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Transmission-line speakers

Vladimir Filevski

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Well, I would not call quarter-wave TL a nonsense, it is just another viable loudspeaker construction. The real TL (closed) has many virtues, but also several downsides - just as you accurately described.
 

pseudoid

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Well, I would not call quarter-wave TL a nonsense, it is just another viable loudspeaker construction. The real TL (closed) has many virtues, but also several downsides - just as you accurately described.
If you ever see an advertisement for that "perfect" loudspeaker design/construction; please don't forget to post it in our "Snake-Oil" ledger.;)
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<< Overwhelming number of choices and tradeoffs.
 
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Kvalsvoll

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I am aware of the difference between anechoic and in-room measurement. Since there is no measurement to be found on the internet for the FB1s (they are over 20 years old now, so I suppose that is part of the reason), I thought I would just provide a quick-and-dirty substitute in the form of a measurement in my room. The point I was trying to make was that given the response I get between 20 and 50 Hz, I find it unlikely that the FB1 in an anechoic chamber would look anything like the Twenty.24i. By the way, it seems that the official PMC spec is 27Hz-20kHz – not sure where that SoundOnSound review got their numbers from.

Anyway, since I do not live in an anechoic chamber :), I am happy with the FB1s. I just need to improve my room and possibly compensate with some DRC – a future project.

Since the general trend of opinions expressed in this thread was quite dismissive of TL speakers, I thought I would share my own positive experience. I am sure there are bad TL speakers out there; I hope never to encounter them.
A TL/acoustic loading speaker can be designed so it does not show the flaws pointed out in this thread, and even if there are some anomalies in the design, the overall result, when listening to music in a room, can still be an improvement over the typical sealed box.

If there are resonances and cancellation in the upper bass range, this will often be masked by room reflections and resonances, so the effect of this flaw may not be noticeable.

The positive is that the loading can reduce cone excursion and improve dynamics. Then we have a fault that may not be noticed, and a benefit that may very well be significant.

Just wanted to give a more balanced view, if my posts in this thread seems to reject TL speakers. There is a reason why all my own subwoofer designs have acoustic loading.
 

dasdoing

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can somebody tell me what effect this "hole" at the bottom of my PA speakers has? the front board just ends, it's not a port.


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TomR

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Solstice Speakers by Parts Express
I am curently building these solstice speakers, they are mass transmission-line Speakers. I have been trying to determine if I should put 1/2 inch Sonic barrier foam on the inside of the speakers. Of course you can't block the port or the air flow. The Solstice Speaker kits come with polyfill, which you have to stuff into the speakers.
 

Everett T

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Solstice Speakers by Parts Express
I am curently building these solstice speakers, they are mass transmission-line Speakers. I have been trying to determine if I should put 1/2 inch Sonic barrier foam on the inside of the speakers. Of course you can't block the port or the air flow. The Solstice Speaker kits come with polyfill, which you have to stuff into the speakers.
I would follow what Bagby speced. Changing any of that will change several parameters. IiRC Jeff didn't set out to do a TL, the bass reflex just happened to work out that way. Those are very nice reference speakers.
 

itz_all_about_the_music

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Agreeing with Everett T, and adding that building them to "spec" (i.e., the baseline any/all/most other reviewers will also be testing) will allow you to learn from both your AND their experiences. Should you in the future open them up and tweak the fill, you can then better discern what sonic differences resulted.
 

Vladimir Filevski

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can somebody tell me what effect this "hole" at the bottom of my PA speakers has? the front board just ends, it's not a port.
I hope I am not too late to answer this (I didn't notice your post on time). It is a simple bass-reflex port - PA loudspeakers have large volume and relatively high resonant frequency, which usually translates to rather short bass-reflex tube/canal, or simply a hole in the front board of the cabinet.
 

witwald

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A TL/acoustic loading speaker can be designed so it does not show the flaws pointed out in this thread, and even if there are some anomalies in the design, the overall result, when listening to music in a room, can still be an improvement over the typical sealed box.
How is that possible? Doesn't a typical sealed loudspeaker enclosure produce essentially a 2nd-order system that will have an excellent transient response? How can a TL system reproduce such a 2nd-order response function? Anything with a response function of higher order will be worse.
If there are resonances and cancellation in the upper bass range, this will often be masked by room reflections and resonances, so the effect of this flaw may not be noticeable.
As a sort of loose corollary to this, if a TL system has resonances and cancellations in the upper bass range, then these will often not be masked by room reflections and resonances, so the effects of the flaw will be quite audible to the listener. It seems rather odd to use the possible defects in a room response function to creatively cover up a fundamental flaw in a speaker's design: if the listener's room is bad, then bad speakers will sound good?
The positive is that the loading can reduce cone excursion and improve dynamics.
Sounds a lot like a QB3 vented-box low-frequency alignment, or even a more standard B4 vented-box low-frequency alignment. The benefits of the former were described by Small back in the 1970s.
 

witwald

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I think the practical application for transmission lines in hifi speakers is a somewhat niche thing. You have to have a bigger and more complex box, and you get less bass.
Plus the commercial "transmission-line" loudspeakers offered by companies such as IMF and TDL were quite low in sensitivity. For their enclosure size, their sensitivity seemed to be quite in line with that of a vented loudspeaker system with the same size of woofer, same size of enclosure and similar low-frequency cut-off.
 

dasdoing

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I hope I am not too late to answer this (I didn't notice your post on time). It is a simple bass-reflex port - PA loudspeakers have large volume and relatively high resonant frequency, which usually translates to rather short bass-reflex tube/canal, or simply a hole in the front board of the cabinet.

thanks, didn't know that a port could be a simple "hole". the strange thing is that it has a steep FIR highpass at 120Hz and the 2 woofers are 6". I wonder if the port has any effect at all
 

Vladimir Filevski

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Port should have positive effect (less cone movement in the lower passband), if designed properly. In your case, resonant frequency should be less than 100 Hz (or equal, depending on how steep the filter is). You can easily determine bass-reflex resonant frequency without measurement equipment - just use any (free) software frequency generator (plus amplifier attached to line out of your PC) and manually choose different signal frequencies, starting with 120 Hz and going down. Watch the cone amplitude movement - on the resonant frequency cone movement will be minimal.
 
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Flak

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DanielT

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So, TL down to 40 Hz. Data on FR and distortion. Plus the size of the speaker then it is this .....?

Observe, I do not diss TL as a construction principle I am just curious to see some good ditto.:)
 
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Kvalsvoll

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How is that possible? Doesn't a typical sealed loudspeaker enclosure produce essentially a 2nd-order system that will have an excellent transient response? How can a TL system reproduce such a 2nd-order response function? Anything with a response function of higher order will be worse.

As a sort of loose corollary to this, if a TL system has resonances and cancellations in the upper bass range, then these will often not be masked by room reflections and resonances, so the effects of the flaw will be quite audible to the listener. It seems rather odd to use the possible defects in a room response function to creatively cover up a fundamental flaw in a speaker's design: if the listener's room is bad, then bad speakers will sound good?

Sounds a lot like a QB3 vented-box low-frequency alignment, or even a more standard B4 vented-box low-frequency alignment. The benefits of the former were described by Small back in the 1970s.
Seems like I missed these comments. Unfortunate.

A TL is not a simple resonator, and its response can be shaped quite freely to adapt to a chosen roll-off, and it is this roll-off that creates the resonance at cut-off. often, the response will have a much steeper roll-off compared to a simple sealed system, and thus the ringing will be much worse, and so the transient behavior should be worse for the TL. But it is not the resonance at roll-off that is problematic for practical speaker designs, it is the dynamic non-linear behavior and room interaction that determines transient performance. A TL can be designed with a very low roll-off, and thus any ringing at resonance will never be a problem, because it is never excited. Acoustic loading can reduce non-linear effects and dynamic compression, when done properly, which means better transient response.

A bad room can mask faults in the speaker, yes, and those same faults will be very audible in a good room, when resonances from the room are removed. So a good room will be better for revealing faults in the speaker system.
 

Everett T

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So, TL down to 40 Hz. Data on FR and distortion. Plus the size of the speaker then it is this .....?

Observe, I do not diss TL as a construction principle I am just curious to see some good ditto.:)
Good ditto?
 

Doodski

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Eh... is not it called that in English? Or did it go wrong, ... ignore the ditto.

An example of a good TL speaker that goes down to 40 Hz. I'm curious to see some example. :)
The issue with pre-made transmission line speakers is everybody wants a very high price for them. There are many kit transmission line speakers but they can get expensive too. I've owned 2 different models of transmission line speakers and they where warm sounding deep bass thumpers. One pair I used in a tri-amp'd system and the other was a set of Mirage that I can't remember the model number it was so long ago.
 

Everett T

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Eh... is not it called that in English? Or did it go wrong, ... ignore the ditto.

An example of a good TL speaker that goes down to 40 Hz. I'm curious to see some example. :)
Plenty have been made, it depends on the design which you're looking for. One of the reasons that they have been used is to extend the frequency response, which you see in single driver systems. You have speaker like the Salk Song Towers that take a 5 1/4 driver and get into the low 40s F3. Many bass reflex units end up being transmission line speaker by accident, Jeff Bagaby had two that he mentioned.

It's not a simple subject, but most agree that no matter the version of a TL, the bass always sounds good.
 

ryanosaur

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The new Philharmonic BMR Towers, as well as the predecessor Phil3 are MLTL designs. Both are rated to mid 20s at the bottom end. Along with several of the Salk builds, all the TL designs are optimized by Paul Kittinger.
 
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