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Transmission-line speakers

beneix

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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.
 

Vladimir Filevski

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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
Because Twenty.24 is very similar to FB1, frequency response of FB1 in a anechoic chamber will be very similar to Twenty.24. No way for 27 Hz at -3dB (or -6dB) anechoically. Usually, if there are no -dB tolerances (as here), than it should mean -10dB (at 27Hz), which is quite possible.

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.
General trend is dismissive because there are so many badly designed TL speakers. Nothing is wrong with the concept of TL speakers, it only needs a good designer.
 

sergeauckland

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General trend is dismissive because there are so many badly designed TL speakers. Nothing is wrong with the concept of TL speakers, it only needs a good designer.
Exactly. A real TL, i.e. one where all the rear radiation is absorbed, none of this quarter-wave nonsense, is in my view, the best possible bass loading. There should be zero resonances, no energy storage and no honky colorations. The downsides are both cost, due to complexity of construction, and size, as a true TL can't be small. Shipping weight is no small issue either.

If I were to choose a 'no-compromise' loudspeaker, or as near to that as current technology allows, I would choose something like the B&W Nautilus, albeit with modern DSP crossover.

S.
 

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.;)
1642618074500.png
<< Overwhelming number of choices and tradeoffs.
 
OP
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.
 
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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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