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Author Topic: Side Vents on Mics  (Read 1923 times)

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Online guitard

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Side Vents on Mics
« on: May 19, 2019, 03:37:50 PM »
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 >:D 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.

« Last Edit: May 20, 2019, 07:15:17 AM by guitard »
Mics: CA-11 (omnis and cards) & AT853 cards
Pre: CA-9100
Decks: Edirol R-07 & Edirol R-09HR

Video: Sony FDR AX100 (4K), Pany ZS100 (4K) & Sony DSC HX50V (HD)
Photo: Canon EOS 60D

A/V software: Sony Vegas Pro 13.0 (build 453) 64 bit / DVD Architect Pro 6.0 (build 237)

Offline EmRR

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Re: Side Vents on Mics
« Reply #1 on: May 19, 2019, 03:39:47 PM »
The vents are a big part of what makes them directional mics.  At minimum you are modifying the pattern.
Mics: DPA 4060 w/MPS 6030 PSU/DAD6001/DAD4099, Neumann KM 131, KMR 81i, Oktava MK 012, Sennheiser MKH 105, MKH 20, MKH 30, MKH 40, MKH 800 TWIN, lots of other studio appropriate choices
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Offline Sevoflurane

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Re: Side Vents on Mics
« Reply #2 on: May 19, 2019, 06:21:33 PM »
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.
Mics: Schoeps MK4s, AT853 (4.7k mod)
Pre: Naiant IPA v1.0
Recorders: M10, A10, TCD-D8
Transfers: Nakamichi CR-3A, Tascam DA-20

Offline MIQ

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Re: Side Vents on Mics
« Reply #3 on: May 20, 2019, 02:47:22 AM »
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...

Offline nulldogmas

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Re: Side Vents on Mics
« Reply #4 on: May 20, 2019, 07:41:00 AM »
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.)

Offline DSatz

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Re: Side Vents on Mics
« Reply #5 on: May 21, 2019, 02:03:06 AM »
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.
music > microphones > a recorder of some sort

Offline borjam

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Re: Side Vents on Mics
« Reply #6 on: May 21, 2019, 02:27:42 AM »
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

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Offline Moke

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Re: Side Vents on Mics
« Reply #7 on: May 21, 2019, 09:17:03 AM »
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).
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Offline Gutbucket

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Re: Side Vents on Mics
« Reply #8 on: May 21, 2019, 10:42:03 AM »
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.
 
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<<

Offline flask

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Re: Side Vents on Mics
« Reply #9 on: May 21, 2019, 11:52:02 AM »
Won't the croaks just act similar to a windscreen?
Mics: AKG C480B CK61/CK63 | AT943C/O
Pre/Pwr: UA-5 wmod | SPSB-1 | Baby nbox
Recorders: TCD-D7 | NJB3 | DR-680mkii | PCM-A10

Offline Gutbucket

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Re: Side Vents on Mics
« Reply #10 on: May 21, 2019, 12:46:55 PM »
:facepalm:
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<<

Offline unidentified

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Re: Side Vents on Mics
« Reply #11 on: May 21, 2019, 12:57:54 PM »
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. 

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Re: Side Vents on Mics
« Reply #12 on: May 21, 2019, 12:59:56 PM »
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.
musical volition > vibrations > voltages > numeric values > voltages > vibrations> virtual teleportation time-machine experience
Better recording made easy - >>Improved PAS table<< | Made excellent- >>click here to download the Oddball Microphone Technique illustrated PDF booklet<<

Offline flask

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Re: Side Vents on Mics
« Reply #13 on: May 21, 2019, 03:44:44 PM »
picked the wrong day to quit coffee apparently
Mics: AKG C480B CK61/CK63 | AT943C/O
Pre/Pwr: UA-5 wmod | SPSB-1 | Baby nbox
Recorders: TCD-D7 | NJB3 | DR-680mkii | PCM-A10

Online guitard

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Re: Side Vents on Mics
« Reply #14 on: May 21, 2019, 04:07:49 PM »
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

Mics: CA-11 (omnis and cards) & AT853 cards
Pre: CA-9100
Decks: Edirol R-07 & Edirol R-09HR

Video: Sony FDR AX100 (4K), Pany ZS100 (4K) & Sony DSC HX50V (HD)
Photo: Canon EOS 60D

A/V software: Sony Vegas Pro 13.0 (build 453) 64 bit / DVD Architect Pro 6.0 (build 237)

 

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