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Speaker Asylum General speaker questions for audio and home theater. |
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Speaker Asylum General speaker questions for audio and home theater. |
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In Reply to: transmission line questions? What should Qts be ? To Jon Risch also... posted by jon B on March 15, 2000 at 10:05:49:
[ IF IT IS UNSTUFFED HOW MANY DBS BOOST at fundamentals? +3DB ? 6DB? ]Boost with respect to what?
Some would say that it is 6 dB. However, these peaks are not as much the problem as are the dips, which occur in between the peaks, and they reach -40 dB or so !! While it would seem that loosing certain bass notes may not be that terrible, you must understand that since this is a casual system, the amplitude response will relate to the phase response and time domain response.
What this means in laymans terms is that the bass charater will not be coherent and will not sound of one piece. The 46+ dB range of amplitude
response shifts will cause the phase to go all over the place, and the time domain response will be correspondingly mucked up.[ There was a test by JAES I believe in 1993 of speakers in a pipe 12' long. They also placed the speaker 1/3 of the way in and 1/4 of the way in, and the speaker on the end (there are no specs on the Fs,Qts, or Vas). The pipe with the subwofer at the end had the most low end extension, but had
cancellation @80hz. The speaker with the drive 1/3 of the way in went the least in 20hz o/p (52 db instead of 60 something db) and had less dip near 80hz. The speaker with the sub 1/4 of the way in had medium extension but was the most flat 40 to 120hz. Looking at their results I did not see the
boosted tuning fundamentals reflected on the graphs. Bose also makes a wavecannon 12 feet long with a sub 1/4 of the way in but they list the freq response down to 25hz, not the F3. I will post the actual journal date tonight near 6:00 Iowa time at break time at work. ]These tubes with the woofer placed inside the pipe are not T-lines, but resonant pipes, essentially two pipe organ columns stuck together. Instead of one note bass, you get two note bass.
[ To Jon Risch: Let me get this straight, If I build a transmission line, It will perform as is it was in a sealed box of the same internal volume. Would it then roll off at the F3 if it was in a sealed box or would the f3 (theoretical) be the tuning frequency of the 1/4 wavelength ?
Thank you for all your informative posts. I believe stuffing will happen only after I am dissapointed with the unstuffed pipe. ]
No, a T-line will not act like a sealed box in terms of acoustic output. I mention the sealed box aspect for tuning purposes. Due to the back pressure of a properly tuned T-line at the resonant frequency of the driver IN THE T-LINE, the woofer will see an air load that is as if the open end of the T-line were closed, hence the need to match the line length and line volume using the woofer T-S parameters so that the line length tuning and the apparent enclosure volume the woofer sees end up in juxtaposition.If you tune the line properly for use WITHOUT stuffing, it will not work correctly with stuffing.
The roll-off of a properly tuned T-line will approximate 6 dB/oct through the tuning region, then the roll-off rate will increase progressively.
For the original posts, see:
http://www.audioasylum.com/audio/speakers/messages/19693.html
AND
http://www.audioasylum.com/audio/speakers/messages/11476.htmlJon Risch
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Follow Ups
- Re: transmission line questions? What should Qts be ? To Jon Risch also... - Jon Risch 10:54:56 03/16/00 (0)
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