Build a mobile sound system for off-grid concerts and dance parties!

This setup uses dual 12" PA speakers. A hidden below-board 24V battery keeps the handling stable. The flat-bed trailer design is easy to build, lightweight and durable. We integrated ~150W amps inside each speaker, but other amp configurations are possible.

Compared to other trailer-based systems this project uses improved layout and components to greatly reduce weight and improve power efficiency without any added cost. Overall power efficiency is 2x to 5x better than alternative setups. Total trailer weight as shown is about 95 lbs, compared to a traditional car-battery-powered setup at 150 - 200 lbs.

This project is brought to you by MonkeyLectric and the Monkey Light bike light

Step 1: What You Need

Bike trailer. Not all trailers may be suitable, check the next step.
1/2" Plywood (2' x 3' piece or so)
2 x 12V, around 15Ah sealed-lead-acid (SLA) batteries (see below)
24V battery charger (see below)
24V amplified speakers (see below)
3.5mm audio cables and Y cable
Small stereo mixer (see below)
mp3 player
mp3 player holder
lots of 1/4" bolts, spacers and washers
15' of 16 gauge wire
electrical crimp connectors (home improvement and auto parts stores have these)
10A circuit breaker

BIKE TRAILER

see next step.

BATTERIES

I'm using 2 x 12V, 18Ah capacity SLA rechargeable batteries (also known as "sealed lead acid" or "gel cell"). Together this is about 1/2 the weight of a car battery and it powers my setup for 8 hours at full volume. Reducing the sound volume just a little it will last much longer. You can use bigger or smaller batteries depending on how long you need to run for. SLA rechargeable batteries are very inexpensive and come in lots of sizes. You can get new ones like mine for $35 each - try batterymart.com or batterywholesale.com or amazon.com. You're looking for "12V SLA battery" or "Sealed Lead Acid battery". This type of battery is often found at recycle centers in good condition, so you can get them for free that way - look for discarded computer back-up power systems. You can use a lithium battery for even less weight, but it will cost around $400 for the same capacity.

BATTERY CHARGER

These are mass produced for 24V scooters so they are only about $15. Amazon.com has lots of them like this one, just search for "24v scooter charger". A 2A charger is perfect here.

24V AMPLIFIED SPEAKERS

Using amplifiers that run directly from your battery (without an inverter) makes your setup a lot more efficient. I have instructions on how to build battery-powered amplified speakers here. We sell those same speakers fully assembled at MonkeyLectric.

AUDIO CABLES

parts-express.com and amazon.com have all the ones you need.

POWERED STEREO MIXER

To get full volume from the setup you need a small powered stereo mixer. The mixer will also let you plug in a microphone and additional sound sources so its convenient. I used a Rolls MX51S, you can get this from parts-express.com or amazon.com, it does the job for about $55. There are some other brands with similar models.

Step 2: Making a Flat-bed Trailer

The key to this project is a simple bike trailer with a flat wood bed that you can bolt everything to. Keeping the trailer simple makes it light weight and easy to make.

You can get a commercial bike cargo trailer for this project, or you can make one using one of the many Instructables. I bought a basic bike cargo trailer from Sears.com, the "M-wave trailer" for $200.
Most commercial bike cargo trailers like the one I got have a fabric or plastic bed which isn't strong enough for our purpose, but can be replaced with a wood bed.
Several of the "how to make a trailer" instructables are easy to put a wood bed onto if you choose to go that route.
If I was doing this project over again I'd probably start with the Wandertec Bongo trailer which looks perfect for this project without any modifications. It's the only commercial trailer i could find with a flat wood bed.

Working with the M-Wave trailer (see photos below):

Remove the side-rails
Check the fit of your speakers on the trailer bed.
Unscrew the plastic bed sheet from the trailer frame
Use the plastic bed-sheet as a template for a new wood bed. Just trace the outline and screw holes of the plastic sheet directly onto a piece of 1/2" plywood and cut out the plywood with a saw. I added about 4" extra wood at the rear of the tralier so my speakers would fit on.
Paint the plywood so it will be waterproof
Screw down the plywood to the trailer frame.

Step 3: Checking the Mounting

The speakers mount on the top side of the bed and the batteries on the bottom side. This is a very efficient use of the bed space and keeps the battery weight low to prevent the trailer from tipping.

Mark the mounting hole locations for the speakers
check the battery fit on the bottom side. the batteries will be held on by a 2nd piece of wood in a sandwich. With 2 batteries we will use 6 bolts to hold them on.
Check that the battery bolts don't interfere with the speakers, and that the speaker bolts don't interfere with the batteries. You can use flat-head screws if anything does interfere, or just move things around a little bit.

Step 4: 24V Amplified Speakers

Choosing the right speakers and amplifiers makes an enormous difference to the amount of sound you will get for the same cost and weight.

I have complete build instructions here for making several sizes and types of efficient setup.

If you want to leave the electronics to us, we sell fully assembled, integrated amp+speaker setups just like the instructions above: MonkeyLectric store

Step 5: Attach the Batteries

The batteries are glued with silicone so they don't rattle or shift around. Silicone is weak enough that we can remove the batteries in the future if they ever need to be replaced. SLA batteries normally last 250 - 500 charges, so that should be years unless you use it every day.

I used 1/4" bolts, about 4" long. I put plastic spacers around the bolts to protect the sides of the batteries from vibration. A strip of rubber should also work.

I cut a piece of plywood about 6" wide and 18" long to go across the bottom of the batteries. 6 bolts sandwich the batteries in place.

Step 6: Attach the Speakers

You need to get a strong connection between the speakers and the bed. Many speakers do not even have mounting holes good enough for this, if yours don't then you will need to drill new holes through the bottom of your speaker and use a bolt and fender washer on the inside of the speaker like this.

The Peavey's that I used have 5 x 1/4" holes which work nicely. The only catch is that the bottom of the speaker is not flush with the wood bed because of the rubber feet sticking out a little. You don't want to just screw into the 5 mount holes that way because it will bend the speaker plastic. Put a couple of hex nuts or washers onto the 5 bolts between the bed and the bottom of the speaker to take up the gap.

After bolting on the 2 speakers I also strapped them together half-way up so they don't jiggle at all (see the final photos)

Step 7: Power Wiring

16 gauge wire should be fine. I put a combination on/off switch and 10A circuit breaker. You can get that from digikey.com part PB1033 for $3. The batteries and breaker all use standard crimp terminals.

Step 8: MP3 Holder

On the handlebar is a Bracketron mp3 holder. It's made for cars so it doesn't attach to a handlebar normally. I used hot-glue to attach the bottom of it to the mount for an old bike lock (a headlamp mount would also work). There are lots of mp3 player holders out there with belt-clips and arm-bands and dashboard mounts. Most of them can be glued onto something that will hold your handlebars. An arm-band type may attach to your stem with little or no modification. Make sure your holder can handle the vibration of riding.

Step 9: Audio Wiring

A 6-foot audio cable plugs into the mp3 player and goes down the bike frame. It ends at the trailer hitch. The hitch has a mating audio cable, so when the hitch is attached you just attach the 2 audio cables.

The cable on the hitch goes to the input of the mixer. The output of the mixer goes to a Y-cable (also called a "dual headphone cable"). Each end of the Y goes to one of the speakers. There's also an input on the mixer for a mic.

The mixer boosts the output from the mp3 player, you won't get full volume just plugging the mp3 straight to the speakers. So you want the mixer even if your only input is the mic.

The mixer runs on 12V so it's wired to just one of the batteries. Make sure it gets turned off along with the main switch.

Alternatively, you could mount your mixer and mic onto the handlerbars. You just need a 2nd power cable to run up from the hitch for that.

Step 10: Engage Your Audience

If you plan to ride a trailer like this around town, a few things to keep in mind:

Mobile sound is a great way to connect with your community. Mis-used, it is a great way to do just the opposite.
Play something that most of the people on the street will like. Catchy danceable music is good because the people you pass will only hear the music for a few seconds. Angry rage music is not going to win you friends.
Smile, wave and dance with the people on the street, so they know its fun for everyone. If they have a bike invite them along with you.
Turn down your volume going through residential areas. Even more when its after 10pm.