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In Reply to: Re: the on-axis hole posted by Duke on February 1, 2007 at 03:58:31:
Hello Duke,IMHO: in the case of a waveguide filled by foam, the origin of such a hole of 3 DB cannot be a reflection of waves from the mouth.
Best regards from Paris,
Jean-Michel Le Cléac'h
Follow Ups:
In my opinion you are incorrect.No mouth can be made such that there is no diffraction and as such this diffraction will meet at the axis in anti-phase at some point. This causes a hole. It is not an HOM effect and is unaffected by the foam. A larger mouth radius makes it smaler and a larger mouth makes it go lower in frequency.
I have tested this all and it is quite correct.
I have also stated numerous times that no waveguide has NO HOM, only some have a lot less. The drivers create HOM even if the waveguide does not.
Duke is dead right in what he says. He has learned his lessons well.
Earl Geddes
Hello Jean-Michel,What do you believe causes the on-axis response hole?
Do you believe it has one cause in the case of your horn (as modelled by John Sheerin), and another cause in the case of Earl's waveguide?
Merci,
Duke,As I told you I don't see myself such a hole on my measurements some of them can be seen at:
http://www.musique-concrete.com/mesures/321direct.gif
due to the precision of the measurement, if this axial hole
exists it is far less than 0.5dB with my horn.I don't want to criticize John Sheerin simulations, there are very enlightning about differences between horn shapes. A limitation to those simulations is that they are only performed in 2D not 3 D (someone will surely say this has a minor influence in the case of an axial horn...)
You know that the profile of horns calculated with my method is curving back at the mouth. Diffraction is therefore reduced. This explains both the very pure wave shape on impulse reponse and the very smooth electric impedance curve.
Most waveguides are cut too sharply at the mouth and for sure diffraction will occur with more evidence.
Best regards from Paris
Jean-Michel Le Cléac'h
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