From: dpierce@cartchunk.org   
      
   Arny Krueger wrote:   
   > "Peter Larsen"  wrote in message   
   > news:506e8ca1$0$289$edfadb0f@dtext01.news.tele.dk...   
   >   
   >>Dick Pierce wrote:   
   >>   
   >>   
   >>>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.   
   >>   
   >>So what you say is that a "transmission line speaker" is a bass reflex box   
   >>with a very small box volume and a very large port volume?   
   >   
   > And a highly damped port.   
      
   At low frequencies.   
      
   The notion of a transmission line involves the concept that   
   at the frequencies and wavelengths involved, the "line" is   
   long compared to the wavelength. At low frequencies, this is   
   decidedly not the case. A so-called "1/4 wave" line simply   
   acts as a system of lumped components. The old IMF Monitor used   
   a Kef B139, with a 25 Hz nominal resonance. At 25 Hz, the   
   wavelength is some 45 feet long, and 1/4 of that is almost 12   
   feet. That line is NOT 12 feet long.   
      
   And even considering absolute optimal absorbtive material that   
   fully converts the internal operating conditions from adiabatic   
   to isothermal (which it is FAR from doing), that's still over 8   
   feet.   
      
   People measure the output of the line and compare its phase to the   
   output of the woofer, and from that derive a bizarrely low   
   propogation velocity. The same can be done with a standard bass   
   reflex, which suggests a small bass reflex has an internal   
   propogation volecity FAR lower than that of sound in normal.   
   This simply ignores the phase rotation one goes through in a   
   normal 2nd-order resonat system.   
      
   At higher frequencies, it's a different story, in the sense   
   that the line is, indeed, long compared to the wavelengths   
   involved.   
      
   > A transmission line speaker can be considered to be a very wastefully   
   > designed bass reflex.   
      
   Well, careful. "Wasteful" is in the eye of a beholder.   
   A side effect of the traditional design of a transmission   
   line (the old IMF Monitor/Studio model) results in two things   
   that are very useful:   
      
   1. Midbass and up, the rear wave is lost, gone, forgotten   
       and never to bother us again,   
      
   2. The cabinet walls can be VERY stiff and inert (the rear   
       wall is problematic, but there are solutions to this).   
       Now, that in and of itself is not a feature unique to   
       transmission lines, but it is a useful feature,   
       nonetheless.   
      
   Some of Fried's designs (notably the ALS-50 and its   
   derivatives) were at best, well, interesting (a 5-legged   
   horse is interesting, too).   
      
   Some of the notions (I hesitate to call them "theories"   
   becasue their predictive power is pretty poor) surrounding   
   transmission lines falls into the same category that inspires   
   people to worry about the effective impedance of their speaker   
   wires, leading to bizarre (and, mostly, very expensive) wire   
   contrivances.   
      
   --   
   +--------------------------------+   
   +         Dick Pierce            |   
   + Professional Audio Development |   
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