A Battery-Powered Sound System
Part of what I want to accomplish with this float involves getting the attention of people who aren't looking at us already, and that means sound. The sound I want is something spooky, and not the sound of a generator or a motor vehicle, so that means I need a battery-powered sound system. I tried to do some research on "how much power do you need for an outdoor speaker?" in various circumstances. This was not easy on the modern internet, and I got all kinds of mixed answers. Best I could find was a rule of thumb that "outdoor party" meant you were looking for 10 Watts "per person". I figured that since what I'm looking for in this application is something that people can hear from inside their homes when my float is more than 30 feet away, I needed 250 Watts or more of power.
The best explanation I could find of how to put together a simple, lightweight, but good-sounding system was on Instructables.
What I learned from this article
- You're not looking for car audio stuff, it's inefficient both in terms of power use and weight.
- You're not looking for stuff that runs on 120V AC, as converting your battery to that wastes a lot of power.
- What you're looking for is portable-ish DJ stuff.
- Get a passive speaker (one without a built-in amplifier) and find a DC amp to match.
- Don't bother with stereo.
What I bought:
Based on the advice in the above article, I purchased the following:
- A demo model of a Behringer Eurolive B215XL 1000W 15 inch Passive Speaker from sweetwater.com (I can't say enough nice stuff about sweetwater, their customer service may be too good.)
- The Rolls MX51s Mini-Mix II 4-Ch Mixer from parts-express.com
- The Sure AA-AB31511 1x500W Class D Audio Amplifier from parts-express.com
- Three 12 Volt, 9 Ah Sealed Lead Acid Rechargeable Battery with B1 Connector from batterymart.com
- And a project box, some jacks and some wire to put it all together.
I did some searching and my best bet seemed to be a building a system that ran on about 36V of DC power. Figuring out how the power of amps and speakers work is a little confusing. As you may recall from your brush with AP Physics, watts measure power, amps measure current, and voltage measures potential. And the equation P=IV may still be in your head. That's Power = Current * Voltage. And when you get into speakers there's like peak wattage and "rms" wattage, which is root-mean-square wattage, which is like a kind of average wattage, given that we're trying to reproduce sound, and that's a wave, and waves are wavy and not the same thing all the time. The higher wattage your speakers, the louder they are, but the stronger your amplifier and power source need to be, according to that equation. How many amps you're moving around determines how thick your wires need to be, and how beefy your fuse is.
The Behringer speaker says it's "rated at 250 watts continuous (1,000 watts peak)". The Sure amplifier says it will do "350W x 1 (3 ohms, THD 1%)" and draw at most 10.8 Amps if you give it 50 Volts of DC power and try to output 500W. It maxes out around 500W, and it'll accept as little as 15V. I decided that giving it about 36V will not endanger this speaker at all. And my back-of-the-envelope calculations say that 250 Watts divided by 36 Volts is 6.9 amps. If that math is right, with these 9 Amp-hour batteries, I figure it'll run this speaker at full power for more than an hour. And so probably I'm in good shape for at least 1.5 hours at the power I'll be operating at. Although I also have a suspicion that maybe power and batteries don't work that way and there's some calculus involved. I decided this was good enough for DIY work and though an actual electrical engineer might do a better job, if the thing runs out of juice after an hour, well, I will have learned something.
The speaker weighs 32 lbs, and the batteries total about 20 lbs. A car battery is about 50 lbs, so we're doing pretty good in terms of weight here.
What else I learned
The wiring up of this is not that complicated, and there's a decent explanation in the Instructables article. It doesn't emphasize the need for switches too much, or talk about where to put them very much. It's important to have switches in there so you can turn the thing on and off. Here's the wiring diagram you do get:
What this doesn't show is that the mixer and the amp are connected by the signal cable, which includes a shared ground. Here, let me show you how my thing ended up looking at first:
I looked at this for a while and started to get a weird feeling, like maybe there was a secret ground loop - it felt a lot like the 12V line running to the mixer was also a potential ground for the amp, resulting in it possibly drawing 24V through a circuit that might be flowing the wrong way through the mixer? It seemed like a recipe for trouble. Weird humming and pops that occurred even when the ground switch was off confirmed my suspicions. I ended up using two switches, one on the 36V hot going to the amp and one on the 12V hot going to the mixer.
I have not worked up the confidence to plug an actual iPhone into the mixer (helps that iPhones don't have headphone output anymore) but I have plenty of old mp3 playing stuff laying around. I'm pretty confident whatever I plug in won't get wrecked - as long as I don't accidentally cause a short circuit by messing with the exposed wiring. The mixer works good for adjusting the volume and the system can get pretty dang loud. Loud enough for my purposes, and has decent bass with that 15" woofer. And, someone could plug a microphone into the mixer and use it for a protest or to announce an outdoor event, or who knows what. Hopefully I'll find additional uses for this in the future.
Right now, everything other than the speaker is sitting in a plastic tub with holes cut in it to get at the knobs of the mixer. A future task will be to either (a) follow the instructable and get the amplifier stashed inside the speaker or (b) find a way to separate the batteries from the rest of the electronics, so they can be kept somewhere less conspicuous and more easily charged.