Internal volume of an AR-3

[Guest Pointwhere]

Hey guys... Does anyone know or can anyone calculate the internal volume of an AR-3 (or 3a)? I have seen some quotes, but they vary and I cannot tell if they are including the space that the drivers take up or not. The number I need is just the volume of the empty cabinet. Any help would be appreciated. Thanks.

[tysontom]

>Hey guys... Does anyone know or can anyone calculate the

>internal volume of an AR-3 (or 3a)? I have seen some quotes,

>but they vary and I cannot tell if they are including the

>space that the drivers take up or not. The number I need is

>just the volume of the empty cabinet. Any help would be

>appreciated. Thanks.

It is pretty close to 1.7 cu. ft. This is the number always referenced in the literature for the AR-1, AR-3, AR-3a, LST and so forth. Some of the later 12-inch AR speakers had a somewhat larger internal volume, but if you plan to use the standard, flat-sided AR 12-inch woofer (ceramic-ferrite or Alnico version), this volume is optimum. You want it neither larger nor smaller. That volume is really the internal measurement of the cabinet, I believe, and does not take the drivers into account. For example, the original prototype AR-1, upon which the patent was applied, had a measurement of 19" x 19" x 11" outside measurement, and if you subtract the plywood thickness, it becomes 17.50" x 17.50" x 9.50" for a total of 1.68 cu. ft. Reference is occasionally made to "1.5 cu. ft.," but I am not sure where that originated unless it did take the drivers into account.

--Tom Tyson

[soundminded]

Tom, have you got any data for the amount and density of the fiberglass? As I recally you once posted that it was about one pound per cubic foot. I assume that this is critical since it controls the aerodynamic velocity related drag coefficient and therefore the resonance damping. Is it uniformly distributed? Any data for AR9? Is it exactly twice the volume of an AR3a? As I recall the fiberglass is only in the upper part of the enclosure. Any data for the weight of the fiberglass there? Thanks.

[Guest Bret]

>Any data for AR9? Is it exactly twice the volume of an AR3a? As I recall the fiberglass is only in the upper part of the enclosure. Any data for the weight of the fiberglass there?<

In the AR-9 information in the forum's reference section gives the cabinet volume at 4.24 cu ft. But then, I don't know if that takes into account the "cabinet in the cabinet" which houses the lower midrange unit.

You probably already realize that there is no fiberglass used in the AR-9. They used poly acoustic stuffing. Looks like 40oz in the woofer section rolled-up into fourths.

You'll want to look at the system assembly drawing:

http://www.arsenal.net/speakers/ar/ar-9/en...ng/Scan12sm.tif

Bret

[soundminded]

Thanks Bret. If I ever have reason to take it apart again, I'll stick a video camera inside with a light and see what it looks like in there. I've always removed the woofers when I had to move them. Less risk of damage...both to the woofers and my back. Amazing how little space is devoted to everything but the woofers. The whole box is practically built around them. Again just today, they never cease to amaze me after 20 years how easily and powerfully they produce distortion free deep bass.

[soundminded]

One thing that has to be considered in the internal volume of the cabinet is the space taken up but the stuffing fibers in addition to any internal devices or projections such as the crossover network and the other drivers/cabinet as you pointed out. It's the remaining air space which controls the K or effective compliance while the geometry, spacing, and nature of the stuffing control the velocity related damping B which is caused by the aerodynamic frictional drag as air is alternatively forced between and sucked through the spaces between the fibers. (The third factor which controls the resonance frequency and peak is of course the mass of the moving cone/voice coil assembly.)

[tysontom]

>Tom, have you got any data for the amount and density of the

>fiberglass? As I recally you once posted that it was about

>one pound per cubic foot. I assume that this is critical

>since it controls the aerodynamic velocity related drag

>coefficient and therefore the resonance damping. Is it

>uniformly distributed? Any data for AR9? Is it exactly twice

>the volume of an AR3a? As I recall the fiberglass is only in

>the upper part of the enclosure. Any data for the weight of

>the fiberglass there? Thanks.

Soundminded,

I think the "one-pound-per-cubic-foot" rule is probably close to correct for the single-woofer AR box, but I am not sure about the AR-9. The density of the fiberglass is pretty much the same that is used for house insulation, but I am sure there is a spec for it. I will look to see if I can find the spec somewhere in the files.

The important thing is to control the speaker's "Q," as you alluded to earlier. The ultimate way (brute-force method) to check the efficacy of the proper "Q" is to measure the speaker's output down to and below resonance, and determine the peak, or attenuation associated with damping, at resonance. Ideally, the AR-3-type system, with a "Q" falling between 0.7 and 1.0, yields a peak of about 1.5 dB at resonance, therefore providing very uniform response to just below resonance, with the predictable rolloff below. This "Q" also results in no appreciable ringing (that is, it can't be improved by lowering the "Q"). Too much fiberglass will lower the "Q" and attenuate the response at resonance. The resultant thin-sounding bass, sometimes referred to as "tight" bass, is actually a form of coloration. Too little fiberglass would underdamp the system, and cause a peak at resonance, hence a "boominess."

--Tom Tyson

[soundminded]

I think 0.707 is "spot on" as the Brits would say. That would be critical damping. I think that is the advertised spec for AR9.

[johnieo]

Dickason's book shows that the peak above flat magnitude is 0 dB for both critically damped (Qtc=0.5) and Butterworth (Qtc=0.707). He shows how fast the peak rises above flat for increasing Qtc, producing a boomy sound. For example, at Qtc=1 the peak is +1.25 dB. An alternate source for these data is "Acoustics," by Beranek (1954).

[tysontom]

>Dickason's book shows that the peak above flat magnitude is 0

>dB for both critically damped (Qtc=0.5) and Butterworth

>(Qtc=0.707). He shows how fast the peak rises above flat for

>increasing Qtc, producing a boomy sound. For example, at

>Qtc=1 the peak is +1.25 dB. An alternate source for these data

>is "Acoustics," by Beranek (1954).

Beranek's *Acoustics* is the one almost always referenced; with a direct-radiator system, the response (e.g., 45 Hz fc AR 12W) with a Q of 1 will cross the zero-refererence line at 43 Hz with no ringing. A Q of 0.7 appears slightly overdamped in this case, but is flatter with less "rise" just above resonance. Importantly, there is *no* less ringing with a Q of .7 than with a Q of 1.0; however, a Q of 0.5 is clearly over-damped and more severly attenuated.

This condition of over-damping is not uncommon with some a/s speaker designs, and it results in a "dryness" in the sound of a direct-radiator woofer. It gives the speaker a sense of "crispness." An over-damped a/s speaker produces low bass effectively in a steady-state manner, but there is attenuation in the attack portion of the waveform, such as the sound of a bass-drum beat. The thinned-out bass fails to support the attack portion of the sound which becomes prominent and "crisp." This phenomenon is then sometimes called "tight" bass, as though it were a more accurate reproduction of the bass sounds, which it is not. Villchur lucidly describes this in his article, "Loudspeaker Damping," *Audio* October, 1957.

--Tom Tyson