How do vents on mics make cardiod/shotgun?
RockyRoad
the more vents on the side, the more sound could "get in" from the sides
and the more "omni" the mic would be.
Yet I notice that these vents always occur on cardiod and shotgun mics,
whereas an omni small-diaphragm condenser has no vents, which I would
have thought would block the sound from everywhere but the front.
Could some kind person explain to me how the physics of these things
work, and how sound from behind an omni mic such as the KM183 can get
around the metal side casing and into the mic.
If sound was coming from directly behind the KM183, would the mic plug,
mic lead, stell case, and various electronics serve to dampen the sound?
Sorry for the dumb questions, but I'd like to know why things arent as
they seem on the surface.
And a related question - I have heard that shotguns are most suseptable
to wind noise. I am wondering why this is so, as shotguns block out the
sound from the side - wouldn't they also block out the wind?
Thank for your help in my education
--
RockyRoad [ignorant but keen high school music teacher]
carpedonut
is a pressure transducer, which means it responds purely to changes in
pressure, regardless of direction. It doesn't have vents because the
capsule is only exposed on one side, hence the pressure sensitivity.
Therefore, as sound reaches the mic, the changing pressure of the
soundwave creates a changing pressure on the outside of the capsule, and
an electrical signal is generated. Since the capsule responds to
pressure only, it theoretically (we all know this doesn't actually
happen since the mic itself acts as a barrier to higher frequencies) can
be aimed in any direction relative to the sound source and still
generate the same signal.
The vents in directional mics (pressure gradient transducers) allow
sound to reach the back of the diaphragm, but this sound is delayed to
create summation or cancellation on the diaphragm depending on the angle
of incidence of the sound. The problem is that it is basically
impossible to create an internal labyrinth that delays all frequencies
equally--hence the phenomenon of off-axis coloration. I realize it may
initially seem counter-intuitive, but check it out in an
audio/microphone book and it will make sense. Or, wait for Scott
Dorsey/Stephen Paul/David Satz or one of a number of other technically
gifted people to explain it in more detail. I myself must go and sleep
off an excess of spiked eggnog...
Happy Holidays.
--
Sean D. Carberry
Assistant Professor
Berklee College of Music
http://www.carpedonut.com
Scott Dorsey
>Because I don't know acoustics, my dumb logic would have thought that
>the more vents on the side, the more sound could "get in" from the sides
>and the more "omni" the mic would be.
>
>Yet I notice that these vents always occur on cardiod and shotgun mics,
>whereas an omni small-diaphragm condenser has no vents, which I would
>have thought would block the sound from everywhere but the front.
>
>Could some kind person explain to me how the physics of these things
>work, and how sound from behind an omni mic such as the KM183 can get
>around the metal side casing and into the mic.
Go right now to www.josephson.com where there is a great tutorial
on how venting affects mike patterns.
Basically, if you have a sealed box with a diaphragm on it, the inside
of the box stays more or less at constant pressure and the diaphragm
moves in response to air pressure, making an omni.
If you have a diaphragm in open air (like a ribbon mike), it moves
in response to the velocity of air in the direction perpendicular
to the diaphragm, making a figure-8. Air moving parallel to the
diaphragm doesn't move it at all, which is where the nulls come from.
Cardioids and hypercardioids are in-between those two extreme cases.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
David Satz
> Could some kind person explain to me how the physics of these
> things work, and how sound from behind an omni mic such as
> the KM183 can get around the metal side casing and into the mic.
[ ... ]
> Sorry for the dumb questions, but I'd like to know why things
> arent as they seem on the surface.
"Why are things not the way they seem?" is a question that I _so_
wish people would ask more often than they do. Most folks seem
to stop noticing that things aren't the way they seem, and start
behaving as if that appearances are all that matter. To me that's
the essence of that form of spiritual death which we in this society
call "adulthood." It's why I believe that only children should be
allowed to vote or own property--but failing that, there should be
a law (or better yet, a general agreement) that grown-ups ought to
answer all honest questions honestly. Then maybe we would not
be such a culture of deception and self-deception, and people
would retain their ability to notice things that don't make sense.
--The replies from Sean and Scott are spot on, but I'd like to try to
help you visualize what these two types of microphone are doing.
Again, the relevant categories are "pressure transducer" (basically
omnidirectional) and "pressure gradient transducer" (basically
figure-8, but by using dual diaphragms and other tricks, any other
first-order directional pattern can be synthesized including
cardioid and super- or hypercardioid).
The model of a pressure transducer is a barometer. It measures air
pressure in the space around it. The simplest, grade-school science
barometer is a sealed tin can with air in it. The lid of the can will
flex in proportion to air pressure changes in the room around it;
you can attach a stick to the lid, and calibrate the stick's motions
in terms of whatever units of air pressure you want to use (inches
of mercury or the standard metric unit, which is "bars").
The thing is, the can will get squeezed by increasing air pressure
or it will expand in times of low air pressure, regardless of which
way you "aim" it. In fact the concept of "aiming" a barometer
doesn't really exist because it's integrating and responding to a
phenomenon that is all around it. You just set it up in whatever
physical orientation is convenient for you, and it works.
You could think of the barometric pressure in a daily weather
report as being the response of the barometer at 0.000011574 Hz
if you want (one cycle per day). Essentially a barometer is a
microphone with response down to DC. And that is a real-world
characteristic of pressure transducers: their low-frequency
response can be extended as far down as you like. Most pressure
microphones have some small vent built in to prevent them from
bursting when transported by air, but they can very well be dead
flat to below 1 Hz or 5 Hz, certainly to any audible frequency.
OK. So the pressure transducer works precisely _because_ only
one side of the diaphragm (the lid of the can) is exposed to the
air pressure that is to be recorded; the air on the other side of the
diaphragm is a constant mass, and the diaphragm flexes in order
to equalize the pressure on both its sides.
The other major category of transducer is pressure-gradient, which
is a fancy way of saying that its diaphragm is exposed to the sound
field both on the front and the back, so it responds to the difference
between the pressure that exists on the front and the pressure on the
back. If the pressure presented on both sides at a given moment is
identical, there is no net motion and no output. If the pressure on
the front is greater than the pressure on the back, the diaphragm
will move toward its backplate (assuming a condenser microphone).
If the opposite is true, the diaphragm will move outwards, away
from the backplate.
The thing is, if you just hang a microphone diaphragm out in space,
it will be pushed around by wind or by air currents of any kind
(including if you just blow on it) but it won't pick up much in
the audio frequency band because it's a thin element and the
pressure from sound waves will tend to be identical on both
sides of the diaphragm, at least until you get up to the high
frequencies (which we'll talk about some other day), and when
the pressure is the same on both sides of the membrane there is
no net movement and no output. But before I explain why this
type of arrangement picks up sound at all, let's observe that
we've actually encountered something that is true of pressure
gradient microphones generally, which is that they are much
more sensitive to wind, breath noise and "popping" of consonants
in vocal pickup than their omnidirectional counterparts are
(when the omnis are pressure transducers).
The trick which makes a pressure-gradient arrangement work
for recording sound is that the sound reaching the back of the
membrane is delayed momentarily, by setting up a delay
chamber in between the back vents of the microphone and
the back of the diaphragm. If you can make the pathway for
sound even just a tiny fraction of an inch longer before the
sound reaches the rear of the diaphragm, then you will cause
a phase shift between the sound reaching the front and the
sound reaching the back. That phase shift will be different
at different frequencies, of course, so there will really be only
one frequency (plus its exact integer multiples) at which a
maximum of difference in pressure will result between the front
and back of the diaphragm. At that frequency the resulting
microphone will have its highest sensitivity to sound. But if
you arrange things so that this frequency occurs somewhere
other than at the very top or the very bottom of the audio
range, you can do other tricks with damping and filtering
so as to flatten the overall response.
The thing is, this more complicated type of microphone is also
sensitive to the direction from which sound is arriving, because
if sound is arriving from in front, it will strike the front of the
diaphragm immediately, then when it reaches the rear input
ports it will pass through the acoustic delay chamber and
eventually reach the back of the diaphragm--so there will
be a continually varying difference in the air pressure on the
two sides of the diaphragm, and that's what moves it and
produces a signal. But if the sound is coming from behind
the microphone, it will reach the back inlets first, and pass
through the delay chamber at the same rate of speed as the
original wave is traveling outside the microphone; by the
time both waves reach the two sides of the diaphragm, they
will be in phase with one another and the result is no net
motion of the diaphragm. (That's if the microphone is a
single-diaphragm cardioid.)
That should be enough to establish a basic viewpoint, I hope.
--best regards
William Sommerwerck
to understand things, rather than just accept them on faith from some
self-proclaimed authority. Fortunately, the answer is not particularly
complicated. To make a long story short...
If a microphone diaphragm were freely suspended -- without any kind of support,
obstruction, or enclosure whatsoever -- it would respond to the velocity of the
air molecules and have a figure-8 pattern (like a simple ribbon mic). If this
same element were fully enclosed, it could respond only to the pressure of the
air, and would be (aside from shadowing effects of the enclosure)
omnidirectional.
By putting holes or slots in the mic enclosure, we can obtain a mix of the
figure-8 and omni "effects." The result is a cardioid pattern, its exact shape
depending on the relative amount of velocity and pressure components.
I'll leave the shotgun answer to someone else, because although I think I know
the answer, I'm not sure.
Don MacQueen
that I don't have, I could close the vents, say by wrapping with gaffers
tape? Or perhaps something stiffer?
And thanks to all of the other informative responses to the original
question.
-Don
in article 3A46E83E...@earthlink.net, carpedonut at
carpe...@earthlink.net wrote on 12/24/00 10:26 PM:
David Satz
> Which implies that if I have a cardiod that I don't want,
> and want an omni that I don't have, I could close the vents,
> say by wrapping with gaffers tape? Or perhaps something stiffer?
It does imply that, but even if you plug up the holes with concrete
you will most likely end up with an omni that you don't want.
The frequency response will not very likely be pleasant and the
microphone will still be subject to handling noise and other
solid-borne sound.
Still, it's not a harmful experiment to try out, and I'd urge you
to do it as long as you're reasonably careful when applying
and removing the tape (or the concrete).
Don MacQueen
(insert little smiley face here).
I admit I wasn't optimistic about the resulting quality.
And thanks for the encouragement; I will try it.
-Don
in article uLNyRArbAHA.266@cpmsnbbsa09, David Satz at DS...@msn.com wrote on
12/25/00 12:20 PM:
Lorin David Schultz
>
> Which implies that if I have a cardiod that I don't want, and want an
> omni that I don't have, I could close the vents, say by wrapping with
> gaffers tape? Or perhaps something stiffer?
Want proof that it works (sounds weird but works)? Just listen to what
happens on a live gig when the singer proves his deep sensitivity by
cupping his hands around the back of the mic while leaning over the
monitors...
The shriek from the monitors (and monitor engineer) will be evidence of
the sudden reduction in the mic's directivity.
Lorin
RockyRoad
> "Why are things not the way they seem?" is a question that I _so_
> wish people would ask more often than they do. Most folks seem
> to stop noticing that things aren't the way they seem, and start
> behaving as if that appearances are all that matter.
heh - well if you lived near me you would soon grow tired of my
questions. :)
--
Rocky Road - in Oz
Richard Kuschel
>I'll leave the shotgun answer to someone else, because although I think I
>know
>the answer, I'm not sure.
>
The shotgun mic derives its directionality by phase cancellation. An sound
arriving at the front of the tube hits the diaphragm at the rear of the tube at
the same time.
Signals arriving at the side of the tube must travel different lengths of the
tube to arrive at the diaphragm. Hence, they are not in phase coherency when
they hit the diaphragm and cancel to varying degrees.
Richard H. Kuschel
"I canna change the law of physics."-----Scotty