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Old 23rd June 2004, 10:00 PM   #1
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Default phase plugs

Is there any info on the net about the theory behind phase plugs. I googled, and I searched here, but I'm coming up empty. I would like to know what the things actually do.
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Old 23rd June 2004, 10:08 PM   #2
RHosch is offline RHosch  United States
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Dissipate heat. At least, IMO, that's the most important good thing they do.
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Old 23rd June 2004, 10:17 PM   #3
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Are their any articles or papers on the net about phase plugs?
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Old 23rd June 2004, 10:26 PM   #4
Ron E is offline Ron E  United States
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No articles on the web that I know of. You cna use a search engine, right

They can help in heat dissipation, partially due to conduction if the phase plug is metal, but largely due to the open area around the phase plug that acts as a leakage path.

It also prevents problems with resonances between the dustcap and pole piece which can reduce the amplitude of the breakup peak before eventual HF rolloff.
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Old 23rd June 2004, 10:37 PM   #5
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does this help?
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File Type: jpeg phaseplug.jpeg (17.9 KB, 412 views)
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Old 23rd June 2004, 11:14 PM   #6
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Hi.

You could try to download three ZIP- files on this page= http://baseportal.de/cgi-bin/basepor...audio/download

They contains parts of Earl Geddes' "Acoustik Waveguide Theory"...

The "Phase Plug Theory" was originally invented by Bob Smith, but Geddes' articles are better, because he revised some of the theories, where Smith made mistakes.

Greetings, Benny
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Old 23rd June 2004, 11:19 PM   #7
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Default Re: phase plugs

Quote:
Originally posted by Dave Jones
Is there any info on the net about the theory behind phase plugs. I googled, and I searched here, but I'm coming up empty. I would like to know what the things actually do.
You'd be best to ignore some of the silly marketing material that has been repeated in this thread.

It's not really what the "phase plugs" do, so much as what they don't do.

If you don't have a "phase plug", then you have a dust cap. The dust cap is typically a plastic or paper dome. It will exhibit a major fundamental resonance at some frequency, typically a few kHz or so. This is chosen so that the boost from this resonance fills in the falling response from the main cone. So really, a dust cap acts more like a "whizzer cone" than anything.

If you don't need the (resonance induced) frequency extension of a dust cap, then a driver with a "phase plug" will exhibit an earlier, smoother HF rolloff. These drivers work better with lower XO points.
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Old 23rd June 2004, 11:26 PM   #8
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Default Re: Re: phase plugs

Quote:
Originally posted by Charles Hansen


You'd be best to ignore some of the silly marketing material that has been repeated in this thread.

It's not really what the "phase plugs" do, so much as what they don't do.

If you don't have a "phase plug", then you have a dust cap. The dust cap is typically a plastic or paper dome. It will exhibit a major fundamental resonance at some frequency, typically a few kHz or so. This is chosen so that the boost from this resonance fills in the falling response from the main cone. So really, a dust cap acts more like a "whizzer cone" than anything.

If you don't need the (resonance induced) frequency extension of a dust cap, then a driver with a "phase plug" will exhibit an earlier, smoother HF rolloff. These drivers work better with lower XO points.
I'd also like to add that there is NO distortion due to the trapped air under the dust cap (the phase plug replaces it the dust cap).

However, the effective piston area is lowered because the phase plug is static. Usually this is negligible unless the coil former (and hence, the pole piece) is huge.
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Old 23rd June 2004, 11:35 PM   #9
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Yes, Charles, you're right.

But another reason for designing speakers with phase- plugs is, that the acoustic waves aren't producing turbulences, whith make the sound of a speaker a little bit more "elegant".

Those turbulences, whitch typically raise right above the cone (as you can see on Timn8tor's pic) are making the sound quality of small speakers sound "squeezed"...

Greetings, B.
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Old 23rd June 2004, 11:42 PM   #10
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Charles is correct as far as he goes, but didn't mention the main usage of phase plugs, which is in horn loaded drivers. In fact, this is where they originated.

In a horn the throat is typically smaller than the diaphraghm that it loads, and segments of the wave created by the diaphraghm at its edges have to travel a longer distance to the throat than those that originate at the center of the diaphraghm. Thus they arrive at the throat out of phase with the other wave segments. If they meet the other wave segments at or near 180 degrees out of phase cancellation of the wave occurs and high frequency response can be severely restricted.

A phase plug in the horn throat forces the wave segments originating at the diapraghm center to travel around it on their way to the throat, to arrive there at the same time, and thus in phase, with the segments originating at the diaphraghm edge. As a result high frequency response can be greatly extended.

Phase plugs in woofers that are not horn loaded are in fact extensions of the pole-piece and do not have the same purpose as those in horns. Their primary purpose is to eliminate the acoustic resonance that occurs behind a dust cap, and in some cases to also serve as additonal heat sinking for the magnet structure. They are referred to as phase plugs not so because of any phase correction capabilities but because they look similar to the phase plugs used in horn drivers.
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