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Old 7th April 2008, 06:10 PM   #1
Colin is offline Colin  United Kingdom
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Default PMC TL or QWT?

Browsing the PMC technology page I was interested in the layout of some of their TLs. Most of them have the driver and/or the exit port offset from the start and/or end of the line. I guess this is to suppress resonances pretty much like the MLTLs on this forum but without the larger mass-loaded port.
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Old 7th April 2008, 07:32 PM   #2
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Hi,

You mean these layouts:
PMC

They use a combination of placing the driver on 1/3 line length and placing the line end at 1/3 line length in some speakers. The smaller speakers use placement at the beginning or at 1/5 line length. I asume this is done to minimize the TL gap and unwanted resonances in the line. The relative (compared to the oldskool 340/ 4 line calculation method) short lines helps to move the TL resonances up in frequency where they can be taken care of more easily with damping material.

I use a 1/3 driver placement in one of my TL designs, this works very well to close the TL gap and to minimize resonances. Do note, the exact 1/3 placement is very sensitive to the amount of tappering in the line. The exact 1/3 place is very critical, change the tappering and the driver "sweetspot" wil move with it. I use AJ Horn to calculate these 1/3 TL's.

I did not yet come up with a nice sounding three letter trademarked word for the line i use in my Azumi tl mini monitor speaker . The small enclosure in the bottom is a small volume coupled to the line with a helmholtz resonator to take care of a small peak in the bass response. The volume of this enclosure effects line length a little, but the line only "sees" it at a small frequency band. So please do not categorize my design under mixture of bassreflex and tl. Bernd Timmermans from the German Diy magazine Hobby Hifi invented this Internal Helmholtz method to take care of small resonances.

I never tried to put the end of the TL on 1/3 line lenght, may work very well. They use a lot of damping material in their lines. I try to use the minimum, just enough to keep the midrange clean. The rest is handled by the dimensions of the line. Using minimal damping makes the speaker more agile at lower levels.

Regards
Roland
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Old 7th April 2008, 08:53 PM   #3
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'Advanced Transmission Line.' You can get away with anything in advertising nowadays can't you?

Basically, PMC's ATLs are Olney Acoustic Labyrinths (1937) with the well-known ROT 1/5 or 1/3 driver offset & occasionally an offset vent.
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Old 7th April 2008, 11:08 PM   #4
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Thanks,

I did not know the work by Olney. I have found this picture of an Olson speaker in an AES paper from Philips about Acoustic Labyrinths:

Click the image to open in full size.

Here's the paper on Olney and TL design the picture came from:
Philips research paper

I have not red the paper yet. But i may be off interest for TL fans. I found out the Olson patent is from 1936 and not 1937 so indeed it's a really old invention, these advanced lines are based upon.

Regards
Roland
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Old 7th April 2008, 11:50 PM   #5
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Here's PMC's ATL (Advanced Transmission Line) layout for comparison with the Olson:

Click the image to open in full size.
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Old 8th April 2008, 05:44 AM   #6
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Using MJK's worksheets you will find that the placement of the driver offset is quite sensitive & does change with taper. The proper placement kills the 1st unwanted TL resonance. The extract from MJK's tables attached below shows the driver offset ratio for various tapers.

There is really nothing advanced with the PMC lines... Martin's modeler has allowed diyers to make much more advanced TLs (rumour has it that the MJK sheets are not unknown inside PMC)

dave
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Old 8th April 2008, 08:08 AM   #7
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Yep. I've heard the same.

Re Olney, he was working at Stromberg-Carlson in the 1930s; the Acoustic Labyrinth was for (yep) a QW pipe, lined with damping material. Bailey basically would later take the idea, change the lining to stuffing & call it, with somewhat dubious accuracy, a Transmission Line. Olney's 'Electronics' article appeared in April 1937, & presented a much refined / modified & usable variation on the basic idea Olson suggested in his patent of the previous year. Good article too -full of measurements.
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Old 8th April 2008, 09:27 AM   #8
Colin is offline Colin  United Kingdom
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Interesting stuff. I knew you lot would be able to shed the necessary light. I look forward to reading the Olney paper.

What advantages or disadvantages do these have over the less-complex MLTLs?
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Old 8th April 2008, 12:03 PM   #9
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It depends what you want. Generally, a TL is good if you want a highly damped line / cabinet performance, & you're wanting the flattest possible impedance curve, while a highly resonant MLTL (more accurately termed an MLQWR), is useful for a lot of FR units, which need all the help they can get in the LF
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Old 8th April 2008, 12:49 PM   #10
MJK is offline MJK  United States
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Quote:
Originally posted by planet10
There is really nothing advanced with the PMC lines... Martin's modeler has allowed diyers to make much more advanced TLs (rumour has it that the MJK sheets are not unknown inside PMC)
That is now three commercial ventures that have come to light in the past couple of weeks using the worksheets under questionable circumstances. This is getting harder and harder to take. I must be a complete idiot to continue trusting people to do the right thing.
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