Thanks,
bret
--- Eric Benjamin <> wrote:
> Bret <> wrote:
> Wahlstrom says "Furthermore, at 1000hz the sound wavelength is 34mm,
> <SNIP>
>=20
> Wahlstrom did write that, but please note that it is a typo. He
> meant to write "10000 Hz" not "1000 Hz"
>
>
> [Non-text portions of this message have been removed]
>
>
>
> ------------------------ Yahoo! Groups Sponsor
>
> "Microphones are not ears,
> Loudspeakers are not birds,
> A listening room is not nature."
> Klas Strandberg
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>
>=20
>
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>From Tue Mar 8 18:27:04 2005
Message: 14
Date: Wed, 25 Feb 2004 11:39:06 -0800 (PST)
From: Bret <>
Subject: Re: the nature of parabolic reflectors
Great comments, Eric.=20
Diffraction at the edge of the parabola is yet another problem not
addressed by any of the papers nor is there a simulation offered.=20
Regarding accuracy of the parabolic shape, I have read that for RF
purposes that you want accuracy to 1/10 wavelength.=20
Regarding dispersal over time, Backman shows this in impulse responses.
There can be a sort of pre-echo.
With all that could go wrong, it is amazing they work as well as they
do.
If you attached a chart, it did not make it through. Please post it at
yahoo groups if you can, or email it directly to me.=20
I hope you will expand on this.
thanks,
bret
--- Eric Benjamin <> wrote:
>
> I must start out by confessing that I've never used or built a
> parabolic microphone. In fact I've never even seen one. But all of
> this discussion has piqued my interest.
>
> In order to achieve the theoretical gain, the actual recording
> circumstances must result in the waves from various directions
> arriving at the focus with phase errors conforming precisely to the
> expectation of theory. Put another way, everything must work exactly
> right, or the gain will be less than predicted by theory. This can
> be seen in Wahlstrom's figures 12, 13, and 14. The measured gain is
> always below the theoretical gain with the exception of one data
> point in figure 14.
>
> If follows that Errors of the following types may occur in practice:
>
> Focal length error
> Non conformance to exact paraboloid
> Aiming error
> Violation of the plane-wave assumption
>
> Focal length error: If the microphone is not located precisely at the
> focal point, then it will be progressively further away from the
> point where the high-frequency maximum is, and the high-frequency
> output will be less than the theoretical optimum. This is shown
> quite clearly in Wahlstrom's figures 9, 10, and 11. One practical
> consideration is that the designer might like to use a tripod
> arrangement to support the microphone capsule from the rim of the
> dish, but for the case where =E1 =3D l this results in a non-rigid
> support system
>
> Non-conformance to an exact paraboloid results in waves from one
> direction arriving with positive or negative phase shifts relative to
> other directions of incidence. Variations of 1/2 wavelength, which
> is only about 8.6 mm at 20 kHz, will result in complete cancellation.
> Not only can there be errors due to manufacturing tolerance, but for
> flexible paraboloids it is extremely unrealistic to expect that they
> will snap back to exactly their original shape. Assuming that the
> errors are not systematic, the result is merely a failure for the
> gain to continue to increase at higher frequencies.
>
> A side implication is that a spherical reflector may work nearly as
> well in practice, and be easier to fabricate.
>
> Aiming error when measuring (or using) the microphone will result in
> a rolled-off frequency response. Because the polar pattern of a
> parabolic microphone becomes progressively narrower with increasing
> frequency,
>
> A violation of the plane wave assumption will occur if the source of
> the sound is not located at infinity. Again, in order to achieve the
> theoretical gain from the paraboloid reflector it is necessary that
> the planarity of the wavefront not be in error by more than about 1/4
> wavelength. Note that this effect increases with the size of the
> dish. A very large paraboloid is not appropriate for recording near
> sources. Assuming that the dish is 0.5 meters in diameter, and that
> the distance is 10 m.
>
> So for a dish of diameter 0.5 meters, 5 meters from the source, the
> "height" of the wavefront entering the dish is about 1.2 cm. This
> will result in cancellation for a source at approximately 14 kHz.
>
> In looking at Wahlstrom's analysis in his appendices, I see that the
> figures do not precisely portray the shape of the gain curve. Using
> Wahlstrom's analysis and the result in his Equation 14, I have
> calculated the theoretical gain curve for a parabolic microphone of
> dimensions the same as that of the Telinga microphone. The
> horizontal axis thus takes on the dimensions of Hz. The principle
> thing that can be seen that is not visible in the figures in
> Wahlstrom's paper is that the oscillating part of the sound field
> affects the response up to high frequencies. The implication of this
> is that the sound is dispersed in time, and indeed this would be
> expected given that there is both a direct sound and focused sound
> component to the microphone signal.
>
> If there is interest on the part of the group, I could expand upon
> this in more detail, and put the results into the files section of
> the Nature Recordist group pages.
>
> Eric
>
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