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Transmission-line speakers

sarumbear

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I could insist that I'm young and handsome, too. That wouldn't make it true. Maybe they are a TL, maybe they aren't. I looked into Stereophile magazine:


quote: "Although PMC describes the fact.8's woofers as being loaded with an "Advanced Transmission Line," the solid trace in fig.1 doesn't look significantly different from the impedance of a conventional reflex loading, with a low-frequency saddle at what would be the tuning frequency of the port." and "As with a reflex design, the output of the port is in opposite polarity to that of the woofers, though the length of the line delays the vent output by several milliseconds."


quote: "In a true transmission line, the vent output is in-phase with that of the line-terminated woofer; by contrast, the output of the PMC DB1i's vent is in opposite polarity to that of the woofer, again typical of reflex behavior."

And a page from someone named Kalman Rubinson. I never met him, but he sounds like a bright kid. :)

I am not disputing what you say. I was being sarcastic as TL alignment is the company’s USP even though, as you correctly said, it is not a TL, nor an advanced one (ATL). It is simply a vented enclosure with a lossy port.
 

sergeauckland

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I could insist that I'm young and handsome, too. That wouldn't make it true. Maybe they are a TL, maybe they aren't. I looked into Stereophile magazine:



quote: "In a true transmission line, the vent output is in-phase with that of the line-terminated woofer; by contrast, the output of the PMC DB1i's vent is in opposite polarity to that of the woofer, again typical of reflex behavior."
In a true transmission line, there's no output from the vent. It doesn't matter whether the vent is there or not. That so-called TLs are vented, indicates they're probably not true TLs.
The only example I can think of, of a true TL without a vent is the B&W Nautilus, which has a line of decreasing cross section, ending in a dead end.


S
Nautilus.jpg
 

Jim Taylor

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I am not disputing what you say. I was being sarcastic as TL alignment is the company’s USP even though, as you correctly said, it is not a TL, nor an advanced one (ATL). It is simply a vented enclosure with a lossy port.

I apologize. I find it difficult to identify sarcasm other than my own. :) :) Jim
 

sarumbear

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In a true transmission line, there's no output from the vent. It doesn't matter whether the vent is there or not. That so-called TLs are vented, indicates they're probably not true TLs.
The only example I can think of, of a true TL without a vent is the B&W Nautilus, which has a line of decreasing cross section, ending in a dead end.


SView attachment 166160
All B&W diamond tweeters have a TL enclosure.
 

Apesbrain

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Now 35 years ago and I still remember loving the sound of the Fried Studio IV. Pretty sure it's a TL design. IIRC, it was powered by a Musical Fidelity amp; the little one with a cooking grill on top.
 
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sarumbear

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Now 35 years ago and I still remember loving the sound of the Fried Studio IV. Pretty sure it's a TL design. IIRC, it was powered by a Musical Fidelity amp; the little one with a cooking grill on top.

aK7WxJatbQrDov-DlO6tEP1ddGa7p4wDIAHr53UD-VBFPOihT_FpGBswA0UeTX9oG_6Ck647E4jGx0pY1qF5MGLtTHiO
With that port at the bottom I doubt it is a TL design.
 

617

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99% of the time a speaker is described as a transmission line, it means that the bass augmentation coming from the rearward radiation of the speaker is dominated by 1/4 wavelength behavior. In reality, the effective length of the line and the characteristics of that augmentation can be altered by dampening, as well as tapering or widening of the line, or the insertion of a port.

Bass reflex, back loaded horn and 1/4 wavelength transmission lines can be thought of as three points on a continuum, with every speaker having openings in the box exhibiting some degree of behavior from each part. Tall tower speakers, for example, are sometimes modeled as bass reflex, but end up behaving like mass loaded transmission lines.

The term 'transmission line' to describe a long channel for absorbing the backwave of a driver is technically correct, but rarely used to convey this meaning. Few speakers attempt these types of transmission lines; the Linkwitz LXMini and the Nautilus are good examples. Fried, IMF, PMC and so on all refer to their 1/4 wavelength speakers as transmission lines, and I believe DCM did with the Time Windows, if we're being really esoteric.
 

Doodski

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With that port at the bottom I doubt it is a TL design.
I have never seen what is claimed to be a TL design that didn't have a port. It seems to be widespread that the nomenclature is being abused?
 
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pseudoid

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...Since this sound is delayed from the sound from the driver diaphragm, the port output and driver output combine in-phase and the total sound output is then increased....
...The port output is delayed and only sums correctly with the sound from the front of the driver across a limited frequency range - there will be cancellation above this range, and also problems with delayed sound output...
What would be the distortion by-products for this design?
 

sarumbear

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I have never seen what is claimed to be a TL design that didn't have a port. It seems to be widespread that the nomenclature is being abused?
If you read the thread you would have seen this:

index.php
 

sarumbear

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The term 'transmission line' to describe a long channel for absorbing the backwave of a driver is technically correct, but rarely used to convey this meaning.
I was trying to be technically correct :)
 

DanielT

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Here is another one who studied this with TL:

The short answer is that transmission lines should be avoided as far as possible. The disadvantages outweigh the disadvantages. For bass purposes, it is in practice about quarter wave pipes (because it is not possible to create a reflection-free finish) and here both closed boxes and bass reflex ditto win by far. Quarter wave pipes are simply hopeless in speaker contexts and should only be used in musical instruments - in organs they fit well. :)

Only when you have a large area and very low moving mass (read belt or electrostatic elements), and the diaphragm thus becomes very sensitive to reflections on the back, can there be reasons to use a transmission line.

The construction would have been significantly more efficient with a different principle for the same element equipment and cavity volume. In order to achieve a fairly constant frequency response, you have to add lots of acoustic attenuation and then the efficiency goes to he.... If you do not attenuate, you maintain the efficiency, but instead obtain difficult peaks and valleys in the frequency response.

My short and well-founded advice after studying the theory behind quartz wave pipes, taking part in various dissertations and trying out the principle practically, is that one should not even think the idea.

...... the more bass reflex and the less quarter wave you use, the better. Optimum is thus at 100% bass reflex. Here we have an example of a mixture of principles, which to say the least is not particularly successful:


And you have of course verified this by making sure that the frequency response is the same in both cases? It is quite possible to tune a bass reflex speaker so that it has basically the same (but smoother) frequency response as your quarter-wave, but then it also has lower distortion and a higher efficiency or a smaller cavity volume. Physics is what it is, no matter what you think of it.

Mr. Ekvivalisator

Technical consultant in acoustics and audio technology





Hm, was not a direct advocate of TL, so to speak.Did not think about that. Cavity volume. It should be taken into account when comparing TL vs other constructions. What gives the most bang for your buck (or the volume in this case).:)

Well, that Alta Audio Alyssa did not measure well, but it was quite expensive, always something. Apparently costs $ 5000

 
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Kvalsvoll

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What would be the distortion by-products for this design?
There can be linear distortion in the form of uneven frequency response, due to the port contribution summing in-phase at some frequencies, and cancelling out-of-phase at other frequencies.

A narrow channel can also introduce non-linear distortion if the velocity is too high.
 

617

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I have never seen what is claimed to be a TL design that didn't have a port. It seems to be widespread that the nomenclature is being abused?

There can be linear distortion in the form of uneven frequency response, due to the port contribution summing in-phase at some frequencies, and cancelling out-of-phase at other frequencies.

A narrow channel can also introduce non-linear distortion if the velocity is too high.

An insufficiently damped transmission line has a telltale and dramatic comb filtering looking phenomena in the bass, but this is easy to avoid. There's a sweet spot where these notches and peaks are attenuated, but the bass isn't diminished.

Having said that, I think all of these discussions of bass boost using the backwave of a speaker reflect a dated view of bass reproduction. I wouldn't design a modern speaker with a 1/4 wave augmentation.
 
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Kvalsvoll

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Here is another one who studied this with TL:

The short answer is that transmission lines should be avoided as far as possible. The disadvantages outweigh the disadvantages. For bass purposes, it is in practice about quarter wave pipes (because it is not possible to create a reflection-free finish) and here both closed boxes and bass reflex ditto win by far. Quarter wave pipes are simply hopeless in speaker contexts and should only be used in musical instruments - in organs they fit well. :)

Only when you have a large area and very low moving mass (read belt or electrostatic elements), and the diaphragm thus becomes very sensitive to reflections on the back, can there be reasons to use a transmission line.

The construction would have been significantly more efficient with a different principle for the same element equipment and cavity volume. In order to achieve a fairly constant frequency response, you have to add lots of acoustic attenuation and then the efficiency goes to he.... If you do not attenuate, you maintain the efficiency, but instead obtain difficult peaks and valleys in the frequency response.

My short and well-founded advice after studying the theory behind quartz wave pipes, taking part in various dissertations and trying out the principle practically, is that one should not even think the idea.

...... the more bass reflex and the less quarter wave you use, the better. Optimum is thus at 100% bass reflex. Here we have an example of a mixture of principles, which to say the least is not particularly successful:


And you have of course verified this by making sure that the frequency response is the same in both cases? It is quite possible to tune a bass reflex speaker so that it has basically the same (but smoother) frequency response as your quarter-wave, but then it also has lower distortion and a higher efficiency or a smaller cavity volume. Physics is what it is, no matter what you think of it.

Mr. Ekvivalisator

Technical consultant in acoustics and audio technology





Hm, was not a direct advocate of TL, so to speak.

Well, that Alta Audio Alyssa did not measure well, but it was quite expensive, always something. Apparently costs $ 5000

This was a good example for showing how a similar principle can be utilized to give very different results. Compare the my 1. post, the picture and graph from the F2. Same type - some kind of channel, ported at the end. But the results are different - can be due to differences in requirements specification, or just wanted to make a different sound. Leaving those resonances like it can be seen in the Stereophile measurement, sure will make this speaker sound unique - maybe you will like it.

But a sophisticated design with channels and ports does not in any magical way increase low frequency efficiency above what can can be obtained using a normal ported box of same size. It just makes is possible to use a different driver with parameters that are not suitable for a typical sealed or ported box, and still get bass.

Tuning to a low frequency also does not give more usable low bass. You can tune it to 5Hz, no problem, but the efficiency follows the tuning so it will never be able to produce sound loud enough to be of any practical use.
 
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Kvalsvoll

Kvalsvoll

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I think all of these discussions of bass boost using the backwave of a speaker reflect a dated view of bass reproduction.
But they are exciting and interesting. Speakers that are different - and some have faults - if everything was the same, that would be boring.

Acoustic loading can also do things that a typical box will have a hard time achieving, when done properly. But that is an entirely different discussion.
 

pseudoid

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What would be the distortion by-products for this design?
There can be linear distortion in the form of uneven frequency response, due to the port contribution summing in-phase at some frequencies, and cancelling out-of-phase at other frequencies.

A narrow channel can also introduce non-linear distortion if the velocity is too high.
I am guessing (w/o 'knowing'); I would except THD #s to be quiet high, along with the other TL pitfalls that you ('knowingly') detailed.
I missed the part whether you are using a cross over or bi-amping... tough to eq its problems out.
 
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Kvalsvoll

Kvalsvoll

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I am guessing (w/o 'knowing'); I would except THD #s to be quiet high, along with the other TL pitfalls that you ('knowingly') detailed.
I missed the part whether you are using a cross over or bi-amping... tough to eq its problems out.
Can not eq resonances because if you remove a resonance by using a cut, this cut also works on the sound radiated from the front of the cone - not possible to eq only the sound that goes into the TL.

If the TL is ported to increase output, this can reduce distortion due to better acoustic loading of the driver, even if the acoustic channel itself introduces some distortion.

This applies only to the sort of TL that really is not a pure TL. A pure TL that only absorbs sound - no port - will behave more like a sealed cabinet, where distortion is determined by the linearity of the driver, and low frequencies require large air volume displacement from the driver itself.
 

bigjacko

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From recent posts I see people saying quarter wave box is not as good as closed or vented box because ported box can get same low end with same box volume. This is new idea to me, I thought quarter wave will get a bit more low end then ported box. @Dennis Murphy does use transmission line for his towe speakers, I think we can get his view on this topic as why he choose to use transmission line.

I am wondering what about tapped horn and paraflex type of box? Those box type are pretty popular, of course we can't make ported that big, but what will the low end boost and box volume ratio comparing those box types?
 

DanielT

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- maybe you will like it.
There you have a point, you have to test and have an open mind.:)

Very good thread by the way. Interesting. Thank you for starting it.

I take a little more from that thread I referred to in my previous post. Interesting with TL. Agree with you that it is fun and exciting with different principles regarding the design of speakers.

A quater wave pipe with the same frequency response as a bass reflex solution also has the same impulse response. The transient properties will thus be the same. The difference, however, is that in order to obtain a smooth frequency response for the quater wave pipe, it must be attenuated so that the efficiency is greatly reduced. In addition, the reduction of the cone movements around the tuning frequency becomes smaller for the quarter-wave pipe, which leads to higher distortion.

Vattenfall diagrams do not provide any additional information as the frequency response is sufficient to describe the system properties. This is a minimum phase system and the behavior in the time plane is directly connected to the frequency plane, m.a.o. a nice behavior in the frequency plane corresponds to an equally nice impulse response.

Incidentally, the higher order modes quater wave pipe leads to them appearing much worse in waterfall diagrams than bass reflex ditto. You do not even want to know what the horror example Alta Audio Alyssa looks like in a waterfall diagram.

The speed of sound in the quater wave pipe decreases as the fibers swing with and add mass. You can see it all as a quasi-fluid that consists partly of air and partly of the fibrous material.

Well, despite a little vague language, you understand what is meant. As I wrote above, you get a new composite "material" in the pipe lthat consists of both air and damping material. The air has "stiffness" via the compression module K and has a certain density rho, while the damping material mainly contributes a little extra density. As a result, the speed of sound decreases according to c = (K / rho) ^ 1/2.

Mr. Ekvivalisator
Technical consultant in acoustics and audio technology



The size of a TL / pipe. The cavity volume. These are not small speakers you throw into the listening room / living room. Do you not want to get the best bang for your bass buck money you can get? For example, 200-300 liter seald or BR suwoofer with double 15 "or 18" in each box. There will be a lot of bass.:).

Or for those who do not need so much bass, double 10-12 "in 100-150 liter boxes.
(provided drivers with low distortion, high x-max, good linearity in well-built, well-tuned boxes)
 

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