ISOBARIC: Clamshell vs Push Pull Slot Loaded - diyAudio
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Old 22nd October 2009, 05:27 PM   #1
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Default ISOBARIC: Clamshell vs Push Pull Slot Loaded

I have confused my self regarding the T/S parameters for Isobaric loading. As I understand things ....

In one version, Clamshell, the woofers are on a common baffle and mounted face to face with the drivers wired electrically out of phase (so the air space between the drivers is isobaric). As I understand it, in this case the Vas is reduced by approximately 1/2; Fs remains the same; Bl doubles; and Mms doubles. The port length would also double (e.g., in a 4th order bandpass). So far so good.

In another version, Push Pull Slot loaded (PPSL), the drivers are again wired electrically out of phase but the arranged "magnet-to-face" into a small volume that is open on one side (the slot). The "other side" would be the back volume of the cabinet. In this case there is a reduction in distortion (via cancellation) that would arise from any non-linearity between the cone moving inward vs outward.

My questions:
A. Are the T/S Parameters for the Clamshell and the PPSL configurations the same?
B. Does the reduction in distortion for the PPSL configuration also hold true for the Clamshell configuration?

My ultimate goal is a clamshell configuration in a sonotube cabinet or a PPSL in to a Tapped Horn design. But first I need to get straight on the T/S parameters.
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Old 22nd October 2009, 06:39 PM   #2
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The magnet-to-face configuartion requires drivers be wired in phase or you will get no bass (it is possible to create a complex duct system to allow magnet-to-magnet out-of-phase wiring but probably not practical)

The slotted chamber version is a more complex build and will not have as tight a coupling as the face-to-face but also means you aren't staring at the back of the driver.

T/S remain the same.

As far as ports, just model the box with a driver of 1/2 the Vas.

dave
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Old 22nd October 2009, 06:55 PM   #3
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Dave, thanks for your response.

Regarding the issue of coupling in the PPSL configuration, I assume the coupling becomes "less complete" as you go down in frequency. I also assume the degree to which there is incomplete coupling means that the Vas is no longer 1/2, but something less. Do my assumptions seem reasonable?
Thanks
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Old 22nd October 2009, 07:25 PM   #4
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The compressibility of the air volume means less complete coupling. Also the larger sizes involved mean that (ignoring complications from listening to the back of the driver) the PPSL will not go as high in frequency.

As to the Vas, differences are probably smaller than driver-to-driver variances and might as well be ignored.

dave
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Old 23rd October 2009, 05:52 PM   #5
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Wait wait wait...

This "slot" area in-between front to back speakers is whats coupled to the outside.
So yes, wiring out of phase here is absolutely the correct way! They are in parallel,
not isobaric in PPSL application. You won't get an isobaric halving of VAS effect. But
also won't suffer a -3dB decreased efficiency, you gain +3db! PPSL will cancel some
2nd order LF distortions, slightly increasing 3rd order distortions in the same process.

If you are loading only front of the front cone, and not the slot? Then like a clamshell,
that configuration is still an isobaric. With all the "benefits?" Isobaric configurations do
allow for a smaller box, but efficiency is rather poor. And with front clammed to back,
there's no chance of cancelling 2nd order distortion.

So, which box are we describing? The one where the slot is loaded to the outside?
Or the one where the front of the front cone is loaded to the outside???
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Old 23rd October 2009, 06:44 PM   #6
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I haven't been able to find my nice drawing... here are the 3 "normal" isobarik configurations, i assummed the slot variation mentioned was the 3rd one

dave
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File Type: jpg isob1.jpg (15.3 KB, 700 views)
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Old 23rd October 2009, 08:36 PM   #7
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Quote:
Originally Posted by kenpeter View Post
Wait wait wait...

This "slot" area in-between front to back speakers is whats coupled to the outside.
So yes, wiring out of phase here is absolutely the correct way! They are in parallel,
not isobaric in PPSL application. You won't get an isobaric halving of VAS effect. But
also won't suffer a -3dB decreased efficiency, you gain +3db! PPSL will cancel some
2nd order LF distortions, slightly increasing 3rd order distortions in the same process.

If you are loading only front of the front cone, and not the slot? Then like a clamshell,
that configuration is still an isobaric. With all the "benefits?" Isobaric configurations do
allow for a smaller box, but efficiency is rather poor. And with front clammed to back,
there's no chance of cancelling 2nd order distortion.

So, which box are we describing? The one where the slot is loaded to the outside?
Or the one where the front of the front cone is loaded to the outside???
Kenpeter, I understand the confusion created by not having a diagram.
Yes, you and I are talking about the same thing when I use the phrase , Push Pull Slot Loaded (PPSL). There is no isobaric volume. The back of driver and the face of the other are mechanically in phase and electrically out of phase. They both into a common chamber that is open to the room via a slot. The other side of each driver fire into a common "back vloume". Isobaric is probably a misnomer here (it used by others also). It is really two drivers in a common box (with one of them mounted "backwards").

The diagrams that Dave presented do not capture the PPSL configuration. There are some posts by DJK, with diagrams of the "PPSL". However, I can't find them right now.
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Old 23rd October 2009, 09:27 PM   #8
taloyd is offline taloyd  United States
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Default Ppsl

Here are a top-view and front-view of DJK's PPSL.
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Old 24th October 2009, 01:53 AM   #9
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Indeed the PPSL has no relation to an isobarik, and the title is very misleading.

The reason to use isobarik (and live with its downsides) is to get smaller volume. Isobarik (clamshell) will have !1/4 the volume of the PPSL, better cancelation of 2nd order non-linearities, and lower efficiency at higher cost.

dave
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Old 24th October 2009, 02:05 AM   #10
tinitus is offline tinitus  Europe
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This probably dont make much sense
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Last edited by tinitus; 24th October 2009 at 02:08 AM.
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