Hi all,
Is anybody knows and has the email address of the
person who was looking for Great Horned Owl
vocalizations for her studies?
Thank you
Antonio Celis
--- Walter Knapp <> escribi=F3:
> Posted by: "Randolph S. Little"
>
> > Yes, think about it some more. The microphone is
> immersed in the
> > acoustic medium (air), such that a passing
> acoustic pressure wave
> > effectively wraps completely around it almost as
> if the microphone
> > were not there. One of the great challenges of
> microphone design is
> > to make that "almost" as complete as possible
> across the entire
> > spectrum of interest. This is especially
> difficult at wavelengths
> > short compared to microphone dimensions, and
> various types of side-
> > and rear-porting are typically used to flatten
> performance as a
> > function of direction and wavelength.
>
> Indeed, mic design is not a simple thing.
>
> Yes I more or less agree with your description here
> of a acoustic
> pressure wave interacting with a microphone. It is
> part of what I've
> been pointing out that argues against partial
> interaction of the wave
> and the diaphragm.
> >
> > This discussion pertains to pressure gradient
> microphones, which are
> > typically what we all use in the field. (I
> apologize to any users of
> > velocity mikes who may feel left out.) The small
> volume of air
> > trapped between the diaphragm and the backplate
> rests at ambient
> > static pressure. As an acoustic wave surrounds
> the mike, the
> > instantaneous pressure on the exposed side of the
> diaphragm
> > oscillates around this static pressure at the
> frequencies of the
> > sound and at amplitude proportional to the
> loudness of the sound.
> > The diaphragm is faced momentarily with unequal
> pressure on its two
> > sides and, being compliant, moves in or out to
> compress or expand the
> > air volume behind the diaphragm to equalize the
> instantaneous
> > pressure. This induced motion of the diaphragm,
> of course, is what
> > the microphone transduces to an electrical analog
> of the acoustic
> > pressure.
>
> So far, so good.
>
> > At all wavelengths much longer than the
> diaphragm's diameter,
> > regardless of direction of travel, the entire
> diaphragm
> > effectively "feels" the same oscillations of
> acoustic pressure, and
> > certainly moves in unison. No problem. However,
> at wavelengths
> > short compared to the diaphragm diameter, any
> off-axis excitation
> > will sweep across the diaphragm, rather than
> driving it all at once.
> > The exact response is dependent on the mike's
> porting, the lateral
> > compliance of its diaphragm, and probably many
> other arcane effects.
>
> You have a little problem here. Think back about
> that planar sound wave
> arriving at the parabolic dish. Take the case of the
> sound from on axis
> and assume it's a 1:1 dish just to take a simplified
> case. And just for
> kicks the diaphragm is pointing in and not
> obstructed (no exotic mic
> design problems).
>
> That wave interacts with all the surface of the dish
> and then arrives
> from all points of the dish simultaneously at the
> mic diaphragm (at
> increased amplitude compared to the wave as it
> entered the dish, the
> gain). It's not a wave from some specific direction
> for the mic, for all
> frequencies it's a integrated pressure wave arriving
> from half a sphere
> of directions (in the case we choose above). Now try
> and sweep that
> across the diaphragm at a angle, not the way it's
> going to happen. That
> is where I have a lot of problem with your
> explanation, particularly
> when you go back up to your first paragraph about
> how the pressure wave
> interacts with the mic.
>
> > Actual measurements confirm the leveling-off of
> gain in the
> > neighborhood of 10 KHz for most of our reflector
> systems.
> > Mathematical "proof" of cause is elusive - not
> because we can't
> > calculate the acoustic performance of a parabolic
> reflector or of a
> > particular microphone, but because we must model
> BOTH elements at
> > once. That's a real challenge!
>
> At 10 KHz you have a wavelength of 1.2" or so. Much
> bigger than most
> diaphragms used, particularly bigger than those used
> in the Telinga.
> Though there you have to deal with multiple
> capsules. I've used single
> tie tac mics which are much smaller too. Are there
> actual measurements
> where diaphragm size is correlated with the falloff?
> Preferably with
> some reasonable statistical confidence level?
>
> I'm not saying there is no falloff. I've seen enough
> graphs that it
> would appear there generally is a falloff in gain
> increase. I'm finding
> it hard to buy the explanation. The graphs I've seen
> of gain falloff
> also don't look like a singular cause for them.
>
> Modern computers should be up to the task. I think
> the primary problem
> is that few are interested enough in the performance
> of parabolic mics.
> Probably no money for the effort.
>
> Walt
>
>
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