My guess is that the "globe" has the same diameter as the wavelength. Phase
problems may occur because of the big (too big) membrane. If the membrane is
smaller than any wavelength present, then no phase problems.
It's not a fact, it's a suggestion, based only on a bit of feeling for it.
Klas.
At 13:57 2004-02-26 -0800, you wrote:
>--- Walter Knapp <> wrote:
>> From: Bret <>
>>
>> > I may be wrong, but I think the size of the globe must be frequency
>> > dependent. I believe this is true especially if you have an ideal
>> > parabolic reflector.
>> >
>> > As you move a given distance from the absolute point of focus, this
>> > distance is a larger phase difference for shorter wavelengths than
>> it
>> > is for longer wavelengths. This phase difference at the off focus
>> > position is one mechanism that would give rise to interference or
>> > cancelation rather than the addition of pressure as occurs at the
>> > absolute focus (because the arrival at focus is in phase from all
>> dish
>> > reflecting points).
>> > bret
>>
>> In real parabolic reflectors it may be unwise to assume that the
>> focus
>> contains everything in phase. Or rely too heavily on phase
>> differences
>> to explain parabolics.
>
>Now you must joke. The fundamental principle of the parabolic
>reflector is that the waves entering the aperature parallel to the
>axis, travel the same distance to the focus, and thus arrive in phase,
>and thus add to provide gain. If they move at the same speed, across
>the same distance, they are in phase. If they are not in phase, how is
>gain achieved?
>
>> You also have the problem that the air molecules that move to carry
>> the
>> sound occupy space as well as having mass. They cannot all arrive at
>> a
>> point, which is what the focus would be in a perfect world. The
>> pressure
>> created would be way too high even before they got to a point. So,
>> there
>> is a pressure limit on the focus spot. Dispersal of molecular
>> movement
>> on out past the focus is going to be somewhat influenced by this
>> interference in the soundpath as well.
>
>As you discussed, the compression/rarefaction wave moves to the focus.
> As you noted, there is a fixed amount of particles, it is not wind,
>not a continuous stream of particles. The particles closest to the
>diaphragm move a bit to the diaphragm, and then move away, again and
>again at the frequencies of the compression/rarefaction wave.
>
>> My only point is that the 'focus globe' idea may be somewhat
>> misleading,
>> too simplistic. It's a concept those of us using parabolics have
>> used
>> for some time, but may not stand up to better measurement. The idea
>> was
>> that there was a zone where the sound amplitude was even that was
>> relatively large, I think better measurement will show this zone is
>> very
>> small, much smaller than was previously assumed. We will have to pay
>> more attention to diaphragms being exposed to varying sound pressures
>>
>> over their area and not rely on a 'focus globe' to avoid that.
>
>Maybe it is too simplistic. Aren't most models of reality more simple
>than the true state of affairs?
>
>If it is misleading, please explain the observed behavior in other
>terms that are not misleading, and not too simplistic.
>
>Surely in your experimenting you have moved the mic element through and
>beyond the focus of the dish along the axis and heard the affect of
>increasing higher frequency content as you center the element(s). I
>have, I recall Klas noted this affect also.
>
>If the globe does not get smaller with higher freqencies, how do you
>explain the drop in gain with a mic at the focus whose diaphragm is
>larger than the wavelength in Wahlstrom's paper?
>
>He states "at 10000 hz the sound wavelength is 34mm, and at this
>frequency the microphone membrane is not exposed to the high
>amplification over its entire surface."
>
>Not exposed over its entire surface for these higher frequencies, it is
>exposed over its entire surface for lower frequencies. That says to
>me the cross sectional area of the pressure wave hitting the diaphragm
>is smaller in diameter for smaller wavelengths. How else would you
>explain his observation?
>
>"This will affect all microphones that are large compared with the
>sound wavelength. The focal point of the reflector with a
>mathmatically correct surface represents a strange sound field at high
>frequencies."
>
>> And in real life that 'focus globe' will be less meaningful as the
>> soundpaths are not all on axis but quite mixed. Even for something
>> that's out there right along the axis line.
>>
>> Walt
>>
>
>The focus is the focus only for waves that are parallel to the axis,
>entering the front of the dish. Other paths of sound will not arrive
>at the focus with the controlled path length (such as the direct path
>to the mic, or other reflections from angular waves hitting the
>reflector), but they may still arrive at the mic diaphragm and
>contribute to the gain response.
>
>Maybe that is where you object to the notion of the focus globe, the
>'other influences' to the pressure wave at the focus caused by other
>paths of sound. Those other influences to the sound pressure at the
>mic membrane will be present whether the mic is at the center of the
>focus globe or not.
>
>In thinking of the focus globe, I was concerned with centering the mic
>element(s) for gain response that closest matches the ideal parabolic
>reflector (continuing 6db/octave gain). Of course moving the mic from
>the globe center can be used to 'tune' the response to achieve a change
>in gain response to get the 'sound' that you desire, as a tool.
>
>bret
>
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>Klas Strandberg
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>
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