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Old 19th October 2015, 11:47 PM   #1
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Default PMC TL Stuffing

Hey all,

It's been a while since I've been on here! I need some help from someone better versed in the TL design than me.

I bought a pair of PMC TB1s super cheap on eBay to add to my studio. I've been struggling with a big 50hz null, which I thought was the room, but after measuring closer to the speaker it looks like it might be the stuffing in the TL They had some black foam covering the rear ports, which was starting to crumble, and completely fell out when I pushed it.

I've experimented with an RTA mic 6cm from the center of the woofer. I took 3 measurements, one without the foam, one with the original foam (which did very little in it's current form) and one with a rolled up tea towel stuffed in the port, quite loosely but enough that it's fairly sealed.

Click the image to open in full size.

They sound much better stuffed, although I feel like they've lost some "life". Could anyone advise me on a better way of going about this? What materials should I look at? Is this port supposed to be fairly open, in which case is this a driver issue? I noticed that my LEAK 3090 TLs only have a bit of speaker grill across the ports, although they are front firing.

Many thanks,
Blindmelon6
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Old 20th October 2015, 02:17 AM   #2
Dissi is offline Dissi  Switzerland
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Have a look at the PMC ATL: What's this ATL all about then? - Carlton Audio Visual - Hi Fi & Audio Visual Store, Melbourne Australia. There's probably much more foam in your speaker than what you see in the port.

The null at 50 Hz is not really an issue, the other side of the coin is the port, which reaches its maximum output at the same frequency. Measure it in the same way as the driver.
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Old 20th October 2015, 05:35 AM   #3
keilau is offline keilau  United States
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Quote:
Originally Posted by Dissi View Post
Have a look at the PMC ATL: What's this ATL all about then? - Carlton Audio Visual - Hi Fi & Audio Visual Store, Melbourne Australia. There's probably much more foam in your speaker than what you see in the port.

The null at 50 Hz is not really an issue, the other side of the coin is the port, which reaches its maximum output at the same frequency. Measure it in the same way as the driver.
In this PMC video that they tried to explain what ATL is, they mentioned IMF and TDL who were the pioneers of the transmission line and aperiodic cabinet. The PMC person mentioned that the sound wave coming out of the port is "exactly in phase" with the front woofer wave. That means the equivalent wavelength of the internal acoustic tunnel is at the half wavelength of the woofer resonance. That's how the original Bradley transmission line speaker was designed. IMF and TDL substituted the long fiber wool with open cell foam and achieve the same phase inverting effect. If PMC achieve what they claim, it should not have the 50Hz null.

The foam at the port indicated that the PMC TB1 is a hybrid TL/aperiodic design. You may need to replace the foam inside the cabinet too. You can find open cell, medium density foam at most fibre and sewing supply store. It is best to contact PMC about the foam replacement. PMC said that their foam was specially manufactured by themself.

The output from the port of a quarter wavelength line is not in phase with the woofer front wave, thus, acting like a bass reflex box instead.

Last edited by keilau; 20th October 2015 at 05:41 AM.
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Old 20th October 2015, 05:55 AM   #4
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Despite what keilau claims Bailey's line, IMFs & TDLs are all 1/4 wave resonators. A 1/2 wave line has to be either open or closed at both ends.

According to PMC it is a 71 Hz line.

Click the image to open in full size.

How are you measuring? Unless you are outside and something over 20 ft up, face up buried in the ground, in an anechoic chamber, or doing a measure with the micon the woofer you cannot get the room out of that measure.

If you are doing the last there will be a null in the woofer at the primary output of the terminus. You'd have to measure that separately and add the 2 together (taking care of all the adjustments you need to make to add them together).

Foam does not make for great stuffing.

dave
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Old 20th October 2015, 08:51 AM   #5
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Quote:
Originally Posted by planet10 View Post
Despite what keilau claims Bailey's line, IMFs & TDLs are all 1/4 wave resonators. A 1/2 wave line has to be either open or closed at both ends.

According to PMC it is a 71 Hz line.

Click the image to open in full size.

How are you measuring? Unless you are outside and something over 20 ft up, face up buried in the ground, in an anechoic chamber, or doing a measure with the micon the woofer you cannot get the room out of that measure.

If you are doing the last there will be a null in the woofer at the primary output of the terminus. You'd have to measure that separately and add the 2 together (taking care of all the adjustments you need to make to add them together).

Foam does not make for great stuffing.

dave
I'm measuring on well isolated stands in a fairly well treated room. Bottom end is a little lumpy on my other speakers, but the main dip is more in the 40hz region.

I'd be inclined to agree with you were it not for the fact that the frequency response is also improved at the listening position, and shows no negative effects in a waterfall plot. I'll post some more measurements this evening.

In the mean time in thinking it might be with trying to contact PMC (why oh why didn't I just shell out for a new pair )
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Old 20th October 2015, 01:30 PM   #6
keilau is offline keilau  United States
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Quote:
Originally Posted by planet10 View Post
Despite what keilau claims Bailey's line, IMFs & TDLs are all 1/4 wave resonators. A 1/2 wave line has to be either open or closed at both ends.

According to PMC it is a 71 Hz line.

How are you measuring? Unless you are outside and something over 20 ft up, face up buried in the ground, in an anechoic chamber, or doing a measure with the micon the woofer you cannot get the room out of that measure.

If you are doing the last there will be a null in the woofer at the primary output of the terminus. You'd have to measure that separately and add the 2 together (taking care of all the adjustments you need to make to add them together).

Foam does not make for great stuffing.

dave
Dave, can you provide a link to the PMC "71Hz line" claim? It is not fair for you to make that statement without substantiating it.

You can deny all the wonderful test and analysis that Bailey and Bradbury did. But their mathematics convinced me that a transmission line has to be 1/2 wave line to work the way they claimed and measured. You can deny science, but it does not make science not true. Every college physics student knows that sound pressure adds when in phase and subtracts when out of phase. 90 degrees out of phase (1/4 wave) is somewhere in between.

If you want to claim that the IMF and TDL speakers do not work the way the manufacturers claimed and open cell foam is a poor stuffing material, you need to provide your own measurement with details on the type of foam that you used and how you measure.

IMF electronics

I heard Bud Fried demonstrated his product many, many years ago and was convinced that his lines work the same way as Bailey's. Bud Fried can longer argue with you in person.

I do know that polyester or glass fiber do not work well in transmission line as demonstrated by Bailey and Bradbury. Polyester or glass fiber do not slow down sound wave at low frequency and attenuate sound wave at high frequency the same way as long fiber wool.

Last edited by keilau; 20th October 2015 at 01:36 PM.
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Old 20th October 2015, 01:36 PM   #7
Boden is offline Boden  Netherlands
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Dear Keilau,

The impedance curves of the IMF and TDL speakers, with their double peaks clearly show the system is resonant in its nature and therefore does not work as their designer seem to claim, their claim being it is basically a non-resonant loudspeaker system.

Wasn't that the title of Bailey's original WW article?

Regards,

Eelco
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Old 20th October 2015, 02:10 PM   #8
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Would you agree that there is something very wrong with the response of these speakers, or am I expecting too much from them?

I plan on trying a couple of things. Firstly a measurement in a different room, then a measurement in as near to anechoic conditions as I can manage (I have 12 x 200mm Rockwool panels to play with.)
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Old 20th October 2015, 02:15 PM   #9
Boden is offline Boden  Netherlands
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Is the straight, non dipped line the "towel-in-the-exit"? I have trouble decyphering your graphs, coz the colors look very alike
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Old 20th October 2015, 02:23 PM   #10
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Quote:
Originally Posted by Boden View Post
Is the straight, non dipped line the "towel-in-the-exit"? I have trouble decyphering your graphs, coz the colors look very alike
Yeah, the straight one is the towel, the dip is with the old foam.

I'll post the in room response as well, which was also greatly improved.
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