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Re: Agreements and disagreements

Posted by EGeddes (P) on January 26, 2006 at 08:55:03

In Reply to: Re: Agreements and disagreements posted by tomservo on January 25, 2006 at 08:18:01:

Tom

"I would describe HOM’s as sort of a “edge radiation” effect within the horn, which is caused when there is a transition in angle where the acoustic size is already beginning to exhibit directivity. When the acoustic dimension is small such a change in angle should be unable to produce HOM’s. A first guess would suggest that a 2 inch exit could not produce HOM’s below about 2200Hz regardless of horn wall angle. Is that about right?"

This is not exactly correct. Diffaction occurs whenever a boundary changes shape. The amount of diffraction is proportional to the second derivative of the boundary curve in the direction of wave propagation divided by the wavelength. So at LF there is little diffraction no matter what the shape is - basically the wall shape is irrelavent and all that matters in cross section rate change, i.e. Websters EQ. In this region the ratio of mouth area to throat area is almost all that matters.

BUT, at higher frequencies any change in the wall shape will cause diffraction. The minimum diffraction would then occur for a shape change that had a second derivative that was constant (or vanishes like in a conical horn, but is not possible when the throat slope has to change to the mouth slope). This equation - a catenoid - is exactly the OS coordinate equation. Which is why I can say that no waveguide shape can have less HOM (diffraction) than the OS one - not exponential not Tratrix, not anything.

But this assumes that the throat is feed by a wavefront of constant magnitude and phase. Anything else will propagate HOM directly from the throat, i.e. the driver. Modern phase plugs are design to produce a constant phase at the exit aperature BUT NOT a constant amplitude - Bob Smith's approach definately deliberately creates a non-constant amplitude at the exit of the driver. Hence the driver itself produces HOM's directly at the interface aperture even though it is small. The amount to which these HOM propagate down the waveguide is acoustic dimension dependent and it is not until about 3 kHz in a 2" throat device that the HOM waves reach the mouth - unless the waveguide is short, then it can happen lower.

The presence of HOM created by the driver is the reason that I created the Refractive Phase foam plug to absorb these HOMs that were created by the driver. It works quite effectively and all indications of a compression diver on a horn are gone.

The CLF data is not very useful except within a room modeler. Thats because you cannot see the direcxtivity versus frequency in thier viewer and the format is proprietary so there is no way to extract the data and plot the polar map. But you should have the raw data and I can make a polar map from that. Looking at single frequency ballons is kind of a waste of time.

Earl Geddes

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