Are Transmission Line speakers fundamentally flawed?

[Kayvon]

I've seen this written more than a few times, not least by a fairly well reputed UK speaker manufacturer of old BBC designs and then I found a more explanatory presentation in this article by AES fellow John Watkinson: http://www.theregister.co.uk/2014/07/02/feature_the_future_loudspeaker_design/?page=4

It seems quite clearly explained by the diagram in the middle of the page but I'm guessing that's not quite the whole story, not least because I'm aware there are hundreds of considerations & compromises when designing a speaker and secondly, well, PMC get plaudits from a variety of different worlds (Cinema post processing/Hi-Fi/audio engineers and indeed SOS).

Links to any papers that tackle this question head on or your own explanations are much appreciated : )

ps I realise that nowadays PMC may be able to control this type of thing with their DSP crossovers. Digital processing for speakers seem to me like the way forward!

Cheers,

Peter

[Hugh Robjohns]

I've seen this written more than a few times...

ALL loudspeaker designs are fundamentally flawed. ALL of them.

Different designs just have different flaws that affect different aspects of sound reproduction. The manufacturers and their customers get to choose which flaws they care about, and which ones they are prepared to ignore.

...not least by a fairly well reputed UK speaker manufacturer of old BBC designs...

No commercial interest or advantage in rubbishing another (more successful) manufacturer's products there then... We could equally well point out a range of fundamental technical flaws in the old BBC designs, too... Nothing is perfect.

...and then I found a more explanatory presentation in this article by AES fellow John Watkinson...

As yes, Dr Watkinson. A brilliant man in many ways. He has always taken a very pedantically negative view of 'transmission line' speakers. A few years back, he spent a lot of his time lambasting them for not being true 'transmission lines' in strict engineering terms. It's true, they aren't, but so what? Many manufacturers call their sealed speakers 'infinite baffle' designs when they are nothing of the sort... but Dr W doesn't seem interested in harassing them in the same way.

It seems quite clearly explained by the diagram in the middle of the page but I'm guessing that's not quite the whole story...

And you'd be quite right. The PMC 'ATL' cabinet only affects the bass operation of the bass driver, and only the very lowest octave of its output at that. At higher frequencies the bass driver behaves as if it is working into a large sealed cabinet. So diagrams and discussions of transients are rather misleading at best and disingenuous at worst. I'm not suggesting that a PMC speaker can perfectly reproduce a square wave signal... but then neither can anything else.

It's all a compromise, but PMC genuinely believe that the ATL cabinet loading design they have developed over the last twenty years or more brings some worthwhile advantages. In particular, lower LF distortion and greater bass extension for a given cabinet size, as well as the slow roll-off and time domain performance of sealed cabinet designs. The designs also maintain the spectral balance at low levels, which ported cabinets seem not to be able to do. As you have noted, PMC has a very large and diverse customer base, so they must be doing something right...

I am the owner of a few pairs of PMC speakers and enjoy working with them. But I also have Neumann and Genelec speakers too.. they all have their own strengths and weaknesses.

Links to any papers that tackle this question head on or your own explanations are much appreciated

PMC haven't and won't publish anything about the details of their ATL designs -- it is obviously commercially sensitive. And there are very few academic papers because it is a very difficult and expensive area to research with very limited options for commercial exploitation. But PMC have done (and continue) a great deal of meticulous research and development on things like the shape and cross sectional areas of the line, the type, form and placement of damping materials, the location of the bass driver within the line, the exit format of the line, and much more besides. Their designs certainly aren't thrown together, and the technology has definitely improved over the years, too, as knowledge and experience grow.

One final point, Dr W isn't quite as independent as he would like to appear. As always in his articles on this topic, he inevitably steers things to the conclusion that omnidirectional speakers with advanced active crossovers are the only way of achieving accurate sound reproduction -- and he presents some very good arguments to support his claims.

What he doesn't do is declare his interests as being the designer of several commercial omnidirectional speakers marketed under the Celtic Audio brand -- a company of which he was also a director (it was bought out by TSL in 2011). And having heard a few Celtic Audio speakers in different situations (including here) I have to say that while I could recognise some benefits, there were also some significant flaws. Funny, that!

H

[Kayvon]

ALL loudspeaker designs are fundamentally flawed. ALL of them.

I forgot the first golden rule of loudspeaker design!

The PMC 'ATL' cabinet only affects the bass operation of the bass driver, and only the very lowest octave of its output at that. At higher frequencies the bass driver behaves as if it is working into a large sealed cabinet. So diagrams and discussions of transients are rather misleading at best and disingenuous at worst.

I was thinking about it this morning, there are so many factors involved with regards to time alignment that until you know what they're actually doing with the whole system it does, like you say, seem like it could be misleading to give such a simple explanation to dismiss the advanced transmission line design out of hand. It'd be fascinating to reverse engineer one and see what's going on. The DSP section may hold some interesting clues!!

One final point, Dr W isn't quite as independent as he would like to appear. As always in his articles on this topic, he inevitably steers things to the conclusion that omnidirectional speakers with advanced active crossovers are the only way of achieving accurate sound reproduction -- and he presents some very good arguments to support his claims.

What he doesn't do is declare his interests as being the designer of several commercial omnidirectional speakers marketed under the Celtic Audio brand -- a company of which he was also a director (it was bought out by TSL in 2011). And having heard a few Celtic Audio speakers in different situations (including here) I have to say that while I could recognise some benefits, there were also some significant flaws. Funny, that!

Ah, now that I didn't know. I read another article by John Watkinson that was explicitly about the human auditory system and omnidirectional speakers and I did find it v interesting. I've got my eye on the Bruel & Kjaer omnidirectional speaker/soundsource and I'm curious to listen to it when I can. Perhaps somewhat removed from studio monitors but it's all speaker design.

Thanks for your reply Hugh, do you have any interesting legacy speakers lying around for fondness' sake or are you ruthlessly modern in your speaker collection? Perhaps you could comment on how flawed and therefore useless the Leslie Rotating speaker is! Cheers

[Studio Support Gnome]

I'll add my tuppence

Transmission lines , in theory, are a fairly workable idea, with perhaps a better set of compromises than most standard port applications…

the trouble is, that to really do what they ideally should, they need to be fairly significant in scale…. this leads to a bit of an issue with practical loudspeaker design…. in that the theoretical ideal size of the line , generally makes for a very very large box…… like….. huge……

(and so does a sealed box design….. to get proper low end…. you need a massive cabinet….. )

where Hugh and i might differ in view is what happens when you downscale from the ideal, to try and produce a workable near field sized box….. it's not physically possible to fit a "properly" working transmission line, in to such a box…..

thus an ongoing decade old friendly Jibe between myself, and a colleague or two, and Hugh, about small PMC's making excellent doorstops….



I LIKE big ones…… (sort of, up to a point ) but little ones, don't really work for me.

( said the actress to the bishop, fnarr fnarrr )

[Kayvon]

So even the 8 inch TwoTwos are too small?

How about the subs?

The floor standing models look interesting but I'd be keen to hear what they sound like with DSP crossovers. Maybe that's just where the ridiculously expensive ones come in?

[Hugh Robjohns]

Perhaps you could comment on how flawed and therefore useless the Leslie Rotating speaker is! Cheers

Funnily enough, there is a big feature already in the system all about the wonders of the Leslie speaker. It should appear sometime next year...

[Hugh Robjohns]

...where Hugh and i might differ in view is what happens when you downscale from the ideal, to try and produce a workable near field sized box….. it's not physically possible to fit a "properly" working transmission line, in to such a box…..

No differing. I completely agree with you. A short line is inherently compromised compared to a long line, just as a small sealed cabinet is compromised compared to a very large sealed cabinet. ...and ported cabinets are compromised whatever their size

... but little ones, don't really work for me.

Fair enough, and I know where you're coming from. I think the active twotwos have now overcome some of the limitations of the earlier small passive designs, but the big ones inherently perform to a higher level. Even so, I'd still take small passive PMCs over a large raft of similarly sized ported speakers. But offer me a good mid-sized sealed cabinet and my head might well turn...

[Studio Support Gnome]

IMHO Even an MB system is verging on the "too small"



but then i'm insane , everyone knows this …..

(said the man with Sealed boxes , possibly slightly bigger than an MB )

[Studio Support Gnome]

But offer me a good mid-sized sealed cabinet and my head might well turn...

and it did./... KHO310 , cough cough…..

of course, maybe i could tempt you with reviewing the SP25MA

[Hugh Robjohns]

…of course, maybe i could tempt you with reviewing the SP25MA

Sure. I'd love to. Have them sent over!

H

[Studio Support Gnome]

careful what you wish for, SWMBO might not be amused…… but , AFAIK, there's set destined for a client install in manchester. that might be able to detour for a reasonable period….. will speak to his nibs…..

[Kayvon]

Funnily enough, there is a big feature already in the system all about the wonders of the Leslie speaker. It should appear sometime next year...

Right on cue! I look forward to it, cheers (:

of course, maybe i could tempt you with reviewing the SP25MA

Looks very fetching. Fetching thousands of pounds out of my price range!!

Obviously one shouldn't make speaker purchase decisions based on technology as opposed to what they actually sound like but it's interesting to demo some of the models with rarer designs. It should not surprise you that I don't earn any of my living through mixing records!

I want to demo Unity Audio's Pebbles due to their aperiodic design and I wouldn't mind checking out some of these passive radiator monitors too even though people refer to say the Mackie's notable group delay. Funnily enough it was this idea of sealed speakers low group delay that got me into speakers. But I'm enamoured with the Bose Mini Soundlink's bass (different kettle of bananas, granted) so that's why that design interests me.

I'm interested to read up about if/how speakers interact with the room in some way. Like whether there is a form of feedback? Can anyone comment on that? Like for instance if a passive radiator would resonate with reflected waves in the room or if the air around the opening of a port would be affected by the moving air molecules post excitation.

Cheers all

[dmills]

The Bose soundlink is an interesting wee box is it not.

I have not had one in the chamber yet, but am planning to do that experiment, I suspect lots of psychacoustic shenanigans is involved, probably a Chebyshev polynomial doing the bass harmonics thing, at the least. I expect the Dscope will reveal wonders.

Speakers do indeed interact with the room, for example placing a speaker within a fraction of a wavelength of a reflecting surface will increase the apparent output, two reflecting surfaces gets you another increase in output, and three gets you yet more (This is a major reason you do not tend to see flown subwoofer hangs (Quite apart from the weight).
The more interesting effect of this stuff however is that will also increase the apparent radiating area (Very important for horn loaded Bass boxes in large PA systems where apparent horn mouth area determines the frequency below which the horn unloads and the driver shreds itself).

Regards, Dan.

[Kayvon]

The Bose soundlink is an interesting wee box is it not.

I have not had one in the chamber yet, but am planning to do that experiment, I suspect lots of psychacoustic shenanigans is involved, probably a Chebyshev polynomial doing the bass harmonics thing, at the least. I expect the Dscope will reveal wonders.

Hmm, I'm not sure what I've started here. That sentence I highlighted in bold piqued my interest more than a little. I then went to the wiki entry for it and, well, daunted but hopeful were the two emotions that came up. I'm doing my first maths/physics equations for 14 years (I'm 33) and tbh I'm having trouble even writing the most basic of equations in my scientific calculator. But I'm gonna have to get into some of this Chebyshev stuff if I'm to understand this speaker aren't I?

I have access to an anechoic chamber which unfortunately is only flat to not far below 100Hz I think (with relatively high background noise in the low octave bands. Tbf it is an old one located right above an underground line). In any case I had planned to take mine in there to do some measurements on it myself along with a few other non spectacular speakers I own.

May I ask what your area of work/study is? Can carry on via PMs if you'd prefer.

[Kayvon]

Speakers do indeed interact with the room, for example placing a speaker within a fraction of a wavelength of a reflecting surface will increase the apparent output, two reflecting surfaces gets you another increase in output, and three gets you yet more (This is a major reason you do not tend to see flown subwoofer hangs (Quite apart from the weight).
The more interesting effect of this stuff however is that will also increase the apparent radiating area (Very important for horn loaded Bass boxes in large PA systems where apparent horn mouth area determines the frequency below which the horn unloads and the driver shreds itself).

Regards, Dan.

Very interesting, I'm listening.

I'm keen to know more about the near field responses of speakers i.e. before the waves get a chance to settle into a more plane wave type of consideration (please correct me if I'm thinking about that wrong).

[dmills]

I design broadcast electronics for a living, and we also have a not very flat chamber at work (The things get stupidly large at low frequency), there is something to said for going out in the carpark and using it as a open field site but the A404 makes that hard in the bottom octaves.

There are some well known psychoacoustic tricks you can play with Bass, a good one being to deliberately produce a whole comb of harmonics, and then remove the fundamental... You still hear the Bass, but the bottom octave has now gone away so you don't tend to run into the excursion limit with a small driver so quickly.

If you decide to explore the Bose thing, look at the harmonic structure in the bottom couple of octaves, not just the frequency response.

I commend www.mathworld.com to your attention by the way, much better then wikipedia for maths.

Regards, Dan.

[dmills]

I'm keen to know more about the near field responses of speakers i.e. before the waves get a chance to settle into a more plane wave type of consideration (please correct me if I'm thinking about that wrong).

Far field is spherical in the absence of reflections causing critical distance issues, (never a safe assumption), and can only locally be considered to approximate a plane wave.
The near field is **Complicated** because the approximations behind concepts like plane and spherical waves assume a point source and that is not the case in the near field, the transition from horn throat to free space is kind of fun to think about.

An interesting case is the modern line array speaker, where the system is long enough to approximate a cylindrical wavefront within some sort of medium field with the spherical nature becoming apparent only once you get far enough back that the source once again approximates a point.
The magic being that because the array is long and thin, the system appears as a point source in the horizontal plane a lot closer then it appear as a point source vertically, so you can get 120 degrees of horizontal pattern with only 10 degrees or so of vertical, and within the vertical near field (Which can extend many tens of metres) you only loose 3dB per doubling of distance (Rather handy that in a stadium gig).

Regards, Dan.

[Kayvon]

There are some well known psychoacoustic tricks you can play with Bass, a good one being to deliberately produce a whole comb of harmonics, and then remove the fundamental... You still hear the Bass, but the bottom octave has now gone away so you don't tend to run into the excursion limit with a small driver so quickly.

I'm familiar with a few but have never been satisfied. The way you explain here is the way I always thought about doing it but the plug-ins I've tried in the past have added too much extra tonality to the muted bass tones I like to use. It would be v interesting to understand in some small way how the Bose is doing it.

If you decide to explore the Bose thing, look at the harmonic structure in the bottom couple of octaves, not just the frequency response.

I'm thinking I may hold off for a bit until I have a bit more of an idea about what I'm doing/what I'm trying to look at. I'm thinking of taking a punt on an Studio Six Digital iTestMic and their suite of apps which have, amongst other useful tools, some speaker test tools. Not quite in the same class as the Sound Level Meters from NTi and Norsonic but then again it would be a £185 outlay to feed my curiosity somewhat and I already have an iPad Mini.

I commend www.mathworld.com to your attention by the way, much better then wikipedia for maths.

Cheers for sharing

[Kayvon]

An interesting case is the modern line array speaker, where the system is long enough to approximate a cylindrical wavefront within some sort of medium field with the spherical nature becoming apparent only once you get far enough back that the source once again approximates a point.
The magic being that because the array is long and thin, the system appears as a point source in the horizontal plane a lot closer then it appear as a point source vertically, so you can get 120 degrees of horizontal pattern with only 10 degrees or so of vertical, and within the vertical near field (Which can extend many tens of metres) you only loose 3dB per doubling of distance (Rather handy that in a stadium gig).

Regards, Dan.

I think I can paint a mental picture of that. I had often wondered what's so special about line arrays. I really haven't ever read into PAs and Live Sound but I am keen on designing a keyboard speaker for live & home use so it's good to hear a bit more about the challenges involved in that world.

The near field is **Complicated** because the approximations behind concepts like plane and spherical waves assume a point source and that is not the case in the near field, the transition from horn throat to free space is kind of fun to think about.

So I gather! The directivity of speakers in the near field (or at least within 1m) vs far (2m plus) is one of the things I am keen on measuring when I get into the anechoic chamber.

Cheers for your contribution to the thread

[Tomás Mulcahy]

I've got my eye on the Bruel & Kjaer omnidirectional speaker/soundsource and I'm curious to listen to it when I can. Perhaps somewhat removed from studio monitors but it's all speaker design.

The dodecahedron? Very lacking in bass, and the amp has proven to be unreliable.

[Kayvon]

I had supposed it might be lacking in bass. Was it a particular revision whose amp has been unreliable? I've seen two different models, I know next to nothing about them so not sure if they changed amps or anything.

I'm not seriously gonna buy one although I did check some prices for s/h models from the States!

[Tomás Mulcahy]

I don't know anything about revisions. I'm sure B+K would be very happy, even concerned, to check out our amp but I don't think that's been done.

[Studio Support Gnome]

useful for acoustic testing to meet required test specifications …….. otherwise……. naaahhhhhh

[Kayvon]

On the rounds I did see this one as well.

As used by the Princeton laptop orchestra.

These type of omnidirectional speakers are an interesting proposition for sure (for more than just testing purposes) but there's a part of me that thinks maybe it's just the natural progression for speaker nerds (guilty as charged).

As a keyboard/synth player who's never been satisfied with any modern mainstream speaker solution I'm certainly interested in a speaker that spreads sound out in a more omnidirectional pattern. Preferably not at the same dB level backwards towards my ears as forward out to the crowd but nevertheless, as keys players I feel we're short changed. I bought the Groovetubes SFX Spacestation MKII and wasn't happy with it and then within the last 6 months bought and subsequently sent back the new MK3 version. Mid and Side speakers still interest me greatly (I wish I hadn't passed up on the Emes Mini Owl I demoed in the summer, at that time I thought it sounded a bit boxy but I would trade that for access to the m/s technology now).

Links to any interesting speaker designs greatly received!

Btw, PMC aren't the only outfit in the Transmission Line game, I'm v interested to hear what one of these sounds like on low digital piano notes, an area which really seems to stretch the distortion of inferior PA speakers/keyboard amps. Hopefully as wizzy(!) as the name suggests

[Hugh Robjohns]

On the rounds I did see this one as well.

There is a fundamental misconception in the blurb for that thing -- and in the claims for most omni speakers: instrumental sources do NOT radiate sound omni-directionally. Pretty much every acoustic instrument ever invented has quite wacky polar patterns in which different parts of their frequency spectrum get beamed in radically different directions and with different coverage angles.

... there's a part of me that thinks maybe it's just the natural progression for speaker nerds (guilty as charged).

Maybe for speaker nerds... But probably not for high quality stereo sound reproduction applications, and certainly not for conventional live sound applications.

I'm certainly interested in a speaker that spreads sound out in a more omnidirectional pattern. Preferably not at the same dB level backwards towards my ears as forward out to the crowd...

So you want an omni speaker that's not actually omni. Hmmm. Some muddled thinking there then! I think what you probably mean is a high quality speaker with a wide dispersion angle which is well controlled across the entire spectrum. It's a standard requirement for decent high end studio monitors. Rather less so in budget PA systems...

Mid and Side speakers still interest me greatly

Another bonkers concept derived from a total misunderstanding of Blumlein's mid-side concept and one that can never work in practice....

Btw, PMC aren't the only outfit in the Transmission Line game

I don't think anyone has claimed that they are, have they? But no one else uses their ATL technology. All the other TL designs I've seen are straight from the simpler 1960s designs.

At the end of the day, quality costs money. There are no shortcuts. Every speaker design has its compromises. There are no magic technologies.

H