Gear / Technical Help > Microphones & Setup
Side Vents on Mics
EmRR:
Consider the balls and baffles used to make omni mics more directional in the highs. I see the croaks as blocking highs from the rear and sides, thereby narrowing the pattern in the highs. They won’t absorb lows so they will remain the same.
--- Quote from: Gutbucket on May 22, 2019, 03:35:17 PM ---It's the other way around.
--- Quote from: EmRR on May 22, 2019, 11:17:39 AM ---[snip..] and the croak on the sides would mainly affect high frequency pattern, narrowing treble pattern relative to lower frequencies.
--- End quote ---
^
It will make the pattern wider in the frequency range in question (but in a less than predictable way, and with other baggage).
Consider the simplified conceptual model:
Same acoustic impedance and acoustic path length to both sides of the diaphragm = bidirectional (fig-8).
Back side of diaphragm completely isolated from front = omni.
The croakie fabric covering the rear vents will increase the acoustic impedance to the back side of the diaphragm, pushing the pattern toward the omnidirectional end of the spectrum. It will do so most significantly in the frequency range for which the acoustic impedance (acoustic resistance) through the croakie material is highest.
--- End quote ---
Gutbucket:
--- Quote ---Consider the balls and baffles used to make omni mics more directional in the highs.
--- End quote ---
That is a very different mechanism of directivity. Ignore the bit about mounting near one's head for the moment (which does constitute a baffle) and consider just the microphone with croakie material covering the vents, verses not having them covered.
--- Quote ---I see the croaks as blocking highs from the rear and sides, thereby narrowing the pattern in the highs. They won’t absorb lows so they will remain the same.
--- End quote ---
This is a pressure-gradient microphone.
Microphones generate signal based on the pressure differential found on opposite sides of the diaphragm. In an omni the backside is a sealed volume not open to the outside. In a figure-8 the backside is as open as the front. In a cardioid the the back side is "less open" than the front by a highly tuned and carefully engineered amount. The rear vent system is part of the pressure-gradient sensing aspect of the microphone. Pattern "nulls" or reduced sensitivity regions correspond to directional orientations of the microphone for which the pressure on either side of the diaphragm is equal (zero pressure differential). There is then no resulting displacement force causing the diaphragm to move. Directions at which maximum sensitivity occurs correspond to those which produce the largest pressure differential across the diaphragm.
If we expect the croakie material will reduce the level of the high frequency sound passing through it more so than low frequency sound, the sound reaching the back of the diaphragm will have the high frequencies attenuated more than the low frequencies. Yet the frequency balance of the sound reaching the front of the diaphragm has not been altered. Because of this, for orientations where we formerly had a balanced pressure situation, we can now expect a increased pressure differential (high frequencies). The system has been shifted somewhat closer to the behavior of an omni at high frequencies.
EmRR:
--- Quote from: Gutbucket on May 22, 2019, 08:22:24 PM ---If we expect the croakie material will reduce the level of the high frequency sound passing through it more so than low frequency sound, the sound reaching the back of the diaphragm will have the high frequencies attenuated more than the low frequencies. Yet the frequency balance of the sound reaching the front of the diaphragm has not been altered.
--- End quote ---
I see your point, but the front has been altered as well, with the material blocking more high frequency sounds from 90º and beyond, possibly less than 90º depending on the accuracy of the deployment. Depends on the size of the capsule housing, and I don't personally know this mic.
Gutbucket:
--- Quote from: EmRR on May 22, 2019, 08:42:19 PM ---I see your point, but the front has been altered as well, with the material blocking more high frequency sounds from 90º and beyond, possibly less than 90º depending on the accuracy of the deployment. Depends on the size of the capsule housing, and I don't personally know this mic.
--- End quote ---
Refer the photo in the original post. The front of the diaphragm has not been altered. It is not covered by the same croakie material. But the reason I suggested the OP might consider pushing the microphone further into the croakie, or placing a piece the same croakie fabric in front of the microphone, is as an attempt to bring the differential system back into its intended balance by providing the same filtering to the front of the diaphragm as has been applied to the back of it. In this highly simplified conceptual model at least, the filtering is the same to both sides of the diaphragm, the difference cancels out eliminating the altercation of polar pattern, leaving only the filtering effect on overall frequency response (similar to use of a windscreen covering both the front and the vents).
It now occurs to me that you may be referring to the absorption or diffraction of sound around the bunched up portion of the croakie. Those are quite different mechanisms of directivity, briefly touched on here:
--- Quote from: Gutbucket on May 22, 2019, 08:22:24 PM ---
--- Quote ---Consider the balls and baffles used to make omni mics more directional in the highs.
--- End quote ---
That is a very different mechanism of directivity. Ignore the bit about mounting near one's head for the moment (which does constitute a baffle) and consider just the microphone with croakie material covering the vents, verses not having them covered.
--- End quote ---
Those mechanisms will have an effect, but will not apply in the same way they do for an omni. Because they affect sound arrival to both the front of the diaphragm and it's backside via the vents, their effect is overly complex to try and speculate about here. It's complicated!
Just the vent-covering aspect on it's own is not very predictable, but at least we can analyze that as a simplified system which is what I've tried to do above.
EmRR:
--- Quote from: Gutbucket on May 23, 2019, 12:17:04 PM ---It now occurs to me that you may be referring to the absorption or diffraction of sound around the bunched up portion of the croakie. Those are quite different mechanisms of directivity
Those mechanisms will have an effect, but will not apply in the same way they do for an omni. Because they affect sound arrival to both the front of the diaphragm and it's backside via the vents, their effect is overly complex to try and speculate about here. It's complicated!
--- End quote ---
Yes, I was speculating. You'd need to run comparative control tests. The two effects may cancel, they may lean one direction.
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