Topic: Side Vents on Mics

[guitard]

How important is it that the side vents on my AT853 cardioids not be covered?  I read in a thread about AT853 windscreens that the vents shouldn't be covered, hence my asking.  I typically record and use these croakie-like mic holders (see photo).  For concealment purposes, I usually pull the mics inside of the holders so that only the very tip is sticking out.  Have I been making a critical mistake by doing this?  If so, I'm going to have to come up with a way to allow them to extend out from the holders, while still keeping them concealed and also pointed straight forward.

Edited to notate my AT853s are cardioid mics.

[EmRR]

The vents are a big part of what makes them directional mics.  At minimum you are modifying the pattern.

[RyanJ]

If they are pointed straight at the source, I don't see this inhibiting much of the sound?

Would love someone to chime in on this as well.

[MIQ]

You have to keep the side vents open on those mics.  They will not function correctly if you don’t.  They work by combining the acoustic wave hitting the front of the membrane with the delayed acoustic wave hitting the back of the membrane coming in from the side vents.  For a cardioid mic, when the sound is coming from directly behind the mic, the front and rear waves cancel each other and you get the deep null in the pattern.  Imagine what would happen if you stopped those rear membrane waves from entering the mic at all frequencies.  Even the tech flex mesh will have a big effect at high frequencies, but would do nothing at lower frequencies...

[nulldogmas]

It's super important to keep the side vents uncovered, as I've learned through bitter experience. I don't know the acoustic properties of that croakie material, but I would try to keep it away from the vents if at all possible. (If not possible, you can try to recover some of the lost frequencies via EQ in post, but YMMV there.)

[DSatz]

What MIQ said. If you block the vents, you'll get a boomy, dull-sounding microphone with generally (but not uniformly) reduced directivity, depending on what you block them with.

[borjam]

If they are pointed straight at the source, I don't see this inhibiting much of the sound?

Would love someone to chime in on this as well.

It's critical.

Covering them turns a directional microphone (pressure gradient) microphone into an omnidirectional. It's the typical singer mistake I hate with a passion. "Embracing" the microphone while singing and, whoa! Here comes the feedback!

Anyone interested on microphones should read this book:

https://www.amazon.com/Eargles-Microphone-Book-Third-Application/dp/0240820754

[mfrench]

The croaky isn't going to provide enough blocking to have any effect. You'd need to wrap it, the capsule, in some form of non-flexible shroud that would eliminate 100% of the side venting in order to have it create a drastic change. You could even go so far as to wrap them in a bit of electrical tape, which would still allow for some pressure change to the rear of the capsule, due to the flex of the tape.  Those vents allows for similar pressure in front and behind the capsule, in varying amounts, depending on the mics intended response.  The croaky is not sufficient enough to change that bit of physics. And, at worse, it might make it a sub-cardioid response (which I dig).

[Gutbucket]

The general impact of blocking vents is well known.  They shouldn't be covered or closed, unless the microphone was specifically designed to provide mechanically switched patterns.  Proper practice is to arrange things so you do not to block the vents or the front of the capsule at all.

The actual effect will not be easily predictable, partly because the croakie weave material covering the vents will not block them completely but present an acoustic resistance that varies with frequency.  In the frequency ranges in which it changes the acoustic impedance through the vent system, it will affect frequency and polar responses.  It is not at all likely to affect things for the better, but comparative listening is the only practical way to determine how much effect it has.


The real world-
You've been recording this way.. how do you like those tapes?
Try recording a few things using the same microphone arrangement but with vents uncovered.  Notice any significant difference?
Weigh that difference against the microphone rearrangement practicalities.

If you notice a significant difference after running the above test a few times, yet need to keep the mics hidden in the croakies:
Try pushing the microphones completely inside the croakie.
-or-
Try placing a layer of the identical croakie fabric across the front of the capsule.  If you do this, keep the fabric in front stretched by about the same amount as the fabric over the vents.
^
Both these methods seek to apply the identical acoustic impedance modification to either side of the diaphragm, such that the effect is simply attenuation of the frequency range affected rather than a difference of impedance on the backside.  This is similar to what happens when you use a windscreen or faux-fur cover over a directional microphone. Compare after a making any necessary high frequency response EQ correction.

Or perhaps modify the croakie so you can continue to run them as you have been doing, but without covering the vents. To do that, I'd heat a small section of metal tubing of the appropriate diameter with a torch to heat and use it like a hot-knife, "cookie-cutter style" to make small windows for each vent hole through in the croakie material.  The material is presumably polyester and the hot tubing should both cut the holes and heat-seal them.  Insert a dowel of the same diameter as the microphone to stretch the fabric by the same amount and to present something to push the red-hot tubing against.
 

[flask]

Won't the croaks just act similar to a windscreen?

[Gutbucket]

[unidentified]

Won't the croaks just act similar to a windscreen?

Nope. Way too thick to be as acoustically transparent as speaker cloth, foam windscreens, etc. 

[Gutbucket]

The general impact of blocking vents is well known.  They shouldn't be covered or closed, unless the microphone was specifically designed to provide mechanically switched patterns.  Proper practice is to arrange things so you do not to block the vents or the front of the capsule at all.

The actual effect will not be easily predictable, partly because the croakie weave material covering the vents will not block them completely but present an acoustic resistance that varies with frequency.  In the frequency ranges in which it changes the acoustic impedance through the vent system, it will affect frequency and polar responses.  It is not at all likely to affect things for the better, but comparative listening is the only practical way to determine how much effect it has.


The real world-
You've been recording this way.. how do you like those tapes?
Try recording a few things using the same microphone arrangement but with vents uncovered.  Notice any significant difference?
Weigh that difference against the microphone rearrangement practicalities.

If you notice a significant difference after running the above test a few times, yet need to keep the mics hidden in the croakies:
Try pushing the microphones completely inside the croakie.
-or-
Try placing a layer of the identical croakie fabric across the front of the capsule.  If you do this, keep the fabric in front stretched by about the same amount as the fabric over the vents.
^
Both these methods seek to apply the identical acoustic impedance modification to either side of the diaphragm, such that the effect is simply attenuation of the frequency range affected rather than a difference of impedance on the backside.  This is similar to what happens when you use a windscreen or faux-fur cover over a directional microphone. Compare after a making any necessary high frequency response EQ correction.

Or perhaps modify the croakie so you can continue to run them as you have been doing, but without covering the vents. To do that, I'd heat a small section of metal tubing of the appropriate diameter with a torch to heat and use it like a hot-knife, "cookie-cutter style" to make small windows for each vent hole through in the croakie material.  The material is presumably polyester and the hot tubing should both cut the holes and heat-seal them.  Insert a dowel of the same diameter as the microphone to stretch the fabric by the same amount and to present something to push the red-hot tubing against.

[flask]

picked the wrong day to quit coffee apparently

[guitard]

The real world-
You've been recording this way... how do you like those tapes?
Try recording a few things using the same microphone arrangement but with vents uncovered.  Notice any significant difference?
Weigh that difference against the microphone rearrangement practicalities.

I'm generally satisfied with the recordings I get with these mics.  I will definitely try to record with the vents uncovered the next time I tape though.

Just for the sake of discussion, here are a couple of samples of recordings I've made with the AT853s with the vents covered.

AT853 cardioid mics --> CA-9100 pre-amp --> Edirol R-09HR; 24 bit dithered down to 16 bit/44.1 kHz LPCM

This was a jazz show in a small venue about 15 feet directly in front of one of the stacks:
http://www.dimeadozen.org/attachments/1/649261/8120292/Moshulu%202019-05-10%20Westland%20audio.mp3

This was in a medium sized venue (holds 1500-2000) and I was near the back of the venue on the floor:
http://www.dimeadozen.org/attachments/7/640587/8038381/Slash%202019-01-13%20Seoul%20audio%20sample%201.mp3

[Gutbucket]

^
Worth trying those options to determine it they make a noticeable improvement, particularly if it provides more peace of mind.

picked the wrong day to quit coffee apparently

No worries, time for an afternoon cup for me (nodding off..)

[morst]

What MIQ said. If you block the vents, you'll get a boomy, dull-sounding microphone with generally (but not uniformly) reduced directivity, depending on what you block them with.

What if you only block some of the vents, like if you have the mics mounted against your skull on one side?

[ilduclo]

Helps a lot if there’s lots of airspace between yer ears!
 

Seriously, though...good discussion.

[EmRR]

I'd expect the baffle size of the head would create a resonant boost at that frequency (somewhere midband), and the croak on the sides would mainly affect high frequency pattern, narrowing treble pattern relative to lower frequencies. 

[Gutbucket]

It's the other way around.

[snip..] and the croak on the sides would mainly affect high frequency pattern, narrowing treble pattern relative to lower frequencies.

^
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.

[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. 

It's the other way around.

[snip..] and the croak on the sides would mainly affect high frequency pattern, narrowing treble pattern relative to lower frequencies.

^
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.

[Gutbucket]

Consider the balls and baffles used to make omni mics more directional in the highs.

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.

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.

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]

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.

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]

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.

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:

Consider the balls and baffles used to make omni mics more directional in the highs.

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.

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]

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!

Yes, I was speculating.  You'd need to run comparative control tests.  The two effects may cancel, they may lean one direction.