PMC TL or QWT?

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 :clown:. 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
Crafty Loudspeakers
 
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:

[IMGDEAD]http://img219.imageshack.us/img219/5605/screenshot265ma8.th.jpg[/IMGDEAD]

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
 
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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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.
 
planet10 said:
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.
 
I can imagine how you feel Martin. However, in fairness to PMC, they are a relatively big & well-known company as far as specialist hifi speaker manufacturers go, & have been around a while. IIRC, they were started by ex BBC employees in the early 1990s & they've tended to stick with TL type cabinets throughout. Think a sort of TL version of Harbeth & you'd be about there -their speakers have generally been well recieved by the hifi press & studios alike, & they should have sufficient funding / profits to do a decent amount of their own research. I've certainly no evidence that they are directly using your sheets for commercial designs, but I have heard that they know of your (& George Augspurger's) work on the subject. It'd be surprising if they hadn't, so I suspect at the least, it'll have had some influence on them in recent years. Whether it's direct or indirect though is something I suspect we'll never know. FWIW, I reckon the latter is the more likely option in this case.
 

rjbond3rd

Member
2007-01-24 2:28 pm
MJK said:


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.

May I make a suggestion, as a software person? There is no shortcut -- you have to appoint someone to act as your representative, who will contact the intellectual property people at various speaker companies and offer them licensing.

This is a normal, everyday thing. It's just compliance vs. non-compliance. It's almost certainly not outright dishonesty so much as laziness and sloppiness. Legal departments (or lawyers on retainer) love to clean up those things -- that's how they justify their existence.

If they reject licensing, leave your options open without stating them. Just one programmer's opinion -- IANAL.

Edit -- by the way, I'm not saying you shouldn't be steamed! An organization stole my software while lying right to my face, and I literally could not sleep until I had shut them down. So I relate.
 

legris00

Member
2008-01-23 5:38 pm
Nice thread to read guys.
Had a look at the link about the ATL system but they did not show a picture of the ALM1's incorporation...

We had it on loan and it truly is a great sounding monitor. Some people didn't like it because of the group delay on the bass but I thought it was really nice and full compared to some other units.
Any thoughts?
 
Hmmm.

Just looked up Arthur Bailey's acoustic transmission line Wireless World text.

No reference to B Olney, nor to Mr D Barlow's prior measurements given in a lecture at the then Brit.I.R.E on 24-01-62.

Appararently H J Leak and J Bolinbroke also examined 'line' characteristics but not known if published

Cheers .......... Graham.