Drivers with flat diaphragms on waveguides?

[454Casull]

Is a phase plug necessary for a flat diaphragm firing into a waveguide?

Put another way, does a flat diaphragm have any phase cancellations (assuming no breakup - i.e. perfect pistonic motion) when firing into a waveguide?

Howard

EDIT: This is assuming a direct radiator which doesn't fire into a small throat like a compression driver.

[454Casull]

Come on, somebody has to know the answer.

[BillFitzmaurice]

If by waveguide you mean a horn with a 1:1 compression ratio the answer is generally no. Phase plugs are used to equalize the path lengths to the horn throat when a portion of the diaphragm is directly exposed to the throat and a portion is not. With 1:1 compression all the diaphragm is exposed to the throat so there are no differing pathway lengths to equalize. One may still use a plug if it's desireable to add some compression, as in a ring-radiating 'bullet' tweeter, but then you technically have made the waveguide into a horn.

[454Casull]

http://www.audioasylum.com/forums/hu...ges/81495.html

Here Dr. Geddes speaks about true plane waves not generating HOMs. How does one achieve plane wave production?

[brummkreisel]

Hi Howard,


yes, if you took a flat diaphragm with no break-up occuring, you would feed a perfectly plane wavefront into the waveguide. You could also use a non-planar diaphragm and construct a phase plug to correct for the path differences, a thing you even would have to do if the diaphragm geometry (e.g cone depth, dome height) was of the order of or larger than the wavelengths in question.

@ Bill, I stumbled over that one several times: where do you take it from that a Geddes waveguide is a compressionless (i.e. 1:1) horn? This is IMO not true, since of course a WG not only modifies the direction but also the magnitude of the velocity (simply due to flux conservation). A rectagular tube (the most basic waveguide) would indeed produce no compression, but a flaring contour certainly does so, doesn't it?


Cheers,
bk

[Mudge]

Surely any waveguide, no matter what the geometry, produces some kind of load on the driving diaphragm? This effect would depend on both frequency and on mouth size, I assume, since for the waveguide to operate, there must be some confinement of the pressure wave

[BillFitzmaurice]

I wasn't aware of a 'Geddes waveguide' as opposed to any other kind, but in my experience the term is generally applied to a horn style device where the throat is not smaller than the cone driving it. And yes, even this will have an acoustic load since there is an air column being driven, but the impedance will be quite low. I do admit that I haven't read much of Earl's work, though from what I have read we seem to be on different paths. He doesn't seem to care much for horns, especially bass horns. I live for them.

[454Casull]

I wonder why Tannoy uses a phase plug for the HF driver in its Dual Concentric configs.

http://www.cortex.westhost.com/tanno...s/dualconc.pdf - page 17