First off, I must say that the design of BeastBox owes a lot to a sound system called Son of Pedals, the designer of which put up a very helpful web page . Without that one page of notes I guess I would have got there in the end, but it would have been a lot harder! I'll refer to Son of Pedals and that one page of notes throughout this Instructable and to save time I'll just use SoP.
BeastBox is a trailer mounted sound system that can just about be towed by a reasonably fit person on a mountain bike. It weighs 54kg and will play seriously loud music with proper gut-thumping bass for 3-4 hours. I borrowed a sound meter from work and clocked it at 120dB(A) at 1m, not bad for something that runs off torch batteries :-D
Someone shot some video of the March 2011 Critical Mass ride and caught me a few times along the way. The guy kindly let me edit the footage and I have uploaded a new video showing just the clips of BeastBox to give an idea of the sound level. The part at 1:30 under the bridge was very loud - you can hear it is overloading the camcorder's microphone!
I have used it on a number of Critical Mass rides and a few other cycle events in and around London over the last 6 months. I nearly always get a few people asking how it works, what it runs off, where I got the plans etc. and a friend of mine suggested I write an instructable for it. This is my first Instructable so I'll welcome any comments, good or bad!
Step 1: It's All About Efficiency
Like SoP, Beastbox uses a 6th order bandpass box for the woofer. This is a two chamber enclosure with the driver mounted in the dividing partition. Each chamber is tuned to a different frequency so that the effect is to get a big bass boost at those two frequencies and some boost over the ones in between. The frequency response comes out a bit lumpy but you get a 5-10dB boost which makes a big difference.
The other important factor is the amplifier(s). The obvious choice of amplifier is some sort of car audio unit. My first sound system used a second hand car amplifier and wow, did it get HOT! All of that heat is wasted energy so an efficient amplifier was a priority. Fortunately you can now get Class D amplifiers which are much more efficient than older units. Until recently you could only get Class D bass amplifiers but there are now full range Class D amps which cover the complete audio range. Beastbox uses two Class D amps, one for the bass and one for the midrange and tweeter.
Step 2: Trailer
Other candidates are frames from child trailers (lots of them on eBay) or even a DIY trailer.
Step 3: Main Enclosure and Bass Driver
Before starting on the box design you need to choose a bass driver because the chamber volumes have to be tuned to the driver you intend to use. I used a Fane Sovereign 15-400 15 inch 400W driver. The 400W is probably a bit of overkill but I could get a more or less unused one cheaply. A better choice would have been a driver that uses a Neodymium magnet like on SoP because these are a lot lighter but they are harder to find in large sizes.
To design the 6th order bandpass box for Beastbox I used a program called WinISD . I don't think this has been updated for a while and I couldn't get it to run under Windows 7 but it works fine in XP mode. If your driver doesn't already exist in WinISD's database you have to define the driver by inputting its Thiele-Small parameters. These will always be available on the manufacturer's website for any serious bass driver. Once you have the driver defined it's basically a process of trial and error adjusting the box and port dimensions to get the chamber volumes correct while keeping the overall size within the range that will fit on your chosen trailer.
One thing to bear in mind is that the chamber volumes are net volumes - i.e. you have to subtract the volume of the batteries, amplifiers and anything else that takes up space inside the chambers (including the driver and the volume of air enclosed by the ports). I found that a simple spreadsheet was helpful to keep track of the weights and volumes of the components and also to work out the chamber volumes for given external dimensions and port areas and lengths. The front chamber is 70 litres and is tuned to 60Hz and the rear chamber is 35 litres and tuned to 130Hz. The design gives a nearly 6dB boost at 77 Hz and over 8db at 150Hz.
Once you have the volumes and dimensions for the box and ports you can roughly work out where all the other components are going to fit. I used Google Sketchup to design the box and it was very helpful. The Sketchup files are attached.
The box is mostly constructed from 12mm birch plywood screwed and glued together. The port tubes are made from 6mm birch plywood. MDF would be another choice but I think it is more dense than plywood so it would make for a heavier box. Whatever you use, the main points to note are that the box must be rigid and there mustn't be any gaps. Rigidity is important because you want the panel resonances to be much higher than the bass frequencies that the box is producing. Basically if you rap on the side of the box it wants to sound "tight" rather than boomy. Gap-free construction is important because the bass driver generates large pressure fluctuations inside the box and if there are holes you will get whistling noises!
I cannot over emphasise how much it helps if you can get the panels accurately cut to size. I have a friend in a woodworking shop who cut mine for me and it makes assembling them much more straightforward if they are all the correct size with clean, square edges. I used 3.5 x 30 wood screws (No 6 x 1 1/4" for any Imperial fans out there) at 100mm intervals along each joint. Pre-drill pilot holes for the screws so that you don't split the panels. Make sure you use plenty of wood glue (PVA) on the joint, you can always wipe off the excess with a damp cloth. A couple of long clamps are very useful to hold the panels in place while drilling the edges and waiting for the glue to dry.
The front chamber of the enclosure has a removable cover which is held in place by the midrange/tweeter mount and some tie downs. This allows easy access to fiddle with the electronics and remove the batteries. The cover is sealed on to the chamber by a strip of neoprene draught excluder tape which runs all around the edge.
Step 4: Mid-range and Tweeter
The tweeter is a second-hand Fane CD140H. Not much to say about that other than it tweets well.
The horns and the tweeter are mounted in a simple wooden frame that sits on top of the main enclosure, held in place by the straps that hold the cover on. Making the horns and tweeter removable is handy if you need to transport or store the system.
Step 5: Mid-range Update
I had to build a new midrange/tweeter mount to hold the new horn and I based the design on the one SoP uses. I again modelled the mount in Sketchup and built it out of screwed and glued 12mm ply the same as used for the main enclosure.
Step 6: Mid-Range Update Dec 2012
In an effort to fix these problems once and for all I did some more searching on the web and finally found a mid range horn which could actually cover the whole frequency range from the bass right up to the highest frequencies. This new mid-range driver is the P-Audio BM2-D750 which can handle 100W RMS. I used a P-Audio PH-2380 horn with the driver. Together they were £130 from Blue Aran. I had to build yet another mid-range mount but realised that I didn't actually need a separate cover for the main unit but the base of the mid-range could form the cover for the main unit. The mid-range mount is made from 12 mm marine ply screwed and glued like the rest of the construction. Because there is now only one mid-range driver instead of a mid range and a tweeter I could replace the current crossover with a single channel one although I haven't done that yet.
Step 7: Batteries
Beastbox uses 36 11,000mAh D size NiMH cells arranged in 3 packs of 12 cells. Each pack was constructed from 3 x 4-cell battery holders connected together so that the 12 cells are in series. The battery holders are connected together with heavy gauge wire so that the cells are all in series then the whole pack is taped together with gaffer tape. I fitted a 10A miniature circuit breaker to each pack to protect against short circuits. Each pack has a Deans-type connector so that it can be disconnected for removal and charging. Remember to put the female Deans connector on the battery pack to help minimise the risk of short circuits.
The 3 packs are connected in parallel to give a total capacity of 33Ah at 14.4V, or 475Wh of charge. This might not sound like a lot but it is enough for at least 3-4 hours running time. Connecting NiMH packs in parallel needs a bit of care because the packs will inevitably have slightly different terminal voltages. If you just connect them directly together the pack with the highest voltage will try to charge the others. Unfortunately NiMH cells have (I think) a negative voltage/temperature characteristic so that as the charging cells heat up their voltage falls and they charge up more. They then take more current from the "donor" pack etc etc. You can see where this is going! The solution is to connect the packs in parallel through Schottky diodes which act like non-return valves, allowing current to flow out of each pack but not back in to it. When you first power up the system the highest voltage pack will supply the bulk of the current until it's voltage falls a bit at which point the others will start to supply current as well. I used 3 x 20TQ045 20A 45V diodes from RS Components (stock no 302-173) mounted on a terminal block attached to the inside wall of the box. I also fitted an isolating switch which disconnects the battery packs from the rest of the electronics as a belt and braces safety measure.
NiMH cells do not like to be deeply discharged, especially when they are connected in series. I searched manufacturers' websites and most of them recommend not discharging below either 1.1V or 1.0V per cell. I fitted a small voltmeter to the switch panel on Beastbox so I could monitor the battery voltage in use and switch off at either 13.2 or 12V depending on which manufacturer I believed on the day.
Step 8: Amplifiers
Beastbox uses a Kenwood KAC-6104D single channel amp for the bass driver and a JVC KS-AR8002D stereo amp with one channel driving the midrange and the other the tweeter. If your bass amp has a subsonic filter make sure this is enabled because a 6th order bandpass box does not provide much loading outside of it's pass band and you can damage the bass driver if you throw low frequencies at it.
You can also buy modular, bare-board Class D amplifiers which would probably be a lot cheaper than using a car audio amp, but I only discovered this late on in the build. The ones I have looked at whilst writing this Instructable need a 40 - 60V power supply (often twin rail). I don't know enough about electronics to design and build a suitable boost converter but if you have the know-how it would certainly be well worth looking at.
Step 9: Crossover and Capacitor
The same car audio nut had a very large (1.7F) capacitor like this one which he said he could "lend" me. At first I thought I wouldn't need it and because it is quite heavy I was going to leave it out. Then I thought about it and realised that if I could smooth out the current peaks drawn from the batteries they should last a lot longer (internal losses in the cells go up with the square of the current) so I decided to install it. It is connected to the batteries just after the isolating switch by a 100R resistor so that when you turn on the isolator the capacitor charges slowly through the resistor. Once the capacitor is charged I have a second switch which connects it directly across the batteries. I don't know how much difference it makes as I have never tried running the system with it disconnected but I measured the current draw from the cells and it certainly seems to be doing something! It also has a voltmeter so you can use it to monitor the battery voltage but to see it you have to peer in to one of the bass ports which is not very convenient (and not very good for your ears).
Step 10: Amplifiers Update Dec 2012
I mentioned earlier that it is possible to buy high power class D amplifier modules very cheaply from websites like eBay. The problem with these modules is that they usually require 45 to 50 V DC to operate and the battery packs in Beastbox only deliver 14.4 V. However, the three battery packs are connected in parallel and rather stupidly I hadn't thought of the option of connecting them in series! Three 14.4 V packs in series gives a useful 43.2 V and in fact when the packs are fully charged their terminal voltage is nearer 15 or 16 V so when they are in series you can get almost 50 V out of them. Once I had realised this I did some searching on eBay and found this amplifier module. It is based on the TAS5630 amplifier chip and delivers 300 W per channel into a 4 Ohm load and runs off 40 to 50 V. The best part is that it only costs £35.42 which is less than half the cost of one of the amplifiers that I was currently using! Plus it is tiny! And light!
I knocked up a harness to connect the three battery packs in series and tested the amplifier on the bench. It sounded pretty good so I stripped out the two existing amplifiers, the capacitor and quite a lot of spare wiring and replaced it all with the new amplifier module. The amplifier is connected to the crossover so that one channel feeds the bass driver and the other channel feeds the new midrange driver (see earlier update). The third crossover channel is redundant now.
The amplifier has three gain settings, low, medium and high. I experimented with all three settings whilst measuring the current coming from the battery packs and decided that the medium setting was probably the one that I needed to use. Bear in mind that it's quite difficult to test Beastbox at full volume indoors because it is so loud. I tried it for the first time on the November London Critical Mass and at the end of the evening the battery packs were only about one third discharged so I have since changed the gain to high.
This change has considerably simplified the layout and wiring and also made the main unit about 6 kg lighter. It also opens up the possibility of using different types of batteries. For example 4 x 12 V sealed lead acid batteries would give 48 V if you wired them in series. Alternatively, a four cell lithium polymer battery gives 14.8 V (and probably a bit more than that under no load when fully charged) so three of them in series could also power the amplifier quite successfully.
The only disadvantage of this new setup is that the crossover still needs 12 to 14 V to operate. A while ago I had also fitted a Bluetooth module so that I could connect my iPod wirelessly and this module also needs 12 V to run. I could have tapped off one of the main battery packs to power the accessories but I decided to fit a separate battery just to power these items. I fitted a 5 amp hour 4 cell lithium polymer battery with an in-line 5A fuse and used two of the existing switches on the control panel so that I could switch on the Bluetooth and the crossover independently. This also means I can use the big isolator switch just for the main battery power.
Step 11: Wiring
The second and third switches connect the positive feed from the batteries to the remote turn on lines to the crossover and the amplifiers. This allows me to power up the system in sequence
- turn on the isolator
- wait until the capacitor has charged
- switch the capacitor to directly connected to the batteries
- turn on the crossover
- finally, turn on the amplifiers
The little switch turns the voltmeter on and off. It has to run off 9V and for simplicity I just mounted a 9V battery under the panel! The big 4 pole connector is for the feed to the midrange and tweeter so that they can be removed for transport or storage. The connector is supposedly waterproof but I haven't put it to the test yet. We shall see what happens the first time I have the system out in the rain!
The audio input is via two RCA connectors on the outside of the enclosure. Although the system is mono and would therefore only need a single input, I wasn't sure how the crossover would react to only having one channel driven so I fitted two input jacks. I set my iPod to mono output so the two channels are the same anyway. The crossover has three outputs - woofer, front and rear. The crossover frequencies between the outputs are adjustable over a reasonable range. The woofer output goes to the bass amp which is set to single channel mode. The front and rear outputs feed the left and right channels of the JVC amp, the outputs of which go to the midrange horn and the tweeter.
I carry my iPod in my jacket pocket and have a long curly lead which is tie-wrapped to my bike frame to connect me to BeastBox. A better solution might be some sort of short range radio link (Bluetooth?) but I haven't investigated this yet.
Step 12: Finishing Touches
To mount the system on the trailer I use three ratchet tie-down straps, one around the front end of the box holding the cover in place, one around the rear end and one over the top of the the midrange assembly which also serves to hold the cover down. The cover needs to be clamped firmly in place as the bass driver generates significant overpressure and will easily lift the cover off if it is not secure. Also bear in mind that your trailer is unlikely to have any suspension and you don't want 50+kgs of woodwork and electronics falling off the trailer in the middle of the road!