[Dick Pierce[_2_]]

gregz wrote:
Dick Pierce wrote:

gregz wrote:

You will also find a delay of the 30 Hz since it's below resonance, the
spider will slow it down and cause a phase shift.

Uh, no. The spider does not "slow it down and cause a
phase shift." Whatever phase shift exists is not because
the "spider slows it down."



I guessed. When I was trying to figure out transmission line speakers, I
experimented with a 6.5 inch driver and a pipe. I was trying to figure out
delays along the pipe with stuffing. I first used the generator as sync and
measured various lengths along pipe. What was predicted was lower
frequencies having longer delays through stuffing. Until I placed a second
mic right behind the driver did I discover the delay was caused by the
driver. This was below resonate frequency of driver. I never found the
predicted longer delay at lower frequencies according to at least one
paper.

And assuming your technique was oging to give you a reliable
measure of actual propogation delay is what's wrong with so-
called "transmission line theory". People have used these
measurements and derived completely absurd conclusions about the
effective speed of sound through such a line. I've seen claims
that the "speed of sound" in a transmission line drops by 75%.
Sorry, but that's just plain nuts.

The MOST the stuffing can do is drop the tranmission speed
by about .7 times, and that's assuming a PERFECT absorber,
capable of complete conversion from adiabatic to isothermal
conditions. And that just doesn't happen. You'll be lucky to
see a 15% actual reduction in propogation velocity.

And to talks about something that's shorter than a wavelength
as a transmission line itself is bordering on absurd. Consider
the fact that at those frequencies, you're actually looking at
the system as a Helmholtz resonator with a whopping large
acoustic inertance, a tinu effective acoustic cimpliance, and
a pretty high absorbtion loss, and then calculate what happens
to the effective pahse shift as you move through that overdamped resonance.

Bud Fried (of, among other things, IMF fame) did more to set back
the amateur's grasp of physical acoustics then damned near anyone
else: he, regretfuylly, was a very persuasive, charming personality
who was otherwise clueless about acoustics.

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+--------------------------------+
+ Dick Pierce |
+ Professional Audio Development |
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