Phase plug necessity in horns/waveguides

[454Casull]

Is a phase plug required to minimize/eliminate cancellation if the throat size of the horn/waveguide is the same as the diameter of the diaphragm of the [compression] tweeter?

Also,

Is there a function with which one can determine the necessity of a phase plug whose variables are diaphragm size, throat size, and frequency? i.e. a 2" compression driver with a 1" throat will require a phase plug at x kHz.

Let's say necessity is defined as when cancellation causes over 2-3 dB drops on an otherwise perfectly flat frequency response.

[John Sheerin]

In one specific case I've been playing with, a center exit from the compression chamber gives a nocth around c/d where d is the outer diameter of the compression chamber (this is all round, axisymmetric). In my case I'm 3dB down a bit above c/d/2, though. How you tune your front chamber will also effect this, as will the breakup of the diaphragm, and maybe your compression ratio (I'm using a high compression ratio). In my example, I pushed the notch higher by using a single slot phase plug, and even higher to ~2*c/d by going to a 2 slot phase plug. Beyond that, you are probably into diminishing returns, as the diaphragm will be breaking up unless you have an exotic material.

[454Casull]

In the case of a direct-radiating tweeter with a horn, there is no compression chamber, correct? (You could use one... but I don't see why)

[Audiophilenoob]

Originally posted by 454Casull
In the case of a direct-radiating tweeter with a horn, there is no compression chamber, correct? (You could use one... but I don't see why)

yes that's correct