-september.org> 0ebb84e2   
   From: zekor@comcast.net   
      
   Dick Pierce  wrote:   
   > 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.   
      
   I used a very short line and your probably right. Typical delay I measured   
   about 7 % . However, at 2.6 lbs per cubic foot, I got up to 35% , and was   
   hard to measure because it was a highly attenuated signal. at suggested   
   stuffing rates, 8 oz per cu ft., more like less than 3% delay. I don't   
   think its a transmission line if there is no signal at the end of the pipe.   
      
   Greg   
      
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