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Bicycle Sound System
Completed: July 2001. Last update September 2005
I have had a variety of sound systems for Critical Mass. The most basic one is designed to fit on any bike that has a rear rack. You could probably fit it to a front rack it would be ugly because of the weight.
I bought the speakers and amplifier from a car stereo shop (Strathfield Car radio in Sydney. In one of their regular sales the speakers were $50 the pair, and $79 for the amp (100W per channel stereo amp). They're cheap but they work pretty well, and the amp runs off a 12V battery. The battery is a 30AH (10kg) sealed lead acid (SLA) on from Jaycar but you can sometimes get batteries cheaper from Oatley Electronics. Wiring it up was easy enough - just remember that most car amps have three power terminals, and two of them go to the positive side of the battery (one is the "power on" connector). I have a switch there, but be a little careful as some amps draw a lot of current when they're "off" (200mA doesn't sound like much, but it will flatten a 7AH battery in just over a day, and wreck it). So unless you can check this, just unplug it when you aren't using it.
Buying a charger for the battery will cost a bit extra - although a cheap one will work it will probably either be very slow or a bit hard on the battery. Mine is a car-type one that charges the battery in about 6 hours. Note that SLA batteries last much longer if you don't fully discharge them. Half discharge is the max - beyond that you're really hurting the battery. So work out what your amplifier actually draws, and work out how long you've got! Roughly, an audio amplifier at full volume will draw half its rated power. Except with bass-heavy music, which it might draw more than it's rated for. So my 200W (100W x 2) amp should draw 100W max, or about 8A (power is volts times amps, so 12V x 8A is 96W). 8A from a 30AH battery gives about four hours at full volume, of which I want to use two hours at most. So I've got enough for Critical Mass, and if I keep the volume down I can go for 3-4 hours without problems. Or an hour of doof-doof music ;-)
I loaned the system to a friend who ran the battery dead flat a few times and charged it rarely. The 30AH battery now runs the system for ~15 minutes. So I used the money from her to buy a 10AH NiMH pack with charger from BatterySpace (via ebay) for $US99. This is much, much smaller and lighter (about 3lbs) and goes for almost as long (10AH usable instead of 15AH), so it's a lot easier to carry on the bike.
To attach the speakers to the rack I just use bent aluminium strips. They're about 5cm wide and perhaps 10cm long before bending. I drilled them before bending, three screws each because the speakers are made of thin chipwood (10mm) so more screws. Once they were bent and I test rode the bike, I drilled a new hole through the hook and put a screw right through it, so that the speaker can't bounce off the rack. My rear rack is not straight-sided, so the hooks tend to straighten and the extra screw stops this too.
I strongly suggest a solid-state player (no moving parts) if you're attaching it to the bike. Even a small hard drive probably wont like the vibration from the bike, and a CD player will skip even with new batteries. Carry those in your pocket instead.
When I finished building Tall Bike Too I thought it was about time to build something that would last. Mark offered me some plywood and gave me a lot of help with sourcing the electronics, as his hobby is playing with such things so he knows where the good deals are.
The new box is made of 9mm marine plywood, shaped to fit exactly into Tall Bike Too. This makes for a nice big enclosed volume, and consequently reasonable low bass response. The speakers are arranged as left and right channels on left and right sides of the bike, but the internal partition runs across the box not along it, so stereo separation doesn't really happen at low frequencies (just like people's ears). All the joints are glued as well as screwed in place. That's a combination of rapid assembly to a fairly robust state, with a solid, air-tight joint once the glue dries. Construction time was about three hours, since I used the full range of power tools.
The stereo part is based on a 250W car stereo amplifier from Strathfeild Car radios which provides a nominal 125W per channel and has a low level input and a gain control. That lets me set the max volume before the lid goes on and helps prevent people blowing things up by fiddling. I've got four 250mm bass drivers each rated at about 50W, a couple of 50mm midrange drivers, and two horn tweeters that came out of my original speaker system. Rough schematic picture
There are two cables running out one side - the audio input (a 3.5mm jack) and power input (a generic two pin connector). Current drain gets to about 10A continuous, and my digital meter won't tell me the peak drain. I suspect my 7AH batteries are the limiting factor here. The sound source is normally a Diva solid state MP3 player rather than a CD player because Mark has had huge problems with those. That gives me about 10 hours music at 128kbits, which is about the battery life of the player, and about 5 times the life of the 12V batteries. I've added a 3.5mm socket to one end of the box as an extra audio input because the cable isn't really long enough when the system is on One Less Ute.
The midrange drivers are not sealed, so I've kind of isolated them using a couple of plastic containers which are silicone sealed onto the inside of the box. For crossovers, there's a 1 ohm resistor in series with the latter two speakers, and a capacitor in series with the tweeter to keep things sane.
In use it does very well as both a doof box and general music player. I've used it as a PA a few times, and am working on getting a mic and mixer of my own. That will basically complete the system. Bass gets solid enough to quite noticeably vibrate the bike frame :-)
I sold this system with Tall Bike Too, as it was custom built for the frame. The guy who bought was going to bring it along to events but I haven't seen him. He loaned it to an RTS and I noticed that he'd cut off a corner of the box and also added a hatch so the batteries can go inside. It's down to about 60% of the original enclosed volume, and it's no longer sealed. What bass is left has turned even more waffly and distorted. It's not what I expected when I sold it, so I think the lesson is: only sell stuff with obligations if you already know the buyer.
This is what I started out with - a speaker box and 30W per channel amp bought cheap from a car radio place, powered by a 7AH SLA battery. With a spare battery just in case. I built a small box to hold the amp and batteries because there wasn't really space inside the speaker box to fit them in. The sound source was a small radio, so I could pick up Mark's broadcast during Critical Mass. It fitted across a standard front or rear rack, or quite well on my double-length tandem rack shown.
This system was gutted to provide parts for other, better systems. I think the horns went into the white box above, the bass cones went into a Wheely Bin and presumably so did the amp.