Phase Plug Question

[jonpike]

OK, it's a pretty basic, simple question... but I haven't found much out there addressing it.

When you calculate the area of a driver's cone, and it has a phase plug, do you remove the area of the phase plug?

I would think the answer would be yes, but on drivers I've been working on lately, I notice the factory cone area works out about the same as what I'd calculate from measuring the cone diameter. I.E. it comes out as the area WITHOUT removing the phase plug area.

[jsr]

Re: Phase Plug Question

You're supposed to subtract the phase plug but perhaps the mfr is using a slightly different radius than you're assuming they're using. The area of the plug isn't huge so perhaps it falls within the error from differences in radius/diameter.

[billfitzmaurice]

Re: Phase Plug Question

The area of the plug isn't huge so perhaps it falls within the error from differences in radius/diameter.

I'd be more inclined to think it's sloppy calculation by the manufacturer. After, they do call it a phase plug, and it isn't. :rolleyes:

It's a polepiece extension. ;)

It would be interesting to see what WinISD says Sd is if you put in Mms and let it calculate Sd.

[thekorvers]

Re: Phase Plug Question

OK, it's a pretty basic, simple question... but I haven't found much out there addressing it.

When you calculate the area of a driver's cone, and it has a phase plug, do you remove the area of the phase plug?

I would think the answer would be yes, but on drivers I've been working on lately, I notice the factory cone area works out about the same as what I'd calculate from measuring the cone diameter. I.E. it comes out as the area WITHOUT removing the phase plug area.

You subtract the phase plug area.

How do you measure the cone diameter? Most people measure from halfway surround to halfway surround as the surround also moves, but not to the degree that the cone does.

[dcibel]

Re: Phase Plug Question

The drivers I've looked at with polepiece extensions have accurate Sd. This would be from Peerless, Seas, ScanSpeak. Unless the voice coil is abnormally large, or the driver is abnormally small, there's not much difference in radiating surface.

[Jeff B.]

Re: Phase Plug Question

I'd be more inclined to think it's sloppy calculation by the manufacturer. After, they do call it a phase plug, and it isn't. :rolleyes:

It's a polepiece extension. ;)

It would be interesting to see what WinISD says Sd is if you put in Mms and let it calculate Sd.

It can indeed function as a "phase plug" if is is shaped correctly. It seems that very few of them are though. I had a driver designer explain the concept, shape, and how it worked to me a few years ago, so I look at drivers that have them a bit differently now.

[Pallas]

Phase Plug Question

It can indeed function as a "phase plug" if is is shaped correctly. ***

Andrew Jones thoroughly debunked that myth with data on diyaudio.com.

To work as a phase plug, a structure needs to cover a substantial area in front of the diaphragm.

[Jeff B.]

Re: Phase Plug Question

Andrew Jones thoroughly debunked that myth with data on diyaudio.com.

To work as a phase plug, a structure needs to cover a substantial area in front of the diaphragm.

Well I can't really respond to the ambiguity of that statement, but I can say that I have personally seen the effects of what this designer was describing by seeing the response of the same driver with a conventional dust cap versus the driver with his "correct" phase plug. The conventional driver had peaks and nulls in the response in its upper range and the one with the plug had very flat extended response. His explanation of how it worked seemed correct from my perspective based on my understanding of drivers and sound propagation. It was, in fact, a driver that is well known and very highly regarded.

[Pallas]

Re: Phase Plug Question

Well I can't really respond to the ambiguity of that statement,

You can flesh out my assertion by reading here.

I think one can assume Andrew Jones knows how to design a "correct" phase plug.

but I can say that I have personally seen the effects of what this designer was describing by seeing the response of the same driver with a conventional dust cap versus the driver with his "correct" phase plug. The conventional driver had peaks and nulls in the response in its upper range and the one with the plug had very flat extended response.

Nobody's saying that different shapes of polepiece extensions can't impact FR. Often, I'd expect a properly designed dustcap to offer better performance, because it can help damp cone resonances in a way that a stiff ring around a plug cannot. But the differences have nothing to do with the fact that pointy polepiece extensions don't act as phase plugs.

A phase plug for a midrange looks more like this:



(Found that searching "phase plug" "AndrewJ" on DIYA)

or this PE product:

[Jeff B.]

Re: Phase Plug Question

That actually accomplishes the same thing as the extended pole piece does, both of them extend the response by blocking interactions between the sound from the center of the cone and sound from the edge of cone from cancelling due to phase differences. Since both provide a similar function it can be appropriate to call both phase plugs or phase shields (as the covers are usually called). But it is just a matter of semantics, the important part is what it does for the driver.

[billfitzmaurice]

Re: Phase Plug Question

Well I can't really respond to the ambiguity of that statement, but I can say that I have personally seen the effects of what this designer was describing by seeing the response of the same driver with a conventional dust cap versus the driver with his "correct" phase plug. The conventional driver had peaks and nulls in the response in its upper range and the one with the plug had very flat extended response. His explanation of how it worked seemed correct from my perspective based on my understanding of drivers and sound propagation. It was, in fact, a driver that is well known and very highly regarded.

It worked because there was no dome. As Pallas noted the plug extension is too small in diameter to do much, if anything, and what it might do isn't what a phase plug does. At some point someone noted the similarity in the shape of a pole piece extension to the dome of a phase plug and started calling it a phase plug, and it stuck.

[Deward Hastings]

Re: Phase Plug Question

But it is just a matter of semantics,

But, but, but . . . if not for "semantics" we'd have to find something serious to argue about . . .

[killa]

Re: Phase Plug Question

Pallas, I would think the pics you posted would be better described as phase shields. Of course the idea is the same.

[billfitzmaurice]

Re: Phase Plug Question

But, but, but . . . if not for "semantics" . .

A phase plug works by forcing the wave from some sections of the diaphragm to take a longer route than others. A pole piece extension doesn't do that. That's not 'semantics'. How phase plugs work is well explained here:

http://www.centauriaudio.com.au/diy/plugs.html

[jonpike]

Re: Phase Plug Question

Wow... wasn't expecting to start a debate, but ah well...

Overall, it doesn't sound like it's likely that (for the scale of a large woofer) the phase plug/pointy pole piece compensates for it's "dead air" area.

I'm going to have to go back and remeasure and recalculate, to see how much difference in effective diameter the pole piece area would subtract. As well as see how much delta it creates in the whole set of T/S parameters, or in back calculating for Mms, and other particular parameters. As I recall, that was another thing that seemed to match better, by using the whole area from diameter, the Mms was closer to the factory value than without the pole piece removed, in my measurements.

Of course, that raises questions of how much variance there would be on the factory Mms, and how accurately did I do my measurements for other T/S parameters.

I'll go thru that again, and see how much difference it makes, for various areas, how much diameter difference the pole piece subtraction makes, what T/S differences those make, etc.