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Old 17th July 2003, 04:18 AM   #111
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A U shaped tube would still work but the bend may introduce a few reflections. Straight is better.

Expect that the sound will bleed through walls anyhow.

Drew
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Old 17th July 2003, 04:21 AM   #112
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Ok, good luck with it.

Eric.

Waiting to hear what the neighbours say.
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Old 17th July 2003, 10:11 AM   #113
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Hi Drew

One fact that keeps me from looking at the cannon as some sort of dipole is the fact that the flattest response is achieved when the distance between listening position and both ends of the line are the same. If you listen to a dipole speaker from the side you will definitely have an SPL minimum.

Furthermore it is possible to use different length ratios than Bose uses that allow a more peaky response but further improved efficiency (2 dB approx). I have a JAES article dealing with horn modeling where this is discussed as well. There the arrangement is called "resonant tube system".

I do however agree that the total mass of the driver will be affected. As this would definitely decrease the efficiency, one has to assume that there is also another mechanism in effect than just increasing mass on the driver (i.e. one that is achieving frequency dependant improvement of loading).

But the mass-increase might be one reason that Bose doesn't use a longer cannon. If you have larger cannons and even need more of them (to achieve the same SPL) then you will end up with a less profitable solution.
I completely agree that Bose often uses the principles that impresses many people the most with the least effort on the technical side.
But having professional users in mind a manufacturer has to take care of their profit as well, if both want to stay in business. P.A. speakers that go down to 30 Hz are definitely available, but quite rare simply due to economical reasons.
The homebuilder however may use cannons, TMLs, horns and whatever as insanely large as he likes and is willing to pay for (and the SO permits ).

Regards

Charles
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Old 17th July 2003, 09:57 PM   #114
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Agreed, the cannon is not a dipole but the time delay aspect of the structure is similar to the time delay related to baffle width, or front to rear distance.


Changing the pipe ratios would alter how much and at what requency the come in and out of phase.

It seems that the 1/4 : 3/4 ratio gives a broader passband with some gain. no doubt other ratios would peak differently and have a differing passband shape.

does the mass loading imposed on the driver by the air column have the effect of lowering Fs (as if the driver diaphragm was heavier)?
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Old 18th July 2003, 08:29 AM   #115
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Quote:
does the mass loading imposed on the driver by the air column have the effect of lowering Fs (as if the driver diaphragm was heavier)?
I do at least have the gut feeling.

Regards

Charles
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Old 18th July 2003, 06:44 PM   #116
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Quote:
does the mass loading imposed on the driver by the air column have the effect of lowering Fs (as if the driver diaphragm was heavier)?
Indeed it does. Air weighs ~1.21kg/m^3, so you can figure the trapped air in the tube in grams and add it to Mms to see how much it affects it, just like when figuring the equivalent weight for a PR disc.

GM
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Old 18th July 2003, 06:46 PM   #117
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After reading through Bose's patent many times I have come up with the following on how Bose designs their wave products: 1. Choose the speed of sound travelling within the pipe
2. Divide the speed by four.
3. Take that value and divide it by the desired cut-off frequency.

Determining the cross-sectional area (ATCR-method (?)):
Ac=effective cone area of driver.
At=Cross-sectional area.
ATCR= constant (below 0.5=Peaky response, 0.5=smooth response, above 0.5=peaky response) *Note: a value of 0.45, for example, may be chosen if a larger Bl factor is chosen {BL relationship yet to be figured out}.

ATCR=At/Ac.

Using math, you can find the diamater or, Length and width values for the pipe after determning the At.

What about the pipe in front of the driver? Well, that is simply one third the length of the pipe behind the driver. Why? It produces a phase reversal at certain frequencies where the rear pipe also reverses the phase. The two waves meet and add up as to increase the effective response range of the pipe.


I haven't tried this method yet, though, I will be soon as I build my surround TL's.
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Old 26th October 2003, 04:43 PM   #118
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I built two of these during the summer of 2001.

I had heard rumors that Bose had developed amazing
bass using a series of acoustic labyrinths (a complicated system
of "sound caves").

I then saw a 30-minute infomercial on the "Bose Acoustic Wave System".

I, too, found the Bose patent on the web (United States Patent 4,628,528). Lacking a suitable viewer, I had no pictures / graphics. (but I had seen the ad showing the folded tube technology).

My initial design was very complex -- thinking that many
folds in the tube might be key.

I quickly realized that I could not possibly build such a
design however (being a hack).

:-)

After many iterations I came up with a simpler design essentially
the same as here (see phase_accurate's) -- "great minds think alike"

I used Georgia Pacific 3/4" void free plywood, 2" deck screws, and
Gorilla (polyurethane) glue (my reasoning -- ww.gatorboats.com -- see FAQ ).

I doubled up to 1.5" on the piece where the 14" driver is attached. Wherever there is a fold I used 45 degree 7" corner braces from excess 3/4" stock. I glued all connecting surfaces
and screwed every 3".

I know this flies in the face of conventional wisdom
(source: David Dlugos' (outstanding) www.t-linespeakers.org
-- see "Perfectionist Audio" under "coffin" design)).

But it IS very solid -- perhaps because there is a bed
and people on top of it holding it down.

;-)

After two years of living with it my wife still says it is the
best bass she has ever heard -- open, airy, and DEEP.

I originally built four 15" x 15" x 30" reflex 3/4" MDF
enclosures with lots of bracing and active equalization to get
same effect. The WAF was low because they took up
lots of space in the room.

Now I am getting a FULL OCTAVE of deeper bass from only two, and they are hidden under the bed (with a much more open and airy sound). Ironically, the bed does NOT shake during special effects -- the mattress absorbs the vibrations.

Pink Floyd, organ, classical, and special effects are AWESOME.
(so is all the other stuff except for Moody Blues, Lenny Kravits,
and Savage Garden -- their stuff is mixed with WAY too much bass).

There is no sound at 10hz, -9db at 15hz, -3db at 18hz, and then fairly flat from 20hz to 80hz where it crosses over to six JBL HLS 610s.

long tube: 15' (18hz)
short tube: 5' (55hz)
tube area: 121.5" (9" x 13.5")
ATCR: 1.07 (JBL LE14a cone area (6*6*pi()))
driver: JBL LE14a (Fs=28hz)
taper: none
stuffing: none
equalization none

(NOTE: The Bose patent claims an optimum ATCR of .5 for
flatest response -- I went with an ATCR of 1 and no
stuffing just like thier Acoustic Cannon. See Jon Risch's TL design guidelines at www.t-linespeakers.org.)

A frequent question -- "why so big?"

Per unit of energy there is 1000 times as much distortion at 20hz
as there is at 20 khz. Subs need to be big.

It is claimed that the Bose Acoustic Cannon is the sonic equivalent to a 250 sq ft corner horn (see www.cardhouse.com/x09/wave.htm).

This the effect I was going for and after two years I am still very happy with the result.

Oh yeah, it was a couple of hundred bucks.
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Old 27th October 2003, 11:51 AM   #119
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Default efficiency

How efficient is the cofiguration as compared to say sealed box, for instance, at the driver's Fs (18 Hz?) ?

Also, am I to understand that you built this design as a labyrinthed rectangular prism as opposed to the straight pipe design that was being discussed?

Really neat to see someone has actually done this!
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Old 27th October 2003, 03:53 PM   #120
qi is offline qi
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Thank you for the encouragement. It is always nice to hear.

Regarding efficiency, I do not currently have testing equipment here except for my ears.

:-|

I believe, however, that the only thing more efficient than an unstuffed tuned pipe (Dr. Amar Bose's Acoustic Cannon) is a horn loaded sub.

Horns are much more complicated and expensive to build, however (see: http://www.servodrive.com/basstech7.html, http://www.klipsch.com/product/product.aspx?cid=2, http://www.carfrae.com/pages/frameset.html for outstanding examples IMO). Horns also need to be HUGE to go below 20hz.
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