Raimund Specht wrote:
> Walt,
>
> I agree, that pressure and reflection are related to the same thing.
> Of course, if you look at single molecules, there will be no
> differences at low and high frequencies. But there will be
> differences when looking at the larger scale...=20=20=20
Which entirely relate to the molecular level. It's the failure to keep
track of that part that results in this flaw in logic about parabolic
reflectors.
>>The only real difference between the microphone diaphragm and a
>
> perfect
>
>>reflector is that the diaphragm absorbs energy to move it. A
>
> perfect
>
>>reflector returns all the energy. I'm sure that any real world
>
> parabolic
>
>>reflector only approaches a perfect reflector.
>
> Unfortunately, the real-world reflector will only reflect the energy
> at higher frequencies (because of its limited size). As you say, the
> parabola is not an ideal reflector nor it is an ideal high-pass
> filter. To some degree, it is similar to an electronic high-pass
> filter. It is impossible to design a filter with an extremely steep
> transition between the stopband and passband. Therefore you will get
> some gain at freqncies below the corner frequency. Also, the
> transition from stopband (at low frequencies) to passband (at high
> frequencies) to is not very smooth (as it would be in an electronic
> filter). There are many mountains and valleys in the frequency
> response at that range. This is one drawback of the parabola
> compared to a shotgun (as long as your subject is located exactly on-
> axis).
This is where you make a great leap and totally neglect the molecular
level. Go back and try again, sound is moving along its direction of
travel on molecules, it cannot help but hit and bounce off any reflector
of the size of a molecule or larger in it's path, regardless of
frequency. It makes no side to side movement at all, it can't miss. When
it bounces off it will do so following the rules of particle physics.
Note that reflection in a microphone, which you do admit occurs, is
exactly the same thing as reflection off a parabolic surface. If it
occurs in a microphone, it definitely occurs off the much larger surface
of a parabolic.
As I've noted over and over, sound in air is not a electromagentic wave,
is not a ocean wave, is not even the sine wave drawn in textbooks. Your
predictions of how it will behave are transplants from electromagnetic
wave theory. They don't apply.
>>Don't believe me? Go read some of the accounts of designing
>
> microphones.
>
>>You will find quite a few comments on controlling reflections from
>
> the
>
>>parts of the microphone. All of which are smaller than the
>
> wavelengths
>
>>involved. If there are reflections there, they are no different
>
> from
>
>>those from the parabolic reflector.
>
> You are right, there are reflections on the surface of the
> microphone diaphragm too (especially at higher frequencies).
> But these reflections are not essential to the operation at low
> frequencies. The air just moves the diaphram (a special kind of
> barometer). As mentioned earlier by others, this principle will also
> work at infrasound. At wavelengths in the range of the of diaphram
> size, reflections will come into play too. But these reflectections
> are not desired, because they attenuate the output signal (loss of
> flatness). This is the reason, why microphones (I mean the classical
> diaphram designs) for high frequencies must have smaller diameters.
Essential or not, there are reflections internally in a microphone at
ALL frequencies. For reflections to occur from any of these surfaces
which are tiny compared to the wavelengths, proves you absolutely wrong
that the vastly larger surfaces of a parabolic reflector will not
reflect things longer wavelength than their size.
And just what do you think air moving the diaphragm is? It is the
collective impact of the molecules involved. That's what happens in a
barometer too. To understand the physics of air and sound, you have to
apply particle physics. There is nothing other than particles. You
cannot neglect the fact that air is particles. And it's the movement of
the particles that moves the sound.
Which is why one of the ultimate limits of self noise for a mic is the
noise caused by the impacts of the air molecules, which in addition to
moving as a result of passing sound energy, have brownian movement which
a sensitive diaphragm can detect. If the particles did not matter, if
pressure was something else, then this phenominum would not be found.
>>Or take a look at a PZM boundary
>>microphone, where only reflected sound reaches the mic. The
>
> boundaries
>
>>are much smaller than the wavelengths involved.
>
> As I know, those microphones must be placed on a large surface (e.g.
> a wall). Then you will then get a larger boundary.
> If you use such a microphoen in a room it will also pick up low-
> frequency sounds reflected by the walls and the ceiling (but these
> are large).=20
Like, for instance, my SASS MKH-110, which has a really gigantic
boundary of a few inches by a few inches. It has considerable gain over
bare MKH-110's at infrasound levels. Which you claim to be impossible,
using exactly the same flawed logic you apply to parabolics.
Want to try and place the two boundary surfaces of a SASS on a wall?
It's not necessary to pick up infrasound at a gain over a bare mic. It's
not necessary to place them on a large surface, though you can increase
the low frequency gain some more by doing so. And I don't happen to cart
around walls when out nature recording anyway. They don't fit in my
pocket very well.
Note that particle physics does not give a parabolic reflector a free
pass either. But it's not so silly as to think there is no reflection,
or some abrupt cutoff point. I believe what it will predict is that
there is a decreasing gain above a bare mic as the frequency decreases.
This has no reason to contain bumps or irregularities and will never,
even at the lowest frequency reach a situation where there is no gain.
System sensitivity will always be a multiplier of the sensitivity of the
bare mic that's greater than 1. That is the experience of myself, and I
have actually used a Telinga for low frequency stuff.
In addition, particle physics will predict the result that the
directionality is not particularly frequency dependent. It is only
dependent in that as the gain decreases the ratio of direct to reflected
sound will change since the direct sound is not influenced by the
reflector. Which, again is the experience of a number of Telinga users,
myself included. A shotgun may transform into a simple cardioid at low
frequencies, but a parabolic does not have that loss of directionality.
I'm being highly amused being told I did not actually record what I
have recorded. I'm reporting actual experience, theory has to sort
itself out to agree, I don't have to toss my recordings to make theory
work out.
Walt
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