PMC clones

[planet10]

Two articles i made into 1 pdf. http://p10hifi.net/TLS/downloads/DalineB110.pdf

I also have files on my HD that say "Daline-revised-crossover.gif” and another that is a text doc “Fris Daline modification". I have attached those.

dave

 

[keilau]

How transmission line speaker works

The best technical paper that explains how a transmission line speaker works was written by Les Bradbury.

L J S Bradbury, "The use of Fibrous Materials in Loudspeaker Enclosures", AES Journal, Vol. 24, No. 3, (April 1976)

A copy can be obtained from the Audio Engineering Society for $25. Or you can go to a good technical library of any university to copy it.

Bradbury based his analysis and experiment on research on sound propagation in fiber material done for submarine warfare in the 40 and 50's. The concept may be new to those who does not work in fluid dynamics, but the paper is easy for any college student taking Advanced Calculus to follow.

An more down to earth presentation was later written by Robert White, with audio technician in mind.

Volume filling of a vented box

It shows that at the bass frequency range, the speed of sound and the amount of attenuation of the speaker back wave are nonlinear with frequency and the effect of a long fiber wool filled tube differs as the frequency changes. In the Bailey TL design, the slow down of sound speed maximize and the attenuation almost diminishes completely at about 25 Hz. Bailey called his design "A Non-Resonant Loudspeaker Enclosure Design".

When filling a vented box with polyester fiber, no back wave attenuation occurs and the box stays resonant.

There is some interesting reading at the PMC website.

 

[planet10]

My understanding was that that work from Bradbury has been shown to be incorrect.

Since Bailey got bass reinforcement as a feature of his line — from using the fundemental of the ¼ wave resonance — his line was resonant, ie not as per the title.

dave

 

[planet10]

Further: by keilau’s definition very few TLs are "true TLs”. Very few are stuffed until they are aperiodic. The Natilus ½ wave line is one of them. Jor Rasmussen did one for the Visaton B200. The Visaton has high Q and is usually associated with OBs. An aperiodic TL gives an opportunity to put it in a box and (maybe) lower the Q. I used it as inspiration for a B200 centre channel. Most aperiodic TLs are actually used for midrange duty. An attempt to suck the entire backwave away and to flatten the impedance curve.

This last is where an aperiodic box is unlike Paul’s descriptor of near sealed.

dave

 

[Scottmoose]

Augspuger, King and a number of others have shown very clearly, with extensive measured data, that Bradbury's aerodynamic drag theory, while a lovely piece of mathematics, is, bluntly, wrong. Simple as that. It doesn't match measured results. There is nothing like the velocity reduction he predicted and his equations (as he admitted himself) become rapidly inaccurate at low frequencies. QED. Here are two pieces from Martin on the subject:

http://www.quarter-wave.com/TLs/Test_Line_Results.pdf
http://www.quarter-wave.com/TLs/Damping_Coefficient.pdf

As Dave notes, Bailey's line was in fact mildly resonant, and was used to extend the LF. It was not a maximally-flat impedance line.

 

[andy19191]

Two articles i made into 1 pdf. http://p10hifi.net/TLS/downloads/DalineB110.pdf

Many thanks. It was the second article I recalled and hadn't seen the first which was an interesting read. I also hadn't seen the suggested mods. I built a pair of these in the 70s but skipped them during a house move many years ago. It was quite a nice design for the time and had some bass unlike sealed B110 designs of the day. It inevitably lacked SPL given the use of a 5" driver with modest displacement and with them flat against a wall sitting 30 degrees off-axis was not ideal.

 

[keilau]

My understanding was that that work from Bradbury has been shown to be incorrect.

Since Bailey got bass reinforcement as a feature of his line — from using the fundemental of the ¼ wave resonance — his line was resonant, ie not as per the title.

dave

Dave,

It is basic physics that wave (sound or otherwise) reinforce each other when they are in phase. 180° is in phase and 90° is out of phase. A open or polyester filled 1/4 wavelength is out of phase.

The beauty of the Bailey/Bradbury design was the use of long fiber wool filling which inverts the backwave to 180° using a quarter wave tube. It is non-intuitive, but is physically true as proved by Bradbury both analytically and experimentally.

PMC uses mid density open cell foam following the example of IMF and TDL both of which are classic transmission line speaker brands.

Martin King stated that Bradbury were wrong, but never showed any analysis or experiment of a long fiber wool filled tube. He just has no understanding of fluid dynamics. Bailey and Bradbury are both highly respected researchers in their field. It is really bad for you to smear them without any technical evidence. I know that you have a business interest to protect. But physics is physics which will not change.

 

[keilau]

Augspuger, King and a number of others have shown very clearly, with extensive measured data, that Bradbury's aerodynamic drag theory, while a lovely piece of mathematics, is, bluntly, wrong. Simple as that. It doesn't match measured results. There is nothing like the velocity reduction he predicted and his equations (as he admitted himself) become rapidly inaccurate at low frequencies. QED. Here are two pieces from Martin on the subject:

http://www.quarter-wave.com/TLs/Test_Line_Results.pdf
http://www.quarter-wave.com/TLs/Damping_Coefficient.pdf

As Dave notes, Bailey's line was in fact mildly resonant, and was used to extend the LF. It was not a maximally-flat impedance line.

Bailey and Bradbury both showed experimentally that they got in phase backwave at the terminus of a long fiber wool filled quarter wavelength TL. George Augspuger and Martin King showed that there was no phase inversion when they used polyester fiber. Both are true due to the complete difference in fiber interaction. The bottom line is that you cannot build a quarter wave TL speaker without using the proper fiber filling. I highly recommend that you read the original Bailey and Bradbury papers.

Martin King insisted that he had tested long fiber wool too, but never published his results on long fiber wool. It is well known to most audio engineer that not all fiber are created equal. See Ken Kantor's discussion, (on fiber filling, not on transmission line).

 

[pkitt]

All fibrous material used to stuff TLs acts the very same way, be it long fiber wool, polyester, fiberglass, etc., only needing different densities to achieve the same effects. The claim that that long fiber wool is better because it has the magical quality of inverting the back-wave by 180 degrees is bogus (rather than non-intuitive), only one of several TL myths, that keep getting repeated, then believed by the uniformed.
Paul

 

[Charles Darwin]

I used log-haired sheep wool to stuff my TLs.

Chose it for a number of reasons, none of them acoustic.

 

[Scottmoose]

As stated, there is nothing mystical about any damping material. They all behave in similar fashions, with detail differences depending on their physical properties. The twaddle about long-hair wool being completely different to any other type is exactly that -twaddle.

The fact is, Bradbury's theoretical predictions frequently do not match measured results very well, especially at low frequencies. Bullock wrote software based upon it, and found just that. This is in contrast to those based on other models e.g. those of King, Augsperger, Hersh & others, which do not use an aerodynamic drag equation / assume moving fibers, but do show good correlation between the measured & predicted results.

I see it has been claimed that Martin King has no knowledge of fluid dynamics. Since his day-job happens to involve fluid-dynamics in the heavy-engineering sector, while many others who are specialists in the subject have found no major flaws with it, that claim has some formidable obstacles in its way. I also note that the tired old 'how dare you criticise a respected researcher' line has been dusted off, and presented with its usual spluttering ire. I am a practicing academic, and it would be a sad state of affairs if I or any of my research colleagues had such an attitude. There are no sacred cows. If you publish something, you expect it to be critiqued, and flaws (or good points) to be pointed out and discussed. That is an integral part of the research process. Publishing an article in Wireless World magazine in 1965 or a paper in the AES journal of 1976 does not place you on an exalted plane that is above criticism. It means you wish to put an idea out for wider debate, for acceptance or rejection as further discussion and data comes in.

 

[Bigun]

+1

 

[keilau]

All fibrous material used to stuff TLs acts the very same way, be it long fiber wool, polyester, fiberglass, etc., only needing different densities to achieve the same effects. The claim that that long fiber wool is better because it has the magical quality of inverting the back-wave by 180 degrees is bogus (rather than non-intuitive), only one of several TL myths, that keep getting repeated, then believed by the uniformed.
Paul

Calling engineering based on solid mathematics and science "myths" does not make it so. I published technical papers during my career. They are subjected to rigorous peer review.

I went through the derivation of Bradbury's formulation of the reaction of a fiber filled tube out of curiosity and found it to be accurate. Bradbury also conducted experiments to verify his analytical results and found good agreement.

You are making wild statements without technical substantiation. Show me your test that disproved the experimental result of Bailey and Bradbury. Bailey was very ingenious to invent his "spark" test. Bradbury was very lucky to inherit the test equipment from submarine warfare research.

BTW, did you read these papers? Where did you get the idea that "All fibrous material used to stuff TLs acts the very same way"?

 

[Scottmoose]

...and multiple people since then have built lines according to Bradbury's theories and found poor correlation between them and the measured results, especially at low frequencies, where Bradbury himself acknowledged progressive deviation. Bullock for example designed software specifically using Bradbury's theories, and considered the result unsatisfactory. These findings have been mirrored by multiple independent researchers and designers for decades. You appear to be in a minority of one in desperately trying to cling to a disproved theory.

 

[keilau]

...and multiple people since then have built lines according to Bradbury's theories and found poor correlation between them and the measured results, especially at low frequencies, where Bradbury himself acknowledged progressive deviation. Bullock for example designed software specifically using Bradbury's theories, and considered the result unsatisfactory. These findings have been mirrored by multiple independent researchers and designers for decades. You appear to be in a minority of one in desperately trying to cling to a disproved theory.

In the early 70's, many authors and readers of the Audio Amateur Magazine (now AudioXpress, found by Edward T. Dell, Jr.) built transmission line speakers stuffed with long fiber wool. I am one of them who found the bass of such enclosure both excellent in quality (non-ringing, fast transient) and quantity (easily extended below 35 Hz). Bailey and Bradbury have very specific specification for the wool to be used. Bullock was a late comer to the Audio Amateur magazine. Bullock did not understand the importance of the wool fiber diameter and stiffness. He used other type of fiber and could not duplicate the result of earlier builders. I discussed some of the history in posts at the AudioKarma forum in 2014.

Martin King was a participant of another AudioKarma discussion. From his plot, you can see that a Dracon filled line can eliminate some cabinet resonant at higher frequency, but does not change the envelope of the response or the impedance curve. That prove a Dracon filled line is essentially a vented speaker enclosure.

Yes, I am in the minority in an era when boomy bass from bass reflex enclosure is fashionable. But I am not the minority of one. I am a technical person in trade. I need analytical AND experimental proof in support of each other before I accept something as technically accurate. Among the minority, PMC is a well respected speaker manufacturer who distinguish TL from bass reflex. (Unfortunately, they also call Aperiodic enclosure TL!!)

Some classical technical paper on why non-boomy bass is desirable, in addition to Bailey's 1965 classic publication.
Loudspeaker Enclosure Design by E.J. Jordan -1
Loudspeaker Enclosure Design by E.J. Jordan -2

E.J. Jordan is generally recognized as the inventor of the aperiodic speaker enclosure. Aperiodic enclosure has similar bass characteristics as transmission line speakers, but does not go down as low using the same woofer driver.

The question on how transmission line enclosure works appears to be an emotional issue to many. This is my last post on this thread. Refer to the links I cited above (6 of them) for more technical details on the subject. Thank you for your understanding.

 

[planet10]

From his plot, you can see that a Dracon filled line can eliminate some cabinet resonant at higher frequency, but does not change the envelope of the response or the impedance curve. That prove a Dracon filled line is essentially a vented speaker enclosure.

All that shows is that Martin chose not to put sufficient Dacron in to push the response there.

...in an era when boomy bass from bass reflex enclosure is fashionable. But I am not the minority of one.

I too am not one to put up with boomy bass. I have been building TLs since i was in Ubi in 1975. i have recently done at least 4 Woden designed ML-TLs that have superb non-boomy bass.

You’d probably call those Bfs — they do have a passing resemblance — But King showed that they are true TLs.

dave

PS: those Jordan pdfs are probably the ones i scanned and put back out for the community. One of my mentors did some of the most advanced work on aperiodic enclosures… one of th eeay cheats to make one is to build a TL with lots of stuffing.

 

[GringoAudio]

In the early 70’s..

Thanks for your contributions to this most informative thread!

 

[pkitt]

Keilau, here's likely my last post, too, in this thread. You said this in Post #12 about PMC:
The PMC builds true transmission line cabinets based on the correct theory.

If that's true, why doesn't PMC use long fiber wool in their TLs instead of simply lining some of the lines' walls with foam? I'm sure I've not seen the innards of all of their speakers, but all of the ones I have seen are made that way. Wouldn't that be diverging from "correct theory"?
Paul

 

[Scottmoose]

Pardon? Is somebody honestly saying that link 'proves' Dacron does not have a damping effect? That's a cracker. I like that. One of the least effective attempts at a disingenuous example I've seen on the subject for months. Well done indeed.

That line was not designed to be a maximally flat impedance design. Ergo, it does not exhibit those traits. That does not demonstrate dacron does not have a damping effect or impact upon impedance curves (which is such utter drivel I'm struggling to comprehend how anybody thought they could get away with stating it: for example, see pages 4 - 7: http://www.quarter-wave.com/TLs/Damping_Coefficient.pdf & there are plenty more out there from many sources). It simply shows that the speaker you have selected to misrepresent the subject was not designed for that purpose.

Bullock knew what Bradbury used perfectly well, and his model used those equations, while his measurements were taken of test enclosures using the materials originally specified (long hair wool and fiberglass). They failed to correlate well. Nor has anybody else been able to match measured results to Bradbury's equations with a high degree of accuracy, whereas other methodologies show measurements matching predictions very well, a fact you have anxiously avoided.

I am glad you hold up PMC as an example of transmission lines with an unreactive impedance curve though. Here are independent measurements (from Stereophile) of the impedance curve of two of their products, the DB1i and IB1S, both exhibiting an LF double-saddle peak in the impedance magnitude trace; a point John Atkinson specifically commented on re the latter, even describing it as 'typical of a bass reflex design'. PMC IB-1S loudspeaker Measurements | Stereophile.com

 

[Bigun]

Of course it has two humps, of course it has traits like a BR. There are only 3 types of boxes aren't there ?

1) a resonant box (BR, TL, BVR, MLTL, Onken etc. with various amounts or extremes of damping)
2) an impedance converter (horn, SLOB, etc.)
3) sealed