Discussion:
tetrahedral mic record
(too old to reply)
Steven Boardman
2018-09-11 13:11:05 UTC
Permalink
Hi Folks

Is it better to align performers to capsule axis, to get the best frequency, and noise performance from those directions, then rotate the b-format encode afterwards?

I am recording at Union Chapel, with tetrahedral mics, and spot microphones.
The final output will be stereo and HOA.
I usually just position tetra mics as normal (upside down and end-fire), but it occurred to me that it may be better to align the capsules to performers (where possible).
All performers will be spread out horizontally on an equilateral triangle, with the main tetra on a corner.
The main decode will be stereo, but I want to capture the acoustic of the place, and do a HOA mix.


Best

Steve
umashankar manthravadi
2018-09-11 13:17:20 UTC
Permalink
I read this many years ago so cannot remember a source, but I think on-axis is not the best direction for a tetrahedral array. I think I was told this because I was trying to design an array with the capsules at 0, 90, 180 and 270 because it would be easier to do (before 3d printing arrived).



umashankar



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________________________________
From: Sursound <sursound-***@music.vt.edu> on behalf of Steven Boardman <***@gmail.com>
Sent: Tuesday, September 11, 2018 6:41:05 PM
To: ***@music.vt.edu
Subject: [Sursound] tetrahedral mic record

Hi Folks

Is it better to align performers to capsule axis, to get the best frequency, and noise performance from those directions, then rotate the b-format encode afterwards?

I am recording at Union Chapel, with tetrahedral mics, and spot microphones.
The final output will be stereo and HOA.
I usually just position tetra mics as normal (upside down and end-fire), but it occurred to me that it may be better to align the capsules to performers (where possible).
All performers will be spread out horizontally on an equilateral triangle, with the main tetra on a corner.
The main decode will be stereo, but I want to capture the acoustic of the place, and do a HOA mix.


Best

Steve
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Eero Aro
2018-09-11 14:34:57 UTC
Permalink
umashankar wrote:
> I read this many years ago so cannot remember a source, but I think
> on-axis is not the best direction for a tetrahedral array.
Funny that you remember that discussion, it must have been more than a
decade ago in Sursound.

I can't remember whoever started the thread, but I asked the same
question as
Steven. I had played with the rotating plugins and came to think that I
could
point one of the SFM capsules directly towards the center of the performers.
I was very soon told that it's not a good idea.

Eero
umashankar manthravadi
2018-09-11 14:43:25 UTC
Permalink
Well, I have been on sursound for a very long time, certainly longer than ten years



umashankar



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________________________________
From: Sursound <sursound-***@music.vt.edu> on behalf of Eero Aro <***@dlc.fi>
Sent: Tuesday, September 11, 2018 8:04:57 PM
To: ***@music.vt.edu
Subject: Re: [Sursound] tetrahedral mic record

umashankar wrote:
> I read this many years ago so cannot remember a source, but I think
> on-axis is not the best direction for a tetrahedral array.
Funny that you remember that discussion, it must have been more than a
decade ago in Sursound.

I can't remember whoever started the thread, but I asked the same
question as
Steven. I had played with the rotating plugins and came to think that I
could
point one of the SFM capsules directly towards the center of the performers.
I was very soon told that it's not a good idea.

Eero
_______________________________________________
Sursound mailing list
***@music.vt.edu
https://nam03.safelinks.protection.outlook.com/?url=https%3A%2F%2Fmail.music.vt.edu%2Fmailman%2Flistinfo%2Fsursound&amp;data=02%7C01%7C%7C4ffb78d8feb048957dbe08d617f3ca4a%7C84df9e7fe9f640afb435aaaaaaaaaaaa%7C1%7C0%7C636722733161712516&amp;sdata=LSnrTJMzyuN23%2Bd0411%2BLKxX5yxkEOb3AQEsu47ytW8%3D&amp;reserved=0 - unsubscribe here, edit account or options, view archives and so on.
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James Mastracco
2018-09-11 13:20:48 UTC
Permalink
I've love to see pictures of the chapel and the mic placement.

JM

> Hi Folks

> Is it better to align performers to capsule axis, to get the best
> frequency, and noise performance from those directions, then rotate the b-format encode afterwards?

> I am recording at Union Chapel, with tetrahedral mics, and spot microphones.
> The final output will be stereo and HOA.
> I usually just position tetra mics as normal (upside down and
> end-fire), but it occurred to me that it may be better to align the
> capsules to performers (where possible).
> All performers will be spread out horizontally on an equilateral
> triangle, with the main tetra on a corner.
> The main decode will be stereo, but I want to capture the acoustic of the place, and do a HOA mix.


> Best

> Steve
> _______________________________________________
> Sursound mailing list
> ***@music.vt.edu
> https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe
> here, edit account or options, view archives and so on.



--
Best regards,
James mailto:***@verizon.net
David McGriffy
2018-09-11 14:27:16 UTC
Permalink
Actually, directly on axis of one of the capsules may be the worst
direction for a tetrahedral array. While it's true that otherwise you are
slightly off-axis for a given capsule, with whatever coloration that might
cause, I believe the non-coincidence problems of the complete array
overwhelm this effect. When pointing directly at one capsule, the distance
to each of the other three is the same, thus the peaks and notches caused
by phase differences all line up at the same frequency and become one
bigger peak or notch. When a source comes in between two capsules, the
distances in the array don't match so the peaks and notches spread out in
frequency and help to smooth each other out a bit. Of course, the
calibration filters for tetrahedral mics attempt to correct for this
effect, but those filters more closely match the diffuse field average of
all directions than the more extreme results in the direction of one
capsule.

David McGriffy
VVAudio


On Tue, Sep 11, 2018 at 8:21 AM James Mastracco <***@verizon.net>
wrote:

> I've love to see pictures of the chapel and the mic placement.
>
> JM
>
> > Hi Folks
>
> > Is it better to align performers to capsule axis, to get the best
> > frequency, and noise performance from those directions, then rotate the
> b-format encode afterwards?
>
> > I am recording at Union Chapel, with tetrahedral mics, and spot
> microphones.
> > The final output will be stereo and HOA.
> > I usually just position tetra mics as normal (upside down and
> > end-fire), but it occurred to me that it may be better to align the
> > capsules to performers (where possible).
> > All performers will be spread out horizontally on an equilateral
> > triangle, with the main tetra on a corner.
> > The main decode will be stereo, but I want to capture the acoustic of
> the place, and do a HOA mix.
>
>
> > Best
>
> > Steve
> > _______________________________________________
> > Sursound mailing list
> > ***@music.vt.edu
> > https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe
> > here, edit account or options, view archives and so on.
>
>
>
> --
> Best regards,
> James mailto:***@verizon.net
>
> _______________________________________________
> Sursound mailing list
> ***@music.vt.edu
> https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here,
> edit account or options, view archives and so on.
>
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Steven Boardman
2018-09-11 19:03:38 UTC
Permalink
Thanks David, that makes sense.

If i kept the performers within the horizontal spread of 2 capsules, rather
than pointing at them, would there be any benefit? Ie 2 capsules axis
parallel to the horizontal plane.
So in effect the performance will be contained within 60 degrees of the 90.

Or maybe its just to much effort for little gain...:)

Sorry if this has come up before, as i didnt get a hit on a search.

Cheers

Steve



On Tue, 11 Sep 2018, 15:28 David McGriffy, <***@mcgriffy.com> wrote:

> Actually, directly on axis of one of the capsules may be the worst
> direction for a tetrahedral array. While it's true that otherwise you are
> slightly off-axis for a given capsule, with whatever coloration that might
> cause, I believe the non-coincidence problems of the complete array
> overwhelm this effect. When pointing directly at one capsule, the distance
> to each of the other three is the same, thus the peaks and notches caused
> by phase differences all line up at the same frequency and become one
> bigger peak or notch. When a source comes in between two capsules, the
> distances in the array don't match so the peaks and notches spread out in
> frequency and help to smooth each other out a bit. Of course, the
> calibration filters for tetrahedral mics attempt to correct for this
> effect, but those filters more closely match the diffuse field average of
> all directions than the more extreme results in the direction of one
> capsule.
>
> David McGriffy
> VVAudio
>
>
>
>
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Fons Adriaensen
2018-09-12 14:21:58 UTC
Permalink
On Tue, Sep 11, 2018 at 09:27:16AM -0500, David McGriffy wrote:

> Of course, the
> calibration filters for tetrahedral mics attempt to correct for this
> effect, but those filters more closely match the diffuse field average of
> all directions than the more extreme results in the direction of one
> capsule.

This is correct. There is nothing special about the on-axis directions
that would make them better or worse than any other direction.

Tetramics are calibrated for best frequency response in the cardinal
directions (X, Y, Z). This corresponds to a good compromise over all
directions.

--
FA
Steven Boardman
2018-09-12 17:08:42 UTC
Permalink
Thanks everyone, not doing it saves a lot of faffing around!

Oh and Paul, no problem with heaters, as we can turn them off.
Only the performers, a cameraman, and I. We don’t have to keep punters warm!

Cheers

Steve



> On 12 Sep 2018, at 15:21, Fons Adriaensen <***@linuxaudio.org> wrote:
>
> On Tue, Sep 11, 2018 at 09:27:16AM -0500, David McGriffy wrote:
>
>> Of course, the
>> calibration filters for tetrahedral mics attempt to correct for this
>> effect, but those filters more closely match the diffuse field average of
>> all directions than the more extreme results in the direction of one
>> capsule.
>
> This is correct. There is nothing special about the on-axis directions
> that would make them better or worse than any other direction.
>
> Tetramics are calibrated for best frequency response in the cardinal
> directions (X, Y, Z). This corresponds to a good compromise over all
> directions.
>
> --
> FA
>
> _______________________________________________
> Sursound mailing list
> ***@music.vt.edu
> https://mail.music.vt.edu/mailman/listinfo/sursound - unsubscribe here, edit account or options, view archives and so on.
Martin Leese
2018-09-11 21:32:12 UTC
Permalink
Eero Aro wrote:

> umashankar wrote:
>> I read this many years ago so cannot remember a source, but I think
>> on-axis is not the best direction for a tetrahedral array.

> Funny that you remember that discussion, it must have been more than a
> decade ago in Sursound.
>
> I can't remember whoever started the thread, but I asked the same
> question as
> Steven. I had played with the rotating plugins and came to think that I
> could
> point one of the SFM capsules directly towards the center of the
> performers.
> I was very soon told that it's not a good idea.

This appears to be part of a long thread with
the Subject "Formal A-format". Eero asked his
question on July 27, 2006.

Regards,
Martin
--
Martin J Leese
E-mail: martin.leese stanfordalumni.org
Web: http://members.tripod.com/martin_leese/
Paul Hodges
2018-09-11 23:22:46 UTC
Permalink
--On 11 September 2018 14:11 +0100 Steven Boardman
<***@gmail.com> wrote:

> it occurred to me that it may be better to align the capsules to
> performers (where possible).

There is no direction in which the output will be just from one capsule
anyway; even if you try to select the same pattern as the capsule
concerned (because of the EQ, etc).

And if you think you have something to gain from recording each
performer on axis to a capsule, aren't you really pretty much asking
for multi-miking?

Paul


PS - I hope you're not recording there on a cold day, or they have
changed things since I recorded in there. Gas heaters on the front of
the balcony produced a lovely hiss, and the track that went onto a CD
needed some careful noise reduction!

--
Paul Hodges
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