Transmission Lines

[Mattrance]

Hi,

Should or are transmission lines within a speaker cabinet be built in order to absorb all of the backward waves from a driver to prevent any phasing issues, or are they more used for turning an out of phase waveform, in phase, to come out of a port/vent adding to the amplitude of the sound. and if this is the case are exact mathematics of the angles through the line essential to get the waveform in-phase?

Im new to this game as you probably figured. As far as i can tell in my research so far there seams to be an argument for both cases which has confused me somewhat. some to get a better/cleaner response from the driver and others to add to its output

Any information would be greatly appreciated

Cheers.

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[wenn_du_weinst 10+ year member]

Well to be 100% both.

One version of t line, and imo the true t line enclosure, has stuffing and is designed to dissipate the rear wave while not interfering with the cone moment or front wave.

The second version, and the most popular on car audio forums, is really a 1/4 length wave guide. This version puts the rear wave 180 out of phase from it's origin to match the front wave and enforce it. To learn how to design check out here

 

[Mattrance]

cheers for the information and the link. looks useful from a first read. If the T line were to do both jobs surely it would be counter productive to be absorbing the wave you are trying to invert?

im currently in my final university year and for my audio engineering project im building a speaker to then test in different acoustical environments. in your opinion would you opt for a closed box or ported design over a transmission line cabinet? and is it worth doing for a full range driver or just subs? still confused as to what frequencies will be inverted?

 

[wenn_du_weinst 10+ year member]

You understood that wrong.

I meant; That there is no answer to that question as both are excepted as t lines. Not that it does both. There is a huge argument over which is the correct way

 

[Mattrance]

o right sorry. i get you now. is it only useful for subs though if your planning to add to the output with your T line, as i woul of thought high end frequencies will just get absorbed. im thinking of using a full range driver but unsure atm.

 

[subzero 5,000+ posts]

Hi,
Should or are transmission lines within a speaker cabinet be built in order to absorb all of the backward waves from a driver to prevent any phasing issues, or are they more used for turning an out of phase waveform, in phase, to come out of a port/vent adding to the amplitude of the sound. and if this is the case are exact mathematics of the angles through the line essential to get the waveform in-phase?

Im new to this game as you probably figured. As far as i can tell in my research so far there seams to be an argument for both cases which has confused me somewhat. some to get a better/cleaner response from the driver and others to add to its output

Any information would be greatly appreciated

Cheers.

What is a transmission line enclosure ?
The transmission line speaker was first described and patented in the 1960's. In a classic transmission line, the sound wave from the back of the woofer is channeled down a long pathway filled with a fibrous bundle of wool or another synthetic material. In a properly designed line, very low frequencies exit the end of the transmission line that extend the low frequency response one half octave below the fundamental resonance of the driver.

In a transmission line enclosure, the back wave of the woofer does not bounce off an interior wall and radiate back into the room through the thin cone of the woofer as in sealed or ported enclosures. These multiple echoes color the sound and can only be eliminated in a transmission line enclosure.

There is no pressure in a transmission line to excite strong enclosure resonances. In a sealed or ported box, enclosure resonances can usually only be controlled, not eliminated as in a properly designed transmission line enclosure.

Together, the extended low frequency response, the lack of multiple echoes from the inside of the box, and the elimination of wall resonance account for the extremely clean, well controlled and powerful sound of a properly designed transmission line speaker.

 

[Mattrance]

cheers man. think i've got it now. whilst were at it though do you know of any good speaker cabinet design websites?

 

[globalminds_ent 10+ year member]

I learn something new everyday. Seems like with a t line u could use a harder wood or something since when using MDF a lot of the sound is going through the box because 3/4 mdf is not not going to be to stop a huge 40hz wave. If a t line utilizes all of the low end seems like u would want something more solid to keep the sound from going through it... Is this true ??

 

[hispls 5,000+ posts]

I learn something new everyday. Seems like with a t line u could use a harder wood or something since when using MDF a lot of the sound is going through the box because 3/4 mdf is not not going to be to stop a huge 40hz wave. If a t line utilizes all of the low end seems like u would want something more solid to keep the sound from going through it... Is this true ??

I would say it totally depends on your power requirements. Even 5/8 MDF is adequate with say a 12" sub under 500W. Geting into competition level stuff running multiple killowats, even 2 or 3 layers of 3/4 MDF can and will flex if not properly braced.

T-line sounds spectacular if done properly.

 

[wenn_du_weinst 10+ year member]

he isn't talking flex, he means pressure loss. I'd use bamboo plywood

 

[Mattrance]

would only bamboo plywood or MDF and not ordinary plywood keep the low end sound pressure within the T line?

 

[duanebro 10+ year member]

I don't think this is correct, pro audio use 1/4" plywood to build horns, (braced every 4") If they can do 130db @ 50hz without cabin gain, then I don't think that you need crazy wood to build a spl box. Just make sure that the panels can't flex and you will be fine. (That can be a challenge though!)

 

[its_bacon12 10+ year member]

Hi,
Should or are transmission lines within a speaker cabinet be built in order to absorb all of the backward waves from a driver to prevent any phasing issues, or are they more used for turning an out of phase waveform, in phase, to come out of a port/vent adding to the amplitude of the sound. and if this is the case are exact mathematics of the angles through the line essential to get the waveform in-phase?

Im new to this game as you probably figured. As far as i can tell in my research so far there seams to be an argument for both cases which has confused me somewhat. some to get a better/cleaner response from the driver and others to add to its output

Any information would be greatly appreciated

Cheers.

From my understanding, a transmission line is used to manipulate the back wave in different ways depending on the design, but it is never used to cancel the back wave. That would just be a large sealed box essentially. Again, this is my understanding of them, as I don't have much experience with TLs. TQWT is the one I've seen most of, which uses a tapered tube (port) in the same way a horn works, to manipulate the back wave to greater output.

 

[wenn_du_weinst 10+ year member]

I don't think this is correct, pro audio use 1/4" plywood to build horns, (braced every 4") If they can do 130db @ 50hz without cabin gain, then I don't think that you need crazy wood to build a spl box. Just make sure that the panels can't flex and you will be fine. (That can be a challenge though!)

it's not for deterring flex, it's to eliminate resonance.

 

[wenn_du_weinst 10+ year member]

would only bamboo plywood or MDF and not ordinary plywood keep the low end sound pressure within the T line?

you can use mdf/plywood if you want.

Mdf I would use resin to seal it inside though.