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the on-axis hole

Posted by Duke (D) on January 31, 2007 at 21:47:05

In Reply to: Re: overlooked topic posted by lecleach@musee.ensmp.fr on January 31, 2007 at 04:32:20:

Bonjour Jean-Michel,

Thanks for the additional information and links.

The high frequency on-axis hole in John Sheerin's simulation is something that I have seen before, but depicted in a different format.

Earl Geddes told me that a round horn or waveguide will always have an on-axis hole in the frequency response. He said it has to do with an axi-symmetric reflection, and is not there in rectangular or elliptical devices.

You can see this sort of on-axis hole by clicking on the link below and scrolling down to page 11. This is a speaker that uses an oblate spheroid waveguide, which is an axi-symmetric device. There you will see a family of off-axis curves, which gives a somewhat less complete, but easier-for-the-layman-to-understand, presentation of the information contained in a polar map.

The curves are taken at 7.5 degree intervals. Notice that the on-axis curve dips below the first few off-axis curves in the range between about 4.5kHz and 9 kHz. I think this is the same phenomenon shown in John Sheerin's simulation, which doesn't go high enough in frequency to show that the on-axis hole closes back up at higher frequencies.

I also have measurements taken of a much smaller round waveguide than the one in Earl's paper, and it shows a similar on-axis hole in the response but at a higher frequency.

Anyway, I prefer to listen to round horns and waveguides from slightly off-axis, as the response is usually smoother there than directly on-axis. Again John Sheerin's simulation doesn't go up high enough to fully justify that conclusion regarding the LeCleac'h horn, but it looks like the trend is there.

Best regards,

Duke

• scroll down to page 11   (Open in New Window)

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