"ludovic mirabel" wrote in message
news
I'd like to solicit the opinion(s) of knowledgeable readers.
As I recounted recently I made two transmission line woofers,
each consisting of two "sonotubes'" (compressed paper cylinders)one
inside the other with a 12" woofer (Electrovoice frame and Altec
Lansing cone). The whole enterprise is very primitive and just within
my very limited aptitude for jigsawing and so on.
The tubes are packed with Dacron pillow fill. They are 24db
Linkwitz Xovered at 400Hz.
Two things happened: 1) From the start I got clean bass and
low treble. And buckets of it. I use Radio Shack SPL meter with the
Stereophile warble 1/3 octave tones from my listening seat as a guide
to equalisation Behringer 2084 digital equaliser) From the start I
was getting more bass esp in the lower octaves, beginning with 100
Hz, than I ever got out of my old closed boxes (originally Magnat
made)
2) Over the succeeding weeks it became obvious
that the bass was getting obnoxiously too much. I remeasured. To cut
the long story short: on the right side of my room which opens into an
L dining room- (so no corner) I had to cut 80 Hz to 30 Hz range-
mostly by 30 db(yes- no typo) to get it to match my starting point of
ZERO db at 1000Hz. Even 20 Hz had to go 10 db down, And believe me I
hear plenty of bass. No mistake.
It would appear that as the filling settled down
the lower octaves got louder. Is it what should happen with
transmission line?
I should say that now the right side is the "strong"
side. When I had closed boxes I would blow the woofers trying to get
50 Hz up to mark. Hence my custom sturdy 12" made by a friendly
speaker repairsman who took pity on me.
The overall quality of my bass is the best I evar had
and possibly the best I can recall having heard. The cost of
sonotubes/pillowfill and one day's work is about the best return for
the expenditure I ever made in audio. But I still would like to know
HOW it works. (Please make it as simple as possible).
Ludovic Mirabel

Suggested reading: G.L. Augspurger, "Loudspeakers in Damped Pipes--Part
one: Modeling and Testing; and Part Two: Behavior," 107th JAES Convention,
24-27 September, 1999, prepring No. 5011.

G.L. Augspurger, Transmission Lines Updated, Part 1 Speaker Builder 2/00

G.L. Augspurger, Transmission Lines Updated Part 2
Speaker Builder 3/00

G.L. Augspurger, Transmission Lines Updated Part 3
Speaker Builder 4/00

J.A. D'Appolito, Testing Loudspeakers, Audio Amateur Corporation
Peterborough New Hampshire, 1998

Also the article in audioXpress from May 2002 describing the design of the
THOR TL kit for SEAS Mr. D'Appolito mentions that "after many months of
operation, the Dacron pillow filler settled in the second half of one of the
lines."

He says that this did not appear to affect performance but can be avoided by
using Acousta Stuf or Dcron Quilt padding. Read the full article for all
the details.