Introduction: Styrofoam Sound Box for Mobile Purposes

Your powers of observation serve you well! It ain't look like a styrofoam box, neither it's made entirely of it in fact but... wait =P

As I researched about “mobile sound systems” for outdoor uses such as Bike Parties I've found a lot of references on the web (many of them here in the website).

However, they were built either with adapted commercial models of P.A. boxes (implying in some disadvantages), either with low power components to reduce weight (limiting the loudness), or either using very expensive kind of batteries such as LiPo or LiFe (that have a very high capacity/weight relation, but out of my budget!).

In my case I was limited by the traditional lead-acid battery (a little better model, but still heavy), so I had to purge some weight from somewhere else in order to tow it with my bike thru a kinda hilly city without killing myself. They inspiration came from compound panels such as drywall, ceiling tiling and some kind of doors. I've basically made a sandwich of two plywood sheets with a styrofoam board between them.

Although the instructable title is about the box, we'll also see about the hardware parts which I found more efficient for the job, so let's go! =)

Step 1: Acquiring Some Information About Sound Transducers and Amplifiers

The objective is to make the most loud, lightweight and long lasting per charge sound system (with good sound quality too). And for so: efficiency.

I'm not going too much into theoretics here cause is not my strong suit, and you can find more reliable stuff through the web, but here are the main start points I've reached:

- It's not just the speakers wattage that counts for loudness, but a factor called SPL (sound pressure level) informed by the manufacturer. A slightly difference of it changes dramatically the dB (decibels) that the transducer can give (loudness \o/).

- For covering most of audible frequencies according the objective we established it's only needed two transducers: one woofer (for low frequencies) and a drive (for high frequencies). A midrange it's not necessary since the frequency they cover are among the most annoying frequencies for the human ears (1 to 5 kHz). Even less a subwoofer, which can be pleasant in a closed room at moderate levels but out loud in the streets serves more as alarm triggers.

- There are many types (classes) of amplifiers and each one has its pros and cons. For our objective the only practicable option is the “class-D” amplifier, that bases on switching (PWM) power. As the switching power supplies, they are extremely lightweight and energy efficient (up to 90%).

- It's imperative to use a crossover to address the specific frequency bands for their respective transducers. The most efficient way is using an active crossover before the amplifier.

Step 2: Defining Key-components


Let's start by the most energy consuming transducer: the woofer. I decided for a 15” model, 400W RMS and 4Ω of coil impedance. As I read, that's the most suitable size for variated music styles, playing well both crispy hits as some extended bass too. The wattage I didn't choose: it was one of the weakest available for a 15”. The impedance I chose to match the output impedance of the amplifier module (to be seen ahead).


Then I looked for the compression (horn) driver, responsible for the high frequencies. I chose a titanium diaphragm type, since it reproduces higher frequencies than the phenolic type (working in part as a super tweeter). For my project, the driver's job was just to put some glitter in the sound, not to make a hysterical screaming thing. Then I chose a 80W RMS model (again, one of the weakest available). The coil impedance, 8Ω, seems to be standard for this kind of transducer and that worried me about matching with the amp's impedance, but it was alright as we'll see next.

Amplifier module

Now it was about finding an amp that fitted to my needs. I found a 800W RMS 4-channel, 200W RMS / 2Ω per channel (4Ω bridged). The wattage and impedance fitted just perfect for the woofer (400w / 4Ω), now about the driver: if I did the same as with the woofer, I was afraid of frying the driver. So I decided to plug it in just one of the channels (200W /2Ω). I did a test and it played very well, even though the driver's impedance is four times the amp's single channel impedance.


The model I found has some features besides the basic filtering. It has a mono/stereo mix switch (useful, since stereo sound does not make sense in a single box), a bass boost knob on the low pass line, a subsonic filter (to suppress frequencies below 30 ~ 60Hz), and an individual on/off switch for each line (useful for testing/tunning).


They say that ordinary car batteries should not be used in cyclic (charge and full discharge) applications, having their lifetime compromised if doing so. Then I found this brand which promises a good durability even when used this way, plus having a better instant discharge rate (it can deliver more amps at music peaks). The D51 model seemed a good choice for the project.

Step 3: Box Size Calculation

Acoustics studies can look like witchery for ordinary people like me, but FORTUNATELY computer and Internet makes our lives easier (ok, sometimes). I found a couple of websites that offers calculation scripts for woofer's required volume, port length and box measurement simulator, here:

The first thing is to discover what is volume required by the woofer. You'll have to fill the form with data provided by the manufacturer. After that you can proceed to the port (vent) calculation and the box construction simulation.

I have a bike trailer already (which I made myself too) and I took it's width as a start point, along with the minimal height necessary to fit the plastic horn, the woofer grid and the port at the bottom to dimension the front of the box. So, the free factor for calculation was the depth of the box.

I've read that the angled wall at the back was somehow better for sound quality. Then I played with the 3 measures left until I reach the calculated volume (watching for the minimal depth for the driver plus horn).

Step 4: Box Assembly

I really hope you do have a nice space for handling with wood, dust, noise and dirt... BUT if you don't, I bet you'll feel motivated by knowing that I've assembled it in the small living room of the apartment that I live with my parents :B

Of course, I tried to make things easier as possible: I bought both plywood sheets and styrofoam boards already cut square in the measurements (or close to them). So I had only a few cuts/adjustments to make with the hand router. Oh, It really helps to draw all of them in some CAD program before! Note that the size of the sheets should vary in a same “sandwich” board, depending of the arrangement you design for joining each other.

To glue the EPS (Styrofoam) to the plywood I used a special contact glue that does not attack/melt it. PVA glue could be used as well (and it's easier to work by the way), but you'd have to find a way to keep it pressed till it dries (about 3 hours), and it would take longer to have the whole thing assembled (a martyrdom for anxious people). I've also employed 1,5cm square wood profiles to improve stiffness at the junction perimeters.

Once you have the sandwich panels plywood-EPS-plywood ready, pre-assemble the box using tape to check the fittings and trim the excess material until they fit nice. Cut the holes for the horn and woofer. Assemble the parts definitely with PVA glue, nails and screws as you find necessary. I predicted a nylon ratchet strap to quick attach the box to the bike trailer, that can also be used to keep things tight while the glue cures.

The full gallery can be viewed at the link below (don't mind the mess)

Step 5: Box Finishings

The edges were rounded with a hand router, and I applied two coats of water based wood sealant all over the box to improve impermeability. The traditional wood sealant could corrode the EPS, plus it has a strong smell for using at a small flat.

Next I applied three coats of black matte paint over the exterior only, synthetic enamel in case. But there are many finishing options as rubberized paints, leatherette, self adhesive vinyl... is more of a matter of taste and budget. Also, I've glued strips of EVA foam at the back cover junctions and at the bottom of the box (to be in contact with the trailer), to attenuate vibration noises.

Again, more details here:

Step 6: Components Mounting & Connections

The woofer and plastic horn were secured with 3/16” bolts/nuts, then the driver was screwed in the horn. The woofer's protection grid was fixed with 1/4” bolts/nuts.

The amplifier module and the crossover were positioned at the back cover, in such a way the connection paths could be optimized.

Wirings for:

Audio signal: shielded 6mm RCA cables (I've assembled myself);

Output audio: 2.5mm² (13 AWG) polarized cable;

Amplifier's power cable: 10mm² (7 AWG) * The owner's manual suggests 16mm², but I guessed that's for longer distance connections (automotive systems)

Other power cables and remote (crossover, mixer, etc): 1 or 2mm² should be fine.

A fuse or/and a circuit breaker must be positioned as close as possible to the battery. I've chosen for only a fuse holder with a 30A fuse. It should be a higher amperage in fact due to the amp's power, but I keep a 50A spare one in case the first melts.

See the connections diagram.

Step 7: Remarks

As it's my first instructable, I'm not sure if I could clearly show what you were expecting to. Then I ask you to give me hints so I can improve it! Even if it's about language mistakes or technical terms that are kinda hard to translate.

Upgrades and notes:

- Don't let the voltage drop under 11V during use! I've lost a speaker once due to - as a technician explained me - some kind of distorted signal that the amp sends trying to compensate the low voltage, frying the speaker's coil.

- Substituted the original noisy mixer for a better quality one, that also gave me more control for aditional inputs and high/low adjustmens (very useful).

- Added a PVC canvas fabric cover, so the weather won't spoil the party anymore ;D

- If you're not gonna ride on a very plain city, consider investing on an electric bike or investing on a lithium battery to power the sound. The lead-acid battery makes things heavy!

March, 2017:

- The Optma YellowTop battery seems not to be made for really deep cycle use. It lasted longer than an ordinary lead-acid, but it's not holding charge as the same when new. So, it does not pay for itself for this kind of use.

June, 2017:

- Flipped the box facing down (downfiring), so the sound can spread out in radius, instead of the original directed to the back. Now every one around can hear at same level, in a volume that's not so aggressive too. Only had to put more gain on the treble.

Some videos:

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