Tech Appendix: More about Mobile Sound Systems

When you’re putting together a sound system, there’s lots to learn about — much more than we can cover in this book. You can spend either a little or a lot of your time and money. Talk to your friends, look online, and read books for tips and techniques to build the best system. What you learn will be useful whether you are setting up a studio or a mobile rig. As you improve your knowledge and skills, you’ll probably redesign and rebuild your system, but that’s part of the fun. We’ve gone through many versions of our system, and we’re still making changes. Here’s some ideas to get you started:


In a venue or studio, you can simply plug in to 110 volt house power. When you go mobile, you’ll need some battery power. A lot of systems use 12 volt car batteries, which are available both new and used. Unfortunately, they are very heavy. In many cases they are bigger than your freestyle session requires, so you might not need all that extra weight. Deep-cycle batteries from boats, RVs, golf carts and other systems are even heavier, but are designed to discharge more of their power before a recharge.

12 volt Gell cells can be found that are a lot smaller and lighter than car batteries. You can find them in different sizes new at electronic supply stores, but you can also get used ones from computer recycling centers for very cheap. These used batteries are often enough to power a moderate sound system for a few hours.

Some cordless drill batteries are 12 volts, and they are usually lithium, so they store a lot of power for their size. You’ll need to play around a bit to be able to connect these to your system. Some smaller systems can run on alkaline AA or D batteries.

Whatever battery system you use, you’ll need to get a battery charger. Make sure your battery charger is matched to your type of battery; the wrong charger/battery combination can destroy your battery and can even be dangerous. Remember to recharge your batteries after a session so they are ready for next time!

If you’re out in the daytime, you could add a solar panel and charge controller to charge up your batteries on the go. We’ve even seen some systems that use pedal power to charge their batteries, but you can’t really ride and charge at the same time.

To gauge how big your battery really needs to be, calculate the total peak amps your system uses times double the number hours your session will last. This is the amp/hour rating your battery should provide. Depending on your system and your batteries, in practice its a little more complicated than this. For more information, get a good book or find someone that knows about 12 volt electrical systems.

It’s a good practice to wire a fuse between each item of equipment and your battery. This can prevent damaged equipment, melted wires, or a fire in the event of a short circuit. Disconnect your battery before working on your wiring! 12 volt power systems and are quite safe, but be careful when working with electricity and tools. 110 volt systems are much more dangerous, and can seriously injure or even kill you.


Whatever amplifier you use, the main requirement is that it will take an auxiliary “aux” input, so you can plug in your mixer or other sound source.

A lot of people use car audio amplifiers for their mobile system. They operate on 12 volts so they can run from your battery directly, and they are easy to find in old cars, but they can be quite heavy and aren’t always that efficient. You can use an old home stereo amp, but since they run on 110 volts AC power, you’ll need a power inverter to convert your 12 volt DC battery power to the 110 volt AC power your amplifier can use.

There’s a selection of digital amps called C-class (also called T-class, tripath, or switching) amps, they are very power efficient and great for mobile systems. Many will run on 12 volts. You can find a selection of them on websites like eBay or Amazon, either as circuit boards or complete devices with a nice case for a little more money. Since you’ll want something rugged for your mobile rig, it’s good to get one with a case.


You can try wiring up any old speakers and see what happens. Your sound might be too quiet or distorted. In the worst case scenario you can blow your speakers or the amplifier. If you’re designing a new system, speaker resistance (ohms) should match the amplifier output. Car speakers are rated for car audio systems, usually 2 or 4 ohm. Home speakers are usually 8 ohms. If you chain multiple speakers on the same output channel, it will change the resistance, depending on if you wire them in parallel or in series. Do some research on how to correctly wire up speakers.

Speakers are also rated for frequency response, with multiple speakers required to cover the audible range. Some two-way or three-way speakers produce good sound for treble, midrange and bass frequencies, but you might want to add a subwoofer for a louder low end. Good subwoofers take a lot of power and are heavily built so they don’t rattle. This makes them inconvenient for a mobile system, so you’ll have to compromise. Some car audio subwoofers can pump out a lot of sound at high power but are inefficient at low power. Others have a high sensitivity (as measured by the SPL rating), and so will be louder at a lower power. There’s “rare-earth” neodymium speakers that are much lighter. These would be ideal for mobile rigs, but they cost more.


If you’re running any equipment that requires 110 volt AC power from a 12 volt battery, you’ll need an inverter to convert the power. We try to avoid using 110 volt equipment with our mobile system, because there’s a few problems with using inverters. Besides having the weight and space of one more item of gear in your system, a common inverter will often add an audible 60Hz hum to your sound. There are “pure sine” inverters available that should avoid this, but they are more expensive and usually built for high power ratings. The other drawback is inverters are only about 85% efficient, meaning that your battery will only last 85% as long as it would compared to the similar system running on pure DC. 110 volt power is also more dangerous, with a higher risk of electric shock.

To find the size of inverter you need, add up the maximum wattage of your amp, mixer and anything else you will run on AC. Make sure the result is less than the inverter’s continuous power rating (not the peak or maximum rating). You can can find simple inverters and many hardware and auto stores starting from about $20.


A few specialty performance amplifiers and portable stereos will include a channel for a microphone. We like to add a mixer to the rig so that you can add one or two mics and mix the volume of the tracks and the different MCs. Almost any audio mixer you find will do the job, if it has channels for two mics and a stereo sound source. DJ mixers are good but usually have only one mic input. Most mixers you find will run off 110 volt AC, so for a mobile system you’ll need an inverter to plug them in. We’ve tried building our own simple mixers but it’s hard to get all the features into a rugged package similar to the commercial mixers. Yamaha makes a small six-channel 12 volt mixer that will run directly off your battery for around $120. You may need to find a special DC power cord to connect it to your battery, or you can cut the cable from the power supply it comes with.

Mics and cables: We try to use two mics at all times. More mics can be fun, but you can also get too many voices going at once and a lot of confusion. Mics take a lot of abuse, so good, solid vocal microphones are best, but old karaoke mics or other mics will do. Our mics include a Shure sm58 (about $130 new) and a Yoga dm250 (found at a thrift store for $5), and they both are rugged and sound great. It’s worth reading up to understand why some mics are better than others, but you don’t need to spend a lot of money on gear. Check with relatives, try a thrift store or ask at a sound equipment rental shop. Mic cables, also called XLR cables, Long mic cables are good. 25 feet in length (or more) helps MCs get creative as they roam around the jam area.

Connectors: There are many different types of cables, so wiring up your system can be confusing. Mics use XLR (or sometimes 1/4″) cables. 1/4″, 1/8″ or RCA cables connect sound equipment to your mixer, and connect your mixer to the amplifier. Speakers are often connected with bare speaker wires, but sometimes use some kind of connector. You may need adapters, 1/8″ to RCA, 1/4″ to RCA, and vice versa. You will find it’s good to have a selection available. For a mobile system, try to secure all cables with cable ties if possible, so they can’t be tripped on or pulled out while you are rolling.

Lights: Now that the sound is bumping, you might think you’re done. When it gets dark, you’re going to want some lights. You can get El-wire, LEDs, lasers that run off the same 12 volts as your sound system, and at night they will really amp your game, but even blinking bike lights or other glowing things from a dollar store will help you be seen and make your show more exciting.


Our own MC Mahi Rahi constructed a big red box on a cargo bike to put our whole system together. The box contains our battery, amp, speakers, some lighting, and on the top we’ve got our mixer. The top of the box is high enough to provide a small table. It looks great. It’s not too heavy, but it is hard to move around. You could use a modular setup with different units for your speakers, battery and amp. We’ve also experimented with having all the pieces together in a plastic tote bin on a bike trailer. The easier your system is to carry around and set up, the more you will use it.

There’s some great powered speakers (like the Traynor TVM10) designed for street performers that contain a battery, speakers, amp, input and mic channels, so you would already have everything you need.

Putting it all together:

There’s a lot of little things that make everything work together. A nice paint job on your gear will make you look good. Wiring should be well done so it’s reliable, easy to troubleshoot and service, and safe. Your mics, mixer and sound device need to be accessible, but also secure so they don’t fall off your rig when you’re on the move, and they don’t get wet if it starts to rain. Velcro straps, hooks, bungee cords (or old bike inner tubes) and tie-downs will help keep your gear where it belongs. Some additional features can include hand holds to make it easier to move your gear, drink holders to keep your beverages safe when you’re rolling, and a rain cover to protect your gear.

Once you’ve got your system built and tested, it’s time to hit the road and freestyle! You’ll probably learn a lot and go through a few versions of your system as you build it out to your needs. There’s a lot of resources out there to help you on setting up audio systems. Have fun!