thinking this might be very useful, as I can use one panel for another project. I could split the fiber board panels in two and incorporate into the pine enclosure I was talking about above. This could potentially save me the effort of cutting circular holes! That's a very big win if each kit I order comes with an extra panel.

The kit comes with two plastic tubes for the ports in the bass reflex system. At 2 cm diameter x 9.5 cm long, these ports are going to blow a lot of air, enough to put out a candle for every thump. This might create unwanted turbulence and hence a whizzing noise. I can start thinking about using some mesh glued on the inside interior end of the tubes to break the turbulence into smaller eddies, and that should quiet the ports a bit. Notice the ports are backward facing. This was done on purpose.

One of two port tubes.

Let's see how far I get today with the build. I'm thinking I have to give some sort of finish to the boards before I assemble the box. For this prototype if I can use stain and varnish. I need to make some finish tests now.

[J. Wilhelm]

Alright. First observation a few minutes ago. The fiberboard is very "drinky" it soaks stain like a sponge. I took one panel and on the inside of the panel I used two very different stains (see below), but I can't even tell them apart! I stained the center with dark red mahogany stain. The sides with yellowish Golden Oak stain. They couldn't be more different, but on the fiberboard you can hardly see any difference. The stain also reveals any tear on the fibers of the boards (see edges of circular holes). I don't think this will be a problem, as long as I don't scratch the boards before staining them, but it makes no difference which color of stain you use.

EDIT: A while later, I have used the second panel to do a test using only polyurethane on the sides and mahogany stain+polyurethane on the sides.

I think I like the plain poly varnish on the fiberboard. It looks very similar, but the finish is not as splotchy and it's sealed against moisture. The poly doesn't show imperfections the same way that plain stain does.. For the center I used both dark mahogany and polyurethane, which gives me a nice chocolate color.

[J. Wilhelm]

So I've assembled the box and spotted a major issue. It's their box design. While it looks standard for speaker construction, the way it's designed, the enclosure can't deal with minor warping without the use of plenty of glue on every single edge. A couple of wood plates which separate the amplifier from the speaker enclosures proper are shorter than they should be, and while you can fix that by using caulk to fill the gap between the top lid and those plates, it means the top lid will have to be shut permanently, which precludes any servicing of the amplifier or replacement of batteries.

The MKBoom enclosure requires all of the edges of the fiberboard to be glued due to the way
the joints are designed; in some cases T-joints which can lead to vibration if not fixed,
hence the caulk. You can see caulking at every junction. Screws don't work well with fiberboard

You can see a crack in the glue showing the strain due to warping. Those are T joints, by the way. Only three edges of the separators fit into slots on the bottom front and top boards. I guarantee that loud music will make those T joints vibrate if not glued, and the height of the plates needs to be incredsed

It'll take time for all the glue to cure right now, before I apply another fillet (not filet) of "caulk" again (it's not silicone caulk, but rather acrylic glue which I always use for strength).

Not being able to open the box to replace the electronics is a major faux pas for this design. Shutting a speaker enclosure with staples and glue is standard for speakers, but not boomboxes.  I've used a pair of self drilling pocket screws to deal with warping of the top lid and bottom board. Not a big deal, but the fiberboard splits and will not tolerate screwing and unscrewing of those fasteners, for more than 2 or three times, because it destroys the fibers in the board. Customers who buy a system without batteries due to shipping restrictions, wouldn't be able to buy and load their own batteries. And if the lithium ion batteries short, you won't be able to remove and dispose of the batteries, making the whole boombox a fire hazard.

So already this box is calling for a major redesign. The electronics can't be wedged between the bass reflex speakers enclosed like that. It's not acceptable. One solution which I saw while making the Mk. II boombox, used by Logitech to service their PC subwoofer unit, is to somehow attach the electronics to the switch plate and then have the switch plate be made removable, so the user pulls the entire guts of the system out by unscrewing the switch plate. In theory, I can build a frame to do just that.

The amplifier of the Logitech z313 pulls out of the enclosure when you remove the back panel

At this point and looking at the finish issues and joint design issues, I'm positive I prefer to use my pine enclosures with a specially designed compartment for the electronics. The finish will look infinitely better, and much more moisture proof than the fiberboard. I may have to work on my pine enclosures to make the design more compact, by way of copying this MKBoom enclosure dimensions, but ultimately the pine enclosures will be better and easier to assemble due to the tolerance to screws that solid wood provides. The box might be lighter as well. This fiberboard is heavier than it looks.

[J. Wilhelm]

Alright. My mistake. I should have rewatched the You Tube video I posted above. For a few seconds you can see someone installing the amplifier with screw posts on the switch plate.  If you blink you'll miss it. I guess I blinked. Which means that the amplifier pulls out from the enclosure by way of the rear panel and the smart charger and battery pack is attached to the same frame.

*Whew*

The instruction manual says absolutely zero about how to install the amplifier. There's an installation pamphlet (see a couple of posts above) showing the connections on the amplifier's mainboard, but that's it.

Now, that still leaves the crossovers trapped inside the speaker cavities. I have to solve that issue because I want to use the crossovers on another box which I'll make immediately after. Also, there's one last issue: ventilation. I have to create an airflow around the amplifier. Even if the power supply limits the output power to say, 30+30 Watts, there will be heat accumulation in that small cavity between the baffles.  I'd prefer to add holes on top and bottom of the enclosure right where the amplifier is when installed to allow convection to take place between the fins of the black heat sink on the amplifier.  If i increased power to say, the 4x100 Watt amplifier module I showed above, the little fan would be useless without proper ventilation to the outside.

[J. Wilhelm]

So, this Monday I built the amplifier on to the switch plate. The process lasted more than 1 hour, not because it was too complicated, but because the seller doesn't provide a good manual and I had to figure out how to assemble the boards and wire the device with the included wire harnesses.

At first impression, the system is well designed and rather clever at being what I'd call "the Arduino or Raspberry Pi of amplifiers." The amplifier boards are standardized to fit other extension boards which may include battery charger packs, preamplifiers and other unspecified extensions. The boards are stacked on top of one another with chromed steel posts or separators which screw into each other using a hex key driver.

You are left with extra parts due to the fact that each set of components in the kit is made for other configurations, so I was left with extra posts and a few extra wire harnesses. And this is where it gets a bit interesting. There's no instruction on how to stack the boards . You have to figure that out yourself, considering how much space you want between the boards, the orientation of the boards and what seems to be your choice on which harnesses you should use, because several of the harnesses look identical to one another, in particular LED lights, and the power and volume control harnesses. They all use the same type of pin jack/ plug arrangement, and you have to be careful not to plug the volume control to the power jack or vice versa. The only thing you have is the pin diagram on the pamphlet provided for the amplifier. There's also a minor error on the amplifier's pamphlet, where one plug is shown as having the wrong number of pins, which is critical, because this is power jack which is carrying significant voltage and current.

Luckily you can double check the polarity of the harnesses by looking at the main boards which have an identification table for each pin. That's how you confirm that the connection you made will not result in a short circuit. The second problem was that one of the chromed LED holders had a nut with a stripped thread. I fixed that last one with a drop of cyanoacrylate/ threadlock. The last issue is that the wire harnesses are way too long. Understandable, because the components are designed to be compatible with other systems, not just the MKBoom. The way the assembly video shows the harness, the builder wrapped it into a ball of wire on top of the heat sink of the amplifier board. That's a really bad idea, because you need to ventilate the heat sink if it's going to keep the amp chip from frying. I solved the problem with a set of miniature zip ties (not included) I had in my doohickey tool box).

Otherwise, I think once you know the setup, for my next assembly, the frame will take me 15 minutes to put it together. The system was designed for panel mount amplifiers that fit guitar speaker cabinets, for example, but I can see this frame fit in a light switch arrangement on the wall of a house, for example. All in all, a pretty clever idea, made possible by the small size of Class D amplifiers. What I like about it is it's expandable design which allows me to improve on the of performance using the same components. I'll try to see if I can put up a similar package, buying individual components without having to buy the fiberboards. The only obstacle is the crossovers, which I might have to march to the one provided for this kit.

What comes next is a quick test of the system by connecting a pair of speakers to the output harness. Then I'll wrap the excess wires in foam so they don't make sound by vibrating on the interior baffles when the speakers are working.

[J. Wilhelm]

I installed the batteries and did a quick run using the subwoofers. It seems to be running well. Bluetooth pairing, battery charging and volume all working. The heatsink at medium power is cool to the touch.  The subwoofers outside of a container shake the table more than make sound (big thumps, those are powerful electric motors), but that's obvious. They need to be put inside an enclosure.

I'm exited to move to the next stage. The amp is extremely quiet, as in no loud pops when turning on, connecting and disconnecting the power supply. The whole assembly is remarkably lightweight. I can already tell this is going to output some serious bass. Mind you, these are the same sub units I used for the Mk. III. The three batteries seem to handle low voltage. Right out of the box, they were charged, and they don't produce much more than 3 volts, so these are high current devices. At 9 volts, that's about half of the voltage of the AC mains power adapter, which produces 4 Amperes of current, so obviously the amplifier won't produce the peak 50 Watts, but more in the 30 Watts range, per channel. So far, so good!

[J. Wilhelm]

Today I looked at the installation of the crossovers.  The solution was rather simple, but it required I go out to buy eight #6 brass screws and nuts.  Making the crossover removable is easy, because you can pull it out when you remove the woofers.  The obvious location then is the back wall of the enclosure.

I ended up using a rather unusual attachment method. I drilled 8 holes on the back of the cabinet and installed the screws backwards, with the slotted head inside the cabinet. This is possible because the crossovers come with vinyl separators  that act like nuts.  Then I make sure the threaded end of the screws protrude from the back and I use the brass nuts to tighten the crossovers to the interior wall of the cabinet.

Not very standard, and I doubt this would be a final configuration, but it's very solid and will make migration to a new enclosure trivial.

The next step is to make a wire harness attached to and crossing the interior baffles, with jacks and plugs, to connect to the amplifier output cables such that all the components are removable in the near future.

[J. Wilhelm]

More photos from the assembly.  Here I show you how I managed the electronics bay between the baffles of the MKBoom. My major complaint is that the electronics bay is too narrow. In fact so narrow that I ran into trouble with a power socket from the battery pack to the mainboard. The electronics fit, but just barely and you don't want to take that frame in and out too many times.

I  installed two phones jack couplers to bridge between the speaker enclosures and the electronics bay.
This represents one extra set of items apart from the kit .Very tight fit.
I had to knick off a tiny corner from the battery pack so the frame didn't hit the battery pack.


The system is set up so you can stick your hand through the woofer hole and unplug the crossover.
It's very easy to replace the crossovers now provided you remove the woofers.

Not all wiring went well. I had to crease a wire harness due to lack of space


Because the bundle of wires had to pass overhead from the electronics frame, I lined
the remaining cavity space with felt so the sound coming from wires hitting the walls are deadened.




The box top was glued in place, now all access to the innards happens from the woofer's holes...
And by removing the back switch panel. The vent pipes don't interfere with servicing.

[J. Wilhelm]

Final Assembly and Analysis of MKBoom, pending redesign for first prototype of Mk. IV.

I've finished the assembly of the MKBoom, and I have a long list of pros and cons to this kit. Today, I'll give you a first impressions review of the kit.

Note: If you don't see any pictures now on this post (see post date), it's because I'm still loading them! I've decided to write the text first and populate with photos slowly (Smartphone Speed ™)

The finished Dayton Audio MKBoom Bluetooth Boombox


My roommate is very proud of his plants *facepalm*

Let me start with the pros:

1. The enclosure is absolutely rock solid. It's extremely dense and rigid. If you punch it with your first, it feels like it's a solid block of wood. It doesn't knock as much as sound like a muffled thud. Acoustic theory states that the most effective baffle (sound barrier) is a wall as dense and thick as possible to allow mass to absorb sound. The enclosure of the MKBoom does precisely that.

2. It's got impressive bass. Really it's just shy of being as bassy as a system with a dedicated subwoofer. Not surprisingly I used the same sub/woofers in the Mk. III horns.

3. The size is very compact. By making the enclosure have a double depth, they managed to enclose a large volume (Helmholtz resonating cavity). The system is as far as size goes, suitable as a portable device.

Now the cons:

1. The enclosure is absolutely rock solid, and that means it's extremely heavy for it's size. I'm not going to say the weight rivals the Mk. III horns, but it might be between the weight of the Mk. II and the Mk. I. It's a porker for its tiny size.

2. It's got impressive bass. Almost too much. And it sounds very boxy, which means standing waves are forming inside the rectangular cavities.

Even though the cabinet is solid and heavy as a rock, it can't avoid transferring sound and converting any flat surface (other than a concrete slab) into a bass radiator, which means that where you place the enclosure will make a huge difference as to how the system sounds; It's happening because of the very intense pressure waves inside the small enclosures.

Suspecting the frequency response of the MKBoom might be too bass-heavy, or mid-range deficient,  I looked at the frequency response using my spectrum analyzer and a source of white noise.

The frequency bandwidth for the MKBoom is very wide and the profile very flat, yet it sounds very throaty.

I can't say I found any evidence of a fault. The frequency profile is very flat as you can see from the snapshot above (you need to mentally smooth out those small fluctuations because this is a real time snapshot); The frequency response is as good as advertised.

I don't think that short legs will help much, as sound radiation will excite said flat surface as a passive radiator. This is however easy to fix:

This enclosure *needs* a suspension to keep it away from flat surfaces. It just can't be placed on or near parallel to a flat countertop. A base with a thin tall column to support the speaker enclosure might help (elevated from the ground). I also tried inclining the enclosure so it's resting on one edge of the box at a 30-40 degree angle from the horizontal, and that helped greatly to prevent sound transmission by contact. It also helped to improve diffusion of sound which is another problem too (read below).

3. The size is very compact. Too compact for it's own good. There's no space for wires on the sides of the inner electronics bay, which required me to get creative with how I wrapped the excess wire lengths of the various harnesses. I had to use felt to create a padded cavity so the wires don't vibrate against the baffles (interior walls).

Also, the woofers are way too close together. It's not just sub-bass that becomes monaural (that's normal due to the wavelengths of sub-bass sound below 120 Hz), but its also killing the stereo separation between Left and Right signals up to 2.5~3 kHz, where the crossover frequency is treble sound by definition. The tweeters are more separated than the woofers which are almost overlapping soundwise, so you don't have stereo sound except for the sound of cymbals.

4. Sound diffusion is zero. Save from the sub-bass which naturally diffuses in any direction, the MKBoom is highly directional. It points all sound straight ahead. I don't know how they managed to do that, but that's a problem too.

Most people will either place the boombox on a countertop horizontally or on the ground, with the speakers pointing at an angle away from the listeners' heads. This is a huge problem for the MKBoom, but there's a solution for that as well:

For the Mk. I Vicky Boombox, I actually angled the satellite speakers at near 45 degrees from the horizontal. This means the speakers are pointing in the direction of most listener's ears. This greatly enhances the perceived frequency response, particularly in the human speech range.  I think the Mk.IV absolutely needs to be inclined in a similar manner. That can be part of the suspension mechanism I was talking about. The MKBoom can easily lay on its rear edge without transmitting too much sound to a surface.


Conclusion

The MKBoom has some serious faults, but luckily these faults are due to it's geometry, not it's components. Individually, the Left and Right speaker assemblies are great, but glued back to back actually they become terrible, mostly because of proximity to each other, and the narrow radiative sound pattern from these drivers, not to mention the weight.

The MKBoom is therefore *crying and begging on its knees* to be split in half and turned into two separate speaker enclosures. I might just do that if I keep this unit for myself. I don't plan to sell the MKBoom as shown above.

If I understand correctly the MKBoom is actually based on a pair of award winning speakers called the "Mk Series" made by Dayton Audio. I do believe that properly separated and with a suspension, these speakers would have an impressive sound. Just don't glue them back to back.

What these observations are doing, is telling me is how to proceed design-wise:

1. There's no doubt that I have to switch to wood, to save on weight. Despite the high density of the MKBoom case, radiation from the box is a big problem. Let the problem remain with at least less weight to carry around.

2. The drivers must be properly separated and pointed upwards at an angle of 30-40 degrees from the horizontal, with a suspension system to keeps the walls of the boombox from radiating or conducting sound to parallel surfaces. This follows the Mk. I design style.

3. The Left and Right speakers can be incorporated into a single box, but only if the box is long enough to separate the drivers far enough from one another, 2 to 3 ft. of space between the centers of the drivers. And a suspension is mandatory when using big-motor subwoofer drivers such as these.

4. If I could get more "spherical" containers, that would help greatly to reduce the size and weight of the unit,  but more importantly it would eliminate the standing waves that make the MKBoom sound very boxy. Ideal Helmholtz resonators are spherical.

Otherwise the MKBoom is a good base for a new boombox. I just have to address the points above. I have some great ideas for boomboxes based on near spherical containers as well as sound bar and traditional bookshelf enclosures... I'll start searching the Internet...

[J. Wilhelm]

Weekend Project: First Iteration.

Seeking to implement some of the recommendations above, I proceeded to calculate new enclosures for the speakers. I made a compromise by realizing that I can still reduce the volume of the enclosures a bit and actually be closer to the design ideal recommended by the Small Thiele parameter theory. The internal volume will be now 3.36 Liters, instead of 3.58 used by the seller. There should be small acoustic change, but not much. I did however notice that the shop ignored the advice of the Small Thiele theory and used shorter ports than recommended, perhaps that was meant to compensate for design changes or perhaps the plastic tubes they get are standardized, and that's what they could use.  Either way, I'll stick to theory and use PVC pipes of the right length and diameter. This time I'll try to fit the ports facing forwards as I did with the Mk. III.

I need a centerpiece to cover the electronics bay. One idea is to fit a tablet there as a console.
Another idea is to have the ports' openings there (pipes external to enclosure)..


I haven't been able to reverse engineer the crossovers so for the moment I'll stick to what they used.
But that does mean I have to buy the whole kit, including the heavy fiberboard parts

Because I bought oversize boards, I tried to give the center electronics bay as much volume as possible, and that helped separate the left and right channels to roughly 40cm or 16 inches center to center between woofers, a more acceptable boombox design. The overall width of the box is now 67½ cm and the overall depth of the box is 16.2 cm with a height of 14 cm which is the standard width of the pine boards (5½ inches). I used the extra faceplate they gave me, which is very practical to avoid having to cut circular holes for the drivers. It's amazing how much work you save by doing that.

There's so much room between speakers that it should be easy to fit the existing frame or upgrade electronics to whatever I need.  The box could even be shortened to a width of 24 inches so I save on plank cuts, but the downside is that I'd have to buy a fifth board to cut the side walls and two internal baffles.  As shown, recycling the fiberboard front baffles, I only need three 3-ft pine boards are they're enough to build all the parts including the center front plate. 3 boards instead of 5. So for a few cuts more you save money and get a better box.

You know you're a Steampunk when all solutions come from the hardware store


There's some bronze finish hardware for a more Steampunk aesthetic.

I've been making the rounds online and in person at hardware stores looking for future speaker enclosure alternatives and hardware. Bathroom towel bars are absolutely excellent handles for a boombox. Both 18 and 24 inch varieties fit this box perfectly.

Really what I want now is get to the point where I like what I hear and like what I see. there's a myriad of derivatives which will come from this prototype. I'm still pursuing finding some plastic imitation ceramic vases around 3 Liters in volume. They'd make wonderful speaker enclosures. I really want to push those fantasy buttons, which I was not able to do with the Mk. III, because of physical constraints and money for the high performance design.

[J. Wilhelm]

Progress pictures. I've implemented a few engineering adjustments, and the boombox will have front facing ports (still not shown in the pictures below. I just drilled them right now.  The project came together rather quickly, though I'm not sure I want this as the final configuration. I used way too many screws to accelerate the build; in that sense, it shows it's pedigree as descendent of the Mk. III. 

I made a small mistake in the height, so I had to recalculate the port length based on the volume lost, and the bottom panel is recessed upwards by 2 mm.  It won't make a difference in the sound, but I discovered it's good to leave the front and back boards and fiberboard panels protrude downwards, because the enclosure's bottom protects the table from getting scratched by the dome "pocket screws" on the bottom pine board. I did have to trim a bit the fiberboard's rebbit joint to fit the new height of the pine box, though.

I would change the end baffles so they could be bolted. They'll be glued on top of square dowel used as a rebbit joint. It's a little bit of a pain, and the end caps can't be bolted. As shown below, they're not installed yet, as I'm keeping the sides and the front open for access while I install the electronics. That gave me the idea of having the front center piece of the boombox be removable, so I can upgrade later on to some fancy front panel or even a tablet. I'm a little afraid of vibrations that might occur with that cover so I'll have to play it by ear, and if I have to glue it, I will. For the moment I was able to stain the whole cabinet, and a few minutes ago I made the perforations for the ports which will use ~8 cm lengths of standard ¾ inch PVC pipe (Note to self: don't stain the box before cutting the port holes).

Hopefully the new shape of the right and left enclosures will eliminate some of that super boxy and boomy sound. So far the volume is more in line with the recommended enclosure volume according to the Small Thiele theory.  I know that at least the box is lighter, it's got a better stereo separation and I have a chance to upgrade it later.

[James Harrison]

Can anybody else see the images?  All I can see are blank boxes.

[Sorontar]

I can see them if I load them as separate tabs (which I normally have to do because I prevent externally hosted files from loading in webpages, so embeds like these from Twitter don't work by default).

[James Harrison]

I can see them if I load them as separate tabs (which I normally have to do because I prevent externally hosted files from loading in webpages, so embeds like these from Twitter don't work by default).

Thanks- if I open them on separate tabs I can see them.  Twitter acting up some more then?

Nice work anyway!

[J. Wilhelm]

I can see them if I load them as separate tabs (which I normally have to do because I prevent externally hosted files from loading in webpages, so embeds like these from Twitter don't work by default).

Thanks- if I open them on separate tabs I can see them.  Twitter acting up some more then?

Nice work anyway!

Thank you!  I'm almost finished with the first iteration. I have added the ports now, and I'll be returning home tonight to finish those pesky cap ends and line the electronics bay with felt. Putting all the guts inside should go very fast. Then the $64 000 question is how will it sound?

I'm comparing to the original speaker enclosures that inspired this kit, and, as suspected, the actual outer measurements of the bookshelf speakers are different. If that fails, then I need to question the crossover specs Dayton Audio used for the original "MK 402" bookshelf speakers.

https://www.parts-express.com/Dayton-Audio-MK402X-4-2-Way-Bookshelf-Speaker-Pair-300-465

I wouldn't put it past Twitter to do something about hosting pictures. I can see it on my smartphone. Perhaps security was updated? Anyhow it won't matter, because they're now in serious trouble. As of Thanksgiving Eve, they fired another tranche of employees, and it was reported that about half of Twitter's most important advertisers had stopped placing ads on Twitter.  I'm afraid this is it folks.  I need to fix my desktop computer to use an old fashioned Mozilla, or I need to find another primitive browser to make HTML copies of my Brassgoggles. Otherwise the whole kit'n kaboodle of my gallery is going bye-bye, sooner rather than later.

[J. Wilhelm]

Slow progress. Three coats of Minwax wood stain, Red Mahogany and Golden Oak + wax, and one coat of polyurethane. Still not done. Very slow to cure.

As an eccentric gesture, I stained the end caps/baffles in a different color; that gives me ideas. I can't wait to install the panel amplifier and the speakers. The dark candy colors Red Mahogany gives off and the gold metallic sheen of Golden Oak are insane, though.

[J. Wilhelm]

First Iteration: Victorian Boombox Mk. IV B. Discussion and Analysis.

I think the modifications were wildly successful!  More below with an in depth discussion of the design changes, but first, the outward appearance.

Outer Finish

The boombox was varnished a total of three times with spar polyurethane varnish spray over hand-rubbed Red Mahogany stain and wax (one step mineral spirit finish) from Minwax. It was fine sanded with 800 grit sandpaper and I chose to finish with spray varnish, as it could do a much better job than I could with a brush.

The finish is not quite as smooth as a piano finish; it still shows the wood grain, and otherwise it's glossy, smooth, and extracted very deep burgundy tones from the pine wood, or "candy shades or burgundy" if you will. I don't know if it's worth my time to sand and coat the box three more times until I reach a glass finish, though. I was anxious to hear the effect of my design changes.

So I transferred the components from the fiberboard box to the new one and I was surprised to see that in spite of being bigger, the pine box is still lighter than the fiberboard. I was pleasantly surprised to see the pine box is very rigid and non-resonant (not a surprise there, as I used the same technique on the Mk. III box.

Results

I think I hit the jackpot on this redesign. In fact, I'm having a hard time justifying all the changes that I hear, in spite that my actions were very deliberate.

The first thing you'll notice between the old box and the new one is the stereo separation. Now you can actually hear stereo in all frequencies instead of just the high tones. Reproducing sound at up to 3 kHz means that you can't place those woofers together like in the MKBoom. It's remarkable how much difference a few inches of separation between drivers make on the perception of stereo!

Gone completely is the boxiness of the sound. This was a very prominent feature of the MkBoom. I do know for sure this is the result of changing the inner proportions of the speakers enclosures. I reduced the volume of the box to fall more in line with the Small-Thiele parameter theory. I noticed the MKBoom had a slightly oversize box and the port legth was different from the ideal configuration as prescribed by Small Thiele for the speaker specifications. I can only assume those were fine tuning changes made by the folks who drafted the design.

Anyhow, I'm fairly sure now that standing waves were bouncing between parallel walls of the box. And the relative proportions between height depth and width may have promoted standing wave formation inside the enclosure. Those standing waves constitute frequencies that should not be present during sound reproduction. They're in fact harmonic distortion. When I made the cross section of the box narrower, it must have eliminated some of those standing waves. The sound is now much clearer than before, something I accomplished on the Mk. III with the non parallel surfaces of the horns.

The other thing that bothered me about the MKBoom was that the port was facing backwards. And many people reviewing the MKBoom noticed that the sound was "gummy," I guess a euphemism for the sound only being good in the center of the room, directly ahead of the speakers. I attributed that to the speakers concentrating sound in only one direction, since the woofers were being forced to produce higher frequency (3 kHz) content, and higher frequencies disperse less than bass frequencies. I reasoned some sound was being thrown in a different direction. A passing thought that I didn't utter loudly was that sound cancellation was possible for higher frequencies. But it worried me, even though you'd need to expect sound cancellation for *overtones* of the Helmholtz resonance frequency for that to happen (yes, you have overtones to the frequency produced by the port! Remember how the Mk. III uses harmonic undertones to transfer energy from a higher resonance frequency to a lower one?).

Ports facing backwards are not new, and extremely common in bookshelf speakers.  But here's where you have to pay attention: the operative word is "bookshelf." Any sound coming from the port is in phase with the sound coming from the front of the woofers. But if the bookshelf speakers are in front of a wall, you can bounce the sound from the port on the wall, returning the sound to the listener. You'd have to modify the length of the ports to ensure phase is consistent. Otherwise you could in theory get a beat pattern in the sound waves, effectively producing a pattern of quiet areas across the room. I think this is why the speakers sounded very different depending on where you were standing in the room. To begin with, I wasn't placing the boombox in front of a wall, and it wasn't sitting on a pedestal above the ground, like on a bookshelf!

Now I know that the position of a boombox typically isn't the same as the position of the bookshelf speakers, and that has an effect on where you place that port in your design.  I knew instinctively I couldn't copy their port length and diameter, and instead chose my own port design, but that was a lucky move, because now I get very little difference in the strength of sound as a function of the location in the room, aside from perhaps standing behind the boombox. But it's remarkable how that phenomenon disappeared altogether.  It was sound cancellation!! It was the electronics shop's error to try to adapt bookshelf speakers to a boombox enclosure. They also increased the volume of the original Dayton Audio Mk. 402 bookshelf speakers. It was a poor adaptation on their part. And they never considered phase issues from the rear ports.

Now what about the tonality? That also improved by leaps and bounds. Now I'm getting very clear mid and high frequencies, and the high bass is still there. It's really astounding how much the middle frequencies just jump at you now. And that's the last point: the sound is very clear now. Crystal clear. If there are midrange frequencies (human voice), you will hear them loud and clear.  Yes, the narrow port blows enough air to put out a candle, but what a difference it makes to point it forward!

I've tried the boombox with various soundtracks I know very well, and I can tell you the sound is very close to that of the Mk.I Victorian Boombox. A bit less in the sub woofer frequencies, but you'd never guess that there's no dedicated amplified subwoofer in there.  Makes me wonder if I could return to designing passive crossovers on the Mk. III boombox. The Mk. IV actually wins in the treble department, on account it has dedicated twitters for frequencies above say 5 kHz to 20 kHz, whereas the Altec Lansing system of the Mk. I has to make due with full range satellite speakers around the subwoofer.

Conclusion

Qualitatively the inverted horn Mk. III is still the top performing system with a full ~40 Hz - 20 KHz flat range, followed by the Mk. I Victorian Boombox, and Mk. IV falls to third place with a ~55 kHz - 20 kHz range, but by so very little under Mk I and Mk. III!! Wonderful news for me, all three boomboxes are spectacularly good and very close to one another in sound quality.  I'm very happy with the project now; it makes up for the disaster that Mk. II was, though Mk. IV will never be as inexpensive as Mk. II, so the market is more upscale and thus limited.

Problem solved and I'll follow with feet for the boombox next, and a proper handle.

[J. Wilhelm]

So, I've added a handle to the boombox. I really had to overthink this one.... Let me explain:

As I came back from work every day I kept an eye out at my local hardware store's website for a couple of weeks now, and I found a $20 "barn style" door handle made from welded steel tube painted with black textured epoxy resin. Really heavy duty. I loved the style, and it was the right size. Very strong with two 8 mm round head bolts.  But it was heavy, and to my mind expensive. So I tried to make a handle in the same style out of wood.

I settled on making a wooden handle from a 1 inch diameter round poplar dowel. A straight handle, cylindrical, about 12 in / 30 cm long. Poplar is very lightweight, very strong and with a straight grain. Looking at running timber bolts through the handle, however, I worried that the weight of the boombox would be too much, for the timber bolts over time. Same for the wood finish on the handle, although it would look spectacular, polished in red mahogany stain and spar varnish.

Then on Sunday night after spending hours at the hardware store just looking for the right timber lag screws for the handle (too many choices, few appropriate), I lost my wallet and then I spent the following Monday and Tuesday cancelling fraudulent charges on one of my debit cards (probably made by an employee at the hardware shop)... Which made me realize that my time is valuable and if I had decided to buy the $20 handle on Friday, instead of spending hours on Sunday at the hardware shop buying two stupid screws, I would have actually saved a lot of time and money.

Moral of the story, don't overthink it. If you find something good the first time don't try to save money looking for difficult alternatives...

So I bought the steel handle. It's really strong and its bolts go through the wood right next to the interior baffles, which work as hard points for the boombox. You can swing that boombox over your head if you want to, it's that strong.

The style of the Mk. IV B reminds me of the modernist "Prairie Style" of Frank Lloyd Wright. It's not quite Art Deco, but leans further back into the 20th century with architectural simplicity. Perhaps not as far back as Arts and Crafts, and definitely not Art Nouveau. But honestly, I think I'm going to call it that: the Frank Lloyd Wright.

I still have to resolve how to add a suspension to the box.  One idea is to simply put 4 short legs right under the hard points of the box, to minimize sound transmission. That is, the two side enclosures need to hang free from the center part of the boombox. Like a bridge, if you will, because the hard points (baffles) of the boombox vibrate the least, relative to other wood panels on the box. The Mk.I has that precise arrangement, because the legs are on the subwoofer part of the box, and the satellite speakers are suspended inside their respective "horns."

It makes me feel like I should have designed the side speakers to swivel up and down... That's something I'll try to implement in the next iteration of Mk IV.  I'm trying to get spherical or cylindrical ceramic vases or pots for that iteration. The idea is for the vases to have the same internal volume as the speaker enclosure.