More on T-Lines

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General speaker questions for audio and home theater.

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An e-mail from someone with an old attempt at a T-Line prompted me to reply with an analysis of the line, and I thought that others might benefit from the response as well. The T-Line was 91.7" long, and had a volume of about 1.54 cu. ft. He wanted to use a Scan Speak 8545 in it, so I used those T-S parameters when calculating the closed box resonance.
Response:
A quick analysis of your T-line. First, the bends do
count in the length, you draw a line down the middle of
the T-line, and the length of that line is the length of
the T-line. In your case, it was 91.7" (or 233 cm).
This means that the 1/4 wave resonance can vary from
about 37 Hz for an unstuffed line, to around 19 Hz for a
fully stuffed line. ((The effective speed of sound can
be slowed to about 1/2 nominal when a line is fully stuffed).
An estimate of box volume reveals that the raw unstuffed
T-line volume is about 1.54 cu. ft. (43.6 liters), and
this in conjunction with your ScanSpeak 8545 would cause
a sealed box resonance at 40.5 Hz. If the line were to be fully
stuffed, this would only lower the resonance down to
about 37.5 Hz. (Fully stufing a line increases the
effective volume by about 1.35 times the nominal volume)
The idea is to get the frequencies to match, and in this
case, the line is too long to allow this. The two ranges
do not overlap at all, and obviously, the two should
match up under the same conditions, preferably when close to fuly
stuffed. The reduction in line length will allow the
volume resonance to catch up and match.
For instance, if the bottom section (40 cm) were removed,
then the line length would be about 193 cm, with a raw
tuning of 44.6 Hz, and a fully stuffed tuning of 23 Hz.
Volume resonance would now be for 37 liters, and the
frequency would be 42.4 Hz without any stuffing, and
around 39.2 Hz fully stuffed. At least now, the two
ranges overlap a little.
This tells me that the line needs to be a little shorter
yet, to allow the line to be close to fully stuffed, so
as to prevent the typical line cancellations.
You want to shoot for an overlap that has the fully
stuffed condition for the two cases very close, so that
minor adjustments of the stufing for the line will allow
the two to match exactly.
End of response.
This is all based on the original T-Line note instructions I posted at:
/audio/speakers/messages/9723.html
Someone might wonder why not create a simple formula that takes into accoun the effective line length vs. the effective line volume, and come up with a sure-fire result? The problem is that the speed of sound and volum increase factors are only aproximate, and both aspects vary differently with different stuffing materials. Hence the need to cut and try IF you want the tuning and stuffing to be optimum.
Jon Risch

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Topic - More on T-Lines - Jon Risch 08:10:54 12/11/99 (4)

Re: More on T-Lines - Bob 11:36:56 12/11/99 (3)

Re: More on T-Lines - Jon Risch 15:03:45 12/12/99 (2)

Re: More on T-Lines - Shawn Harvey 19:43:25 12/12/99 (1)

Re: More on T-Lines - Jon Risch 11:09:04 12/13/99 (0)

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