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midfield 6th February 2007 02:59 AM

The craziest project
 
Ok folks, here's a strange one for you. Sorry for the long post, I
have thought about this a lot and so I figured I should just write
down what I've thought about and solicit comments from you experts!

The story: I'm trying to build a sound system for my bicycle. Yes, I'm
crazy, and it's a long story. Basically it needs to be 1) loud enough
to dance to, 2) somewhat durable, 3) portable, and 4) cheap enough so
i won't care that much if it gets broken or stolen. (I'm not planning
on keeping it on my bike all the time, just for special occasions.)
Being audiophile etc is not as important as being able to produce good
volume especially at the low end. (This is going to be my soul-train
bicycle...)

The amplifier is a T-amp, built from the AMP6-BASIC kit from
www.41hz.com, which is totally awesome. The output power is around
2x25W RMS maximum, into 4 ohms at 10% THD+N. Up to about 2X15W, the
THD+N is below 0.1%. It's super efficient, I power this sucker with
8-10AA batteries and it lasts forever, and it's tiny, audiophile sound
quality, etc.

I've gone through many ideas as far as speaker design (TQWTs out of
ABS pipe, etc.) As an experiment I even got a pair of Fostex FE166E's
and built a 288 cu in cabinet out of double-ply cardboard fitting
perfectly inside the main triangle of the bicycle. These are
wonderfully efficient and loud but didn't have enough bottom end, even
in a bass-reflex (cardboard) cabinet. Also the exposed cones and cost
of the drivers made me a little afraid of using them.

So I've decided to go with a sub/sat system to get the bass I want.
For the satellites I'm going to use the Sonic Impact Soundpads
attached to cardboard triangles fitted into the main triangle of the
bicycle. (For those of you unfamiliar, these are $30 devices which
turn any relatively rigid surface into a speaker.) They actually
produce good volume in the mid / mid-high range, of course without
nearly the clarity of the FE166E's and even less bass. But they're
cheap, light and they can hide inside the cardboard triangle.

For the sub, I'm thinking about building a 4th order bandpass sub. The
candidate driver is the $21 Dayton SD215-88 8" shielded DVC subwoofer

http://www.partsexpress.com/pe/pshow...number=295-480

*Power Handling: 80 watts RMS/120 watts max *VCdia: 1.5" *Le: 1.2 mH
*Impedance: 8 ohms per coil *Re: 3.00 ohms *Frequency range: 30-2,000
Hz *Fs: 28 Hz *SPL: 89.0 dB 2.83V/1m *Vas: 1.40 cu. ft. *Qms: 3.40
*Qes: .47 *Qts: .41 *Xmax: 6 mm *Dimensions: Overall Diameter: 8-1/2",
Cutout Diameter: 7-1/8", Mounting Depth: 4-1/4".

The enclosure I'm thinking about is 8 in diameter (really 8.5 in
diameter) cardboard tubing I found at home depot (for pouring concrete
pillars, I think.) I tried some of those 4th order bandpass web
calculators and they gave me on the order of 5-12 inches of total
length for the tube (numbers below); this can easily be strapped to a
rear luggage / pannier rack. I think it should give me the bass I
want, and be relatively small, cheap, and without exposed parts.

Now for my questions:

1) Am I crazy? Is this a good design for what I want?

2) Should I be thinking subwoofer in bandpass + soundpads, or woofer
in bass reflex + soundpads? Or something else? Will I get killed on
the low-mids? Is that a problem?

3) Any ideas on how to design filters / a crossover for this setup?
I'm fairly happy soldering caps / inductors to things, but I have no
idea how to design a circuit.....

4) Any other comments, or than on my sanity?

Thanks in advance,
Ben

ps numbers from the bandpass calculator

http://www.carstereo.com/help/Articles.cfm?id=27

Vf = 0.235 to 0.461 cu ft, or 1.78909 to 3.5096679 inches of 8.5 in tube
Vr = 0.213 to 0.689 cu ft, or 1.6216 to 5.245469 inches of 8.5 in tube

so about 5-12 inches of tube combined, taking into account the volume
of the speaker itself. (conservatively guessing the speaker itself is
about .22 cu ft which adds about 1.675 inches of length.)

davidlzimmer 6th February 2007 03:20 AM

I can only attest to the sub speaker. It is quit a performer. You will be amazed.

And YES, you ARE crazy! :D

Have fun and send pics!

Variac 6th February 2007 03:32 AM

going to Burning Man?:devilr:

braden 6th February 2007 05:04 AM

Quote:

Originally posted by Variac
going to Burning Man?:devilr:
...exactly what i was thinking. or some type of "reclaim your public space" event.

Ang 6th February 2007 05:28 AM

I think you should create a system that runs of pedal power.

In interests of space and still being able to ride the thing, how about a couple of planars in a dipole arangement on a baffle that fills the main triangle? I suppose you could put the tweeters there too, but you may want to look into some sort of suspension (gel enclosure?) for them to deal with some of the vibration they'll be subjected to.

While I'm at it, it would be kind of cool to mount the tweeter on your helmet, like a light - flush mount it and you'll be minimizing diffraction effects. (just use long wires for this) Then when you stop to talk to people you can kind of beam the treble at them.

For the sub, unless you put a small enclosure in a backpack and wear that too, perhaps you could just stick a sub inclosure in a basket on the front or rear, (in elegant, I know, but simple).

You could make the pipe longer if you mount it along the top tube - use a pvc bend and double it up so the port is also front firing. It would just be a matter of stabilizing 16" of stacked tube, but it shouldn't be too hard.

Apogee 6th February 2007 05:42 AM

Burning man was my first thought... :smash:

I'd biamp the whole setup...

Go with a piece of 12" sonotube (the home depot concrete stuff) cut about 24" long. Cap both ends with plywood. In one end, install a 10" driver (can be cheap). This end will face to the rear of the bike (think rocket engine).

Inside the tube, I'd house a lawnmower starting battery for power with another switching amp. This wouldn't be too heavy for a bike rack.

From the other end of the cap, I'd run two 2" or 3" rubber flex tubes (or abs plastic pipe) under the seat, along the top bar and drop them into a resonance chamber at the front up near the steering head. The chamber would encompass the entire triangle area beneath the top bar. Paint them silver so they look high-tech. In the resonance chamber, you could mount a couple of small passive radiators if you wanted...

I'm thinking that it'd actually get fairly loud, would run for a while, and you'd actually have some bass response...

You could install a circuit in the resonance chamber with a bunch of led's for a moving light show that was sync'd to the music...

I'm like'n this... LOL!

:D

Ang 6th February 2007 05:46 AM

I'm wondering how big of a "chamber" you can have in the triangle and still pedal comfortably

Apogee 6th February 2007 05:55 AM

I would guess it could be 3 or 4 inches wide without much problem.

He could use 1/4" hardboard with 1x2 edges and bracing. This way he'd maximize the internal volume.

I'd buy flat passive radiators and mount them to the outside.

Something like a pair of 6" or 8" would be perfect. A quick search on ebay led to these:

http://cgi.ebay.com/1-Pair-Sony-Pass...QQcmdZViewItem

It'd also be great if he could find a long excursion 10" for the main woofer but that might blow the budget.

I'd also go with an amp that has more power the bottom end. You can find small used car audio switching amps everywhere - try craigslist. Just don't buy a high wattage one or you'll pull the battery down too fast.

I would just do a simple crossover between the source and the amps.

Lastly, I'd mount the passives as far from the chamber "inlets" as you can...

Apogee 6th February 2007 06:30 AM

Running with this a bit further...

Just make sure that whatever you buy isn't too deep.

A quick search of ebay also turned up these:

http://cgi.ebay.com/2-Infinity-RS-53...QQcmdZViewItem

Or as an alternate, you could just cut the holes and glue these on the outside. I'd try to find smaller ones if possible though.

http://cgi.ebay.com/10-Passive-Radia...QQcmdZViewItem

Also, wouldn't be a bad idea to roll-off the bottom end so you are not drawing lots of juice pushing air that nobody will feel. I would think it would be better to have it sound good and run for longer rather than attempting to push very low bass. Low bass takes lots of power, especially outside, and that would shorten your runtime considerably

Have fun!

poynton 6th February 2007 07:12 AM

How about redesigning and rebuilding the bike with wider diameter tubing so that the batteries and tweeter could be built in or a couple of TB 3" drivers???

Andy


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