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(Don Hills) wrote:

In article ,
(Nousaine) wrote:

Actually slowing down sound has nothing to do with it this effect (I've
conducted experiments that show up to 36% apparent enclosure size increase
----
1.5 lbs per cubic foot @ 1.5 ft3 enclosure.) It has to do with keeping
thermal
conditions in the enclosure closer to isothermal as opposed to adiabatic.

Er... No. I refer you to the seminal works on the topic:

[1] A. R. Bailey: "Non Resonant Loudspeaker Enclosure", Wireless World,
October 1965, and "The Transmission Line Enclosure", Wireless World, May
1972.

Bailey's work was with open enclosures.

[3] L. J .S. Bradbury: "The Use of Fibrous Materials in Loudspeaker
Enclosures", JAES Vol. 24 No. 3, April 1976.

Both found (Bailey by experiment, Bradbury by theoretical work) that about
0.5 lb per cubic foot was optimum, and reduced the speed of sound to
around half of its free air value. This amount of reduction cannot be solely
due to isothermal versus adiabatic effects, as the theoretical maximum
reduction in apparent velocity is only on the order of 15%.

I'll bow to Bradbury. Haven't read that for several years now.

Someone did some
research into the reasons for the discrepancy, and it is this paper which
I'm currently looking for in what I jokingly call a filing system. Dick
Pierce did some work on this, maybe he knows - are you listening, Dick?

For the apparent enclosure size increase, your experimental figures are
quite close to the theoretical maximum (about 40%).

Indeed they were; which probably meant that the highest stuffing density may
have
somehow added mass to the system. Which is why I use 25% as a rule of thumb.

1 to 1.5 pounds per cubic foot appears to be optimal and stuffing tupe
(fiberglas, rockwool, wool or polyester) idicates that polyester is as good
as
the others and has no disadvantages.

As the man with a wooden leg said, "That's a matter of a pinion." grin

--
Don Hills (dmhills at attglobaldotnet) Wellington, New Zealand

I'll amend that to 1 lb/ft3.