[Dick Pierce[_2_]]

Arny Krueger wrote:
"Peter Larsen" wrote in message
. ..

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.

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+ Dick Pierce |
+ Professional Audio Development |
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