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Old 18th August 2011, 03:29 AM   #1
goody75 is offline goody75  United States
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Default Transmission Line Tuning Question

Hypothetically, lets say I have a single FR driver with an Fs of 40Hz and a 3db roll off @ 60 Hz.

In a TL tuned to 40 Hz, I understand that I will get a boost @ 40 Hz and the associated harmonics but will I get any assistance between 40-60 Hz?

Likewise if I tune the TL to, let's say 30 Hz, other than getting a boost @ 30 Hz (below the drivers natural resonant freq) will I get any help between that and the natural roll off?
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Old 18th August 2011, 05:28 AM   #2
GM is offline GM  United States
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Assuming the TL's net Vb is sufficiently large for the driver's specs, then undamped, the pipe will have significant gain for ~[2] octaves then will have a huge dip at its 3rd harmonic. Damp down the dip though and pretty much all of the pipe's gain will be gone also, creating a ~ infinite baffle response down to below Fs; so lower the tuning, lower the dip and if it's below Fs, then the driver will have less protection.

GM
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Old 18th August 2011, 07:05 AM   #3
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The advice I received on here (and other forums) is that you can go below the Fs only if the driver has a high Qts. The roll-off of a TL is more gradual than a BR. You can also place the driver one third distance along the line length, which helps flatten the frequency response.
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Old 18th August 2011, 08:09 AM   #4
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What GM said.
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Old 18th August 2011, 01:41 PM   #5
goody75 is offline goody75  United States
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To both GM & Bill,
So just so that I understand both of you correctly...

The Eminence Alpha-15A that is so popular for OB's would be a good candidate for a TL tuned below its 41 Hz Fs (Qts = 1.26). It would be a good candidate disregarding practicality because it would be an absolutely MONSTEROUS cabinet! But I digress....

The 3rd harmonic of 40 Hz is 120 Hz. So since this is used as a subwoofer, putting the low pass filter below the 120 Hz covers the huge dip - is this correct? - Now that I think about it, the 3rd harmonic that GM refers to is probably the 3rd harmonic of the tuning frequency not the Fs. But either way, if you tuned the cabinet to 20Hz and set the low pass for just below 60 Hz, would be a solution to the issue?
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Old 18th August 2011, 02:20 PM   #6
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In my experience, when the driver Qts goes over .7 you get a big hump in the upper bass that can't be removed by tuning. The Delta-15A would be ideal.
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Old 18th August 2011, 02:31 PM   #7
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An end driven undamped TL is a good solution for a bass only speaker. You can tune below Fs, but generally only to .707*Fs. You will most likely get significant room lift at the bottom and maybe gain another half octave. But, you will have to get out before the 3rd harmonic sets it because the FR returns back to the original SLP rather quickly. A steep XO is called for, so this is only economically plausible with an active XO. Also, you might consider a high pass filter at Fp, because the pipe unloads rapidly below Fp and the driver is in danger if there is any content down there.

BTW, 100dB at 20Hz runs a serious risk of pulling the drywall off of the ceiling.

Bob
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Old 18th August 2011, 02:39 PM   #8
goody75 is offline goody75  United States
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Quote:
Originally Posted by Bob Brines View Post
An end driven undamped TL is a good solution for a bass only speaker. You can tune below Fs, but generally only to .707*Fs. You will most likely get significant room lift at the bottom and maybe gain another half octave. But, you will have to get out before the 3rd harmonic sets it because the FR returns back to the original SLP rather quickly. A steep XO is called for, so this is only economically plausible with an active XO. Also, you might consider a high pass filter at Fp, because the pipe unloads rapidly below Fp and the driver is in danger if there is any content down there.

BTW, 100dB at 20Hz runs a serious risk of pulling the drywall off of the ceiling.

Bob
It would make for a pretty good Youtube video though.
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Old 18th August 2011, 06:26 PM   #9
DrBoar is offline DrBoar  Sweden
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Damping of resonances is most efficent at the velocity maxima. For the fundamental that is at the opening at the pipe. The third harmonic has maxima at 1/3 and 2/3 of the pipe. If you damp the first half of the pipe you will attenuate the harmonics, that you do not want and still keep the fundamental. The strict location of maxima is relly just for straight pipes, folding and tapering shift the locations somewhat. Still the harmonics will have at least one maxima in the first half of the pipe.

There are two TLS schools.
One that use it similar to a bass reflex with low Q drivers and selective damping (PMC speakers come to mind).
The second one aim to lose all back radiation in the line and tend to use higher Q driver.

I really see no pint in the latter unless to salvage a highQ driver for some reason.
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Old 18th August 2011, 06:59 PM   #10
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Re the latter, for flattest impedance. Note that I'm simply stating that to be the case; it doesn't mean I'm promoting it. Or the reverse, for that matter.
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