Songtower transmission line design

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cacophony777

Songtower transmission line design
« on: 11 Jun 2010, 04:46 am »
In the Audioholics review (http://www.audioholics.com/reviews/speakers/floorstanding/salk-songtower-qwt/songtower-setup-and-measurements) Tom Andry claims that the Songtower is not technically a transmission line design:

"The QWT in the SongTower QWT name stands for Quarter Wave Transmission. The design is claimed to be a quarter wave tube variant of a transmission line cabinet but it doesn’t seem apparent to me. Without getting too technical, a quarter wave transmission line speaker design is all about increasing bass response above and beyond what you might expect from a ported design through the use of an extended tube. If you open up a conventional transmission tube speaker, you'll see a labyrinth with bends and corners everywhere (and usually only one driver but that's beside the point). One of the functions of all these turns is as a low pass filter… wait, I'm getting technical again. Let's put it this way, regardless of twists and turns inside of the box, in order to be considered a transmission line, it needs to have an open end and a closed end. If you stretch this definition, you could argue that any enclosure with a length is a transmission line. It'd be hard to make but you could make it (and not a lot of people would agree with you). The fact is that the SongTowers were supposed to be based on Martin King's work. The presence of a flared port pretty much negates this and makes the SongTowers just a calculated bass reflex design (like most other speakers)."

I tried looking at Martin King's site, but it's a bit over my head. Does Tom have a point or is he misinterpreting transmission line design?

rlee8394

Re: Songtower transmission line design
« Reply #1 on: 11 Jun 2010, 06:08 am »
It's not that he is misinterpreting transmission line design, I think he just failed to consider and understand the various types of transmission line design. The Salk Song Towers are a MLQWTL design. THe Mass Loaded Quarter Wave Transmission Line (MLQWTL) is one where the tuning of the enclosure uses a port to "mass load" the enclosure to provide the required tuning for a physically shorter length line. The diameter of the port, the length, and the position along the line relative to the driver position, are all related in the proper operation of the MLQWTL design. Once you understand the operation of this design, you'll understand how it differs from a bass reflex design in that a MLQWTL design operates on quarter-wave resonance as opposed to a bass reflex design operating on Helmholtz resonance. If you don't understand the distinction of this design with respect to operation, and just judge it by physical observation, it is easy to misinterpret the Song Tower as a bass reflex design based on the fact that it has a port similar to a bass reflex design. See Jim's detailed response here:

http://forums.audioholics.com/forums/showthread.php?t=47528&page=3

Actually, read the entire thread. Also read all of the documents from Martin King's site on the operation and design for various types of transmission line designs. He provides Mathcad worksheets for these different quarter-wave designs.

Hope this helps.
Ron

cacophony777

Re: Songtower transmission line design
« Reply #2 on: 11 Jun 2010, 06:32 am »
Thanks Ron! That's exactly the level of detail I wanted.

planet10

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Re: Songtower transmission line design
« Reply #3 on: 11 Jun 2010, 08:00 am »
Also read all of the documents from Martin King's site on the operation and design for various types of transmission line designs.

In Martins seminal paper on the ML-TQWT he has some diagrams generated in ANSYS that clearly show the differences in what is happening inside an ML-TL vrs a BR.

dave

Paul K.

Re: Songtower transmission line design
« Reply #4 on: 11 Jun 2010, 02:47 pm »
That's not the first time, nor will probably not be the last time, a reviewer will make that comment.  Their confusion is understandable since they assume a TL can only be one with a really long line, with a hole in the wall for the terminus, whose 1/4-wavelength resonant frequency of its length is the sole determiner of the shape of the system response curve and the system's overall tuning frequency.  They also are bothered by there being a port tube and the impedance curve having two peaks, just like a vented system.  Of course they either don't know or won't admit that ALL TLs will have twin impedanc peaks until stuffing is added, meaning TLs are inherently 4th-order systems just like vented system.  The SongTowers as an ML-TL uses the internal height of the cabinet as the line's length.  That length is long enough to allow its 1/4-wavelength resonant frequency to contribute to part of the system's overall tuning frequency, and the rest is contributed by the mass-loading port's dimensions.  And, the port's specific location is chosen to optimize the system response.  It's quite easy to demonstrate that an ML-TL is not "just" a vented system via modeling its response.  Theoretically, if it's just a vented system, one could cut the cabinet height in half, double the cabinet depth to maintain the exact same cabinet volume, and keep the same port dimensions, resulting in the system's tuning frequency and overall system response not changing.  It won't happen!  In the end it comes down to mostly semantics, definitions and perceptions, but that doesn't change this fact:  the SongTowers perform extremely well.
Paul (designer of the SongTowers' ML-TL)

HAITIMAN

Re: Songtower transmission line design
« Reply #5 on: 11 Jun 2010, 03:32 pm »
In the Audioholics review (http://www.audioholics.com/reviews/speakers/floorstanding/salk-songtower-qwt/songtower-setup-and-measurements) Tom Andry claims that the Songtower is not technically a transmission line design:

"The QWT in the SongTower QWT name stands for Quarter Wave Transmission. The design is claimed to be a quarter wave tube variant of a transmission line cabinet but it doesn’t seem apparent to me. Without getting too technical, a quarter wave transmission line speaker design is all about increasing bass response above and beyond what you might expect from a ported design through the use of an extended tube. If you open up a conventional transmission tube speaker, you'll see a labyrinth with bends and corners everywhere (and usually only one driver but that's beside the point). One of the functions of all these turns is as a low pass filter… wait, I'm getting technical again. Let's put it this way, regardless of twists and turns inside of the box, in order to be considered a transmission line, it needs to have an open end and a closed end. If you stretch this definition, you could argue that any enclosure with a length is a transmission line. It'd be hard to make but you could make it (and not a lot of people would agree with you). The fact is that the SongTowers were supposed to be based on Martin King's work. The presence of a flared port pretty much negates this and makes the SongTowers just a calculated bass reflex design (like most other speakers)."

I tried looking at Martin King's site, but it's a bit over my head. Does Tom have a point or is he misinterpreting transmission line design?

This is a cutaway of a "conventional" TL loudspeaker from the PMC website. The port is in the front. This might help to give a visual representation for those like me who are a bit challenged by the theory behind the design and it's variants.

Derek