Bullitt Boom Box

Bullitt’s are awesome, but sometimes awesome can be more awesome when you’re playing your favorite tunes… and what better way to do that than by building a stealthy speaker system for your Bullitt, of course!

Images: Various views of the speaker enclosure.  (Right) a stealthy pump and puncture repair-kit holder, next to the speaker enclosure.

The premise of the design is that because we tend to ride bikes on hard surfaces (i.e. sealed road) the sound waves will bounce off the road and be audible. I can confirm through experience that this is definitely the case – more on that in a later section. As such, the speakers are mounted under the cargo deck, facing directly at the road.

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This location is significant: it means the cargo deck is virtually* unconstrained by any sound system paraphernalia – so it won’t interfere with any load; the largest part of the system – the speakers and their enclosure – is neatly tucked in under the cargo deck, meaning there’s no impact on the bikes performance; and it’s visually obscure, so not an obvious target for theft. So you can essentially leave it on the bike all the time: simply jump on, turn up the vibes, and turn heads.

* The only impact is the bolt-heads that protrude above the deck, and even then there are mitigations.  For me these are very small so there’s no practical impact on load carrying.  See the “Speaker Enclosure > Bolt-Heads and Their Impact on Load Carrying” section below for more info.

Disclaimer: the ideas and information provided here are provided “as-is”, no warranty is provided or implied.  Building a system such as the one described here involves various risks, both during implementation and operation.  If you damage yourself, someone else or any property, through directly or indirectly being influenced by this content, that is entirely your responsibility.

88x31  This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.  Want to commercialise this?  I’m open to discussion – get in contact.

 

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Above, the main on-Bullitt components, including speakers, amplifier and battery.

The Recipe

You will need:

  1. Amplifier.
  2. Speakers (water resistant).
  3. Speaker cabinet / enclosure, with mounting nuts & bolts (assumes you have appropriate mounting holes in your Bullitt’s cargo deck).
  4. Battery.
  5. Battery Charger.
  6. Battery charge/voltage meter.
  7. Speaker cable (approx. 1.5 metres x 2).
  8. Speaker cable bungy (x2).
  9. Old bicycle inner tube (speaker cable protector).
  10. Various cables including: battery to amp, battery to charger.
  11. Carry pouch for amp & battery.
  12. Connectivity between the amp and your phone / music player.
  13. Speaker rear cover-plate (optional).

Amplifier

The heart of the system is the amplifier – so choosing this first is recommended. You want an amp that has good power output, and is relatively small and light-weight. Another critical consideration in amp selection is how you’ll power it (i.e. what it’s power requirements are – more on that later).

You may want to consider amp and speaker in combination, since it’s important to ensure these are well matched in term of power per channel and speaker impedance. In my case I bought the amp first, based on guidance from a friend and colleague of mine, Steve*, and selected other components based off that.

* He used to work for Sony fixing electronics. One day a VIP client brought their favorite Walkman in for repair – my colleague was selected to repair it. The client was Princess Diana. So, needless to say, I trust his judgement. 🙂

Steve recommend a digital “D-Class” amplifier, because the overall characteristics of these amps are well suited for use on a bike: they are lighter and more robust that a valve-based amp, and have very efficient power usage (which is important as we’ll be running off a battery).

The actual amp I use is: “TPA3116 Mini Power Amplifier ISSC Bluetooth HIFI Stereo Audio Digital AMP 50Wx2”, purchased from China via e-bay.

Images: (Left) Promotional photo.  (Middle) Rear of the unit, with hand for scale.  (Right) Mounted on spacer-board, and with power on/off switch.

Amp specifications:

  1. Work efficiency: 90%
  2. Rated output power: 2 x 50W
  3. Frequency response: 20Hz to 20KHz
  4. Operating voltage: DC18V to DC24V
  5. Maximum output current: 3A
  6. Bluetooth receiving range: 10 meters
  7. Size: depth 108* width 90* height 39mm (without antenna)

References:

Speakers

I discovered that Pioneer make “Marine” speakers – i.e. for use on boats etc. These are designed specifically to operate in wet conditions; the manual says “Marine use, water-proof design … UV and corrosion-resistant design”.

I have a pair of these, specifically: TS-MR1640’s, which are a good match for the amp.  I had intended to get the TS-MR1600’s but StreetSoundz (where I bought them) kindly offered me the next model up for the same price 🙂

Images: (Left) Spec’s.  (Inner-left) Rear of a speaker, as mounted in the enclosure.  (Inner-right) The business end of the speakers as mounted (note, enclosure unmounted in this photo).  (Right) Speaker with cover removed, showing grime build-up after several months use.

Another great thing about these speakers is that they are a decent size – about 6 inches, so not too small yet fit nicely under the Bullitts deck, which is key to the overall design.

You could use non-marine speakers, but it would be somewhat limiting; if you’re like me, and operate your Bullitt in all weather conditions, then the marine speakers mean you can still pump out the vibes even in the rain.  I’ve painted mine using some oil paints I had lying around (didn’t have black), mainly to make them less visible.

TS-MR1640 Speaker Specifications:

  1. Maximum music power: 100W.
  2. Nominal power: 25W.
  3. Impedance: 4 Ohm.
  4. Woofer diameter: 160mm (~6 Inches).
  5. Sensitivity: 90dB.
  6. Frequency response: 30Hz – 20kHz.
  7. Weight: 790g (per speaker).
  8. Depth: 56mm.
  9. Speaker cover: 28mm high, 168mm diameter.

References:

Speaker Enclosure

I made the speaker enclosure myself, mostly using pine and plywood. The enclosure you see here is my second version.

Images: Various views of the speaker enclosure (unmounted).  Note the additional ply (also 7mm) around the speakers to provide a secure anchor for the speaker mounting screws.

The essentials of the design are:

  1. A plywood (7mm) plate to mount the speakers. Note I have used a second layer of ply inside the enclosure to provide extra strength for the speaker mounting screws – the last thing you want is for them to come loose through all the shocks and vibrations associated with riding.
  2. Light-weight and strong sides/ends. I’ve used untreated 60mm x 10mm clear dressed pine.
  3. Bolts running vertically through the enclosure, so that the open / rear side of the enclosure is held flush against the bottom of the main cargo deck – which is also 7mm ply.

My design is like this:

Speaker Enclosure Design

You’ll notice that the middle bolt hole is off-center.  The reason for this is the lack of space between the speakers along the center-line, plus the presence of the Bullitt’s first central cross-bar.  Initially I had a symmetrical design – 4 bolts with two in the middle, but I have since dispensed with one – three bolts in total is enough.

I have a custom 7mm thick plywood cargo deck on my Bullitt, plus the enclosure’s 7mm speaker plate, plus the 60mm high sides, which is an overall depth of 74mm. The enclosure is bolted to the deck using 80mm M6 bolts and Stainless Steel Nyloc Nuts.  The initial assembly is fairly tight on the bolts, but you’ll find that the plywood is somewhat malleable, so you will get secure assembly.  (You’ll want to keep the bolts short as possible so they don’t protrude down too far and catch on anything).  After 8+ months use I find that the bolts will happily extend past the top of the nut, as the plywood molds to the pressure of the washers/nuts. Initially you might find they end of the bolts only go to flush with the top of the nut – which is fine if you use lock-nuts. Just check them regularly to make sure they are not coming loose.

Plywood is an excellent material for the plates, as it’s light-weight and strong; clear pine is fine for the sides and ends – again because it’s light. 10mm provides enough strength to maintain integrity, and because the design has the bolts passing through the plywood – and taking all the weight – the pine is not really load-bearing.

The Bullitt has two central horizontal cross-bars running across the cargo deck – the length of the speaker enclosure means you’ll need to cut parts of the sides away, so that the enclosure fits snug around this cross-bar, and flush against the bottom of the cargo deck.

You’ll notice the angular joinery at one end; this isn’t part of the acoustics, it’s pure decoration, I thought a jet engine air intake type of look might be cool.  All the joints are sealed internally with a wooden sealant from the hardware store.

Images: Various views of the speaker enclosure.  (Middle) Note the recess that accommodates the frame’s cross-bar, allowing the enclosure to be flush against the bottom of the cargo deck.  I’ve wrapped recycled inner-tube around the steering rod for protection from scratches.  (Right) note the inner-tube protecting the speaker cables.

Bolt-Heads and Their Impact on Load Carrying

The bolt heads are only a few mm high, which (in my experience) aren’t an issue for any loads.  I often use large fish bins; the design of these includes a small rim around the bottom, meaning there’s a thin recess underneath them – which is enough to accommodate the bolt heads.

You might find them an issue though in some specific cases.  There’s a few mitigations you can use, some of which affect the design:

  1. Use packing blankets or some other soft material to protect your load.
  2. Make a solid spacer that acts as the cargo deck, which has holes cut out for the bolt heads.  This could be a separate piece of wood which you use as needed or you could build it into/onto the deck as a permanent fixture.
  3. Use bolts / screws that result in a flush finish with the top of the deck.  Note: you’ll want to make sure there is enough strength to hold the bolts even after months of use / wear & tear.  I don’t think 7mm ply is safe to do that.
  4. Use a different mounting system – e.g. screwing the enclosure directly to the bottom of the cargo deck.  Note: this will make maintenance a lot harder, but not necessarily protect you from thieves.

Acoustics

I’m not an audiophile, so you may want to do your own further research the overall acoustics and the designing of speaker enclosures.

There’s no doubt that some types of music sound better than others. I tend to prefer Techno, Ska and Roots/Reggae/Dub; some Classical can work well too (a spot of Rostropovich, perhaps), and some Pop.  Interestingly I’ve found House music doesn’t fair quite as well.  Your mileage may vary.

The sound is generally clear, but some wavelengths can get a bit lost.

Because the speakers face into the ground, sound is emitted in 360 degrees. This means that you provide some fun and vibes for those around you – whether they like it or not. Interestingly, it also lets people know your coming, which can be useful form a safety perspective – more on that in the “Operation” section.

References:

Battery System

This is a huge and complex subject, so I’ve posted the details separately here: DYI 18650 Battery Pack

Speaker Cable

You’ll need two lengths, around 1.5m each.  This runs from the amp, mounted in a pouch hanging off the back of the cargo deck frame, down the frame and along under the main frame to the speakers.

Do yourself a favor and get good quality cable.

Speaker Cable Bungy (x2)

Use these, or something similar to lash the speaker cable to the frame so that it doesn’t foul on anything.  I have one on the side vertical, and one under the deck.

Old Bicycle Inner Tube

I use an old bicycle inner-tube to protect the speaker cables from getting scratched / cut, sunlight, and it’s a lot less obvious than speaker cable (i.e. theft) – mine was bright copper in a clear rubber seal, so bit of an attraction.

Various Cables

As with everything in do-it-yourself systems like this, you’ll need to ensure you can connect everything together.  These are also covered in the DYI 18650 Battery Pack post.

Carry Pouch for Amp & Battery

Army surplus stores are great for random bags and pouches, usually at good prices. The bags I have are old rifle ammo pouches – the dimensions are perfect for mounting on the read cargo frame of the Bullitt, and holding the amplifier and battery. The canvas is nice and thick (protective). They are rated as “shower proof”, the canvas lids have sides that provide decent protection and clips for holding them down.

How you mount the amp and battery is entirely up to you. What I like about the pouches is that they fit perfectly at the rear of the frame – which means the volume knob on the amp is easy in reach, so I can change volume even when riding.

Images: (Left) Army surplus ammo pouch.  (Inner-left) The amp and battery, as it sits within the pouch.  (Right) The amp in its operational position; volume knob removed to avoid accidental volume change.  (Right) Battery and amp.

For my design, I have attached the amplifier to a light-weight wooden board, which serves two purposes:

  • As the ammo pouch is bigger than the amp, it pushes the amp up towards the top, so that the amp is easy to access, even when riding.
  • By pushing the amp up, it also helps create a space where cables can sit without getting crushed.

The amp is basically just taped on to the broad, but if you look closely at the photo’s you’ll notice a small piece of wood bolted to the main board, which holds the weight of the amp when it vertically in the pouch.

Lastly, think about how you’ll mount the pouch to the Bullitt – in terms of managing shocks and vibrations.  I’ve used bungee cord to suspend the pouches in place, to help minimize vibrations and shocks.

Speaker Enclosure, Rear-Plate

This is optional. It’s purpose is to cover the back of the speaker enclosure, if you want to use the speakers off the bike. The plate serves two purposes: protects the speakers, and acts as a baffle so that rearward generated soundwaves don’t distort those emitted by the front.

Amp-Smartphone Connectivity

This really depends on the amp; I had intended to simply use an old-school style lead: 3.5mm headphone jack to twin RCA connectors – RCA connectors are very common on amps. The amp I got also had Bluetooth, and I’ve ended up just using that exclusively. If you need them, cables like 3.5mm jack to twin RCA should be easy to buy off the shelf.

Operation

People

The most interesting point I’ve noticed is how people around you react, because the idea of audible music coming from a bike is not one most people are familiar with.

In one case, I was cruising along the waterfront in a zone which is mixed pedestrian and cyclists, next to this is a road. I was coming up behind an older gentleman, and as he heard the music (some techno) he instinctively looked towards the street – no doubt expecting some “youths” to go cruising past.

I’ve also sat at the lights with pedestrians nearby looking around, unable to sense where the music is coming from.

There’s also an interesting safety angle. There’s been a few times, especially around the CBD where pedestrians have been just about to step out in front of me – and they’ve heard the music and stopped.

Battery Management

Managing the battery charge is pretty critical – naming not letting it run down too far.  make sure you check the behavior of your amplifier – as I mentioned above, mine has a small current draw even when switch “off” – approximately 0.025A.  If you leave them connected to long you can risk discharging the cells to unsafe levels.  Having some sort of protection is a good idea.  I use an additional switch to completely break the battery-amp circuit, and keep in the habit of using it.  Whilst I haven’t done extensive research on this, it’s likely you can find or build basic BMS’s that would cut-off in low voltage.  The basic voltmeter shown above has a siren built into it, which will go off once you configured voltage is reached – but it has to be plugged in all the time (itself a very small power drain), and it’s only useful if you can hear it (so if you’re not in earshot…).

More info in the DYI 18650 Battery Pack post.

Security

One aspect to carefully consider with the design of your system is security. You will need to judge for yourself what the risks are, how likely they are, and how to deal with them.

The speaker enclosure design is a key component of this as it’s easily the single largest component. How it balances convenience and robustness with security is a key point to consider. My line of thinking is that the space under the cargo deck is perfect because the speakers are effectively out of sight; and out of sight = out of mind. You may want to be careful in how you operate – e.g. having music blaring out, and then turning the sound system off in a really obvious way, may attract unwanted attention.

I like the straight-through bolt design as it’s very strong; having the speaker enclosure come off whilst you’re doing 40Km per hour downhill would be… sub-optimal.

The other bonus is that it’s relatively straight-forward to remove the speaker enclosure – i.e. for cleaning and maintenance, or to use in the traditional sense – pointing directly at you and your friends rather than bouncing off the road.

My assumption is that as long as most people don’t know exactly where the speakers are, it’s relatively safe. There’s also the issue of needing the right tools, and time, to remove them.

Same logic applies to the amplifier and battery. You can design a system that is less obvious, and relatively less secure once it has been detected, or a system that is more secure but perhaps less convenient.   Its really up to your personal preference.

Big Shout-Outs to:

  • Pete for building many of the version 1 electronics – battery, cables, etc, and advice on the battery design and charging approach.
  • Steve for the amplifier selection and other advice.
  • Street Soundz for providing a good advice and price on the speakers, speaker cables and RCA connectors: https://www.streetsoundz.co.nz/

 

1 thought on “Bullitt Boom Box

  1. Pingback: DYI 18650 Battery Pack | Peruse Muse Infuse

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