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Re: ambisonic

Subject: Re: ambisonic
From: "Dan Dugan" dandugan_1999
Date: Sun May 26, 2013 8:49 am ((PDT))
> Ambisonics is usually a particular technique for recording surround sound=
,
> especially with a tetrahedral mic array and a lot of sophisticated time
> domain processing. Calling any array "ambisonic" is muddling.

Agreed. Ambisonic is a particular technique using four mics in a tetrahedra=
l array. It has a cult-like following. Personally I've been unimpressed by =
demonstrations I've heard.

At the NSS Workshop June 21-23 Steve Sergeant will present a case for recor=
ding height information to make nature ambiences usable with the new theate=
r surround systems like Dolby Atmos.

> Any use of the word "distortion" to downplay other types of mic has to be=

> precisely defined. Harmonic distortion and intermodulation distortion
> (enharmonic) is very low in any quality microphone. "Frequency distortion=
"
> is better described as frequency response.

Yes, the Mitra promoter throws "distortion" around indiscriminately.

> Noise in an array inevitably rises with the number of mic elements.

The Telinga experience teaches the opposite for coincident multiple element=
s.

> It does
> need a reference to have any meaning, and with careful design, the forwar=
d
> sensitivity can rise to at least partially mask the noise with reference =
to
> the forward sensitivity. However, I record in stereo with much of the sou=
nd
> field being off-axis. As for the MKH series being noisy, all I can say is=

> that you can pick up HF thermal noise in very quiet locations on them.
>
> The off-axis frequency response of any directional mic is far from flat, =
and
> I include cardioids.

The more directional the mic is, the worse the off-axis (and ambient) respo=
nse is. When I teach about mics I say that the important differences betwee=
n paying $100 and $1000 for a cardioid will be 1) noise and 2) quality of o=
ff-axis response. I don't think it's physically possible to get good off-ax=
is response with anything more directional than a hypercardioid.

> I have yet to see a published off-axis frequency
> response for any highly directional mic or array.

Published frequency responses at all seem to be going away. A result of the=
 decline of engineering participation in recording in general.

> This is important, because
> in any stereo rig the prime forward sound image will be 30 to 60 degrees=

> off-axis.

A good argument for MS technique is that the object you're pointing the arr=
ay at is on-axis to the M mic.

Has anyone measured the polar plot(s) of a Jecklin array? I hear a distinct=
ly different coloration of the ambience from my Jecklin disk (my front chan=
nels) compared with the same omni mics by themselves (my rear channels).

> Has anyone got information on the wind sensitivity of any mic array? How=

> easy are they to windshield and what effect does the windshield have on t=
he
> phase response of the array? The standard basket and hairy cover has an
> audible effect with gunmics, in particular to the off-axis response. Also=
,
> for outdoor recording, the last thing you need is an extended bass respon=
se.

I like to have it for when the wind has died down...but we know that in the=
 real world of collecting sounds high-pass saves your butt. My little exper=
ience with Schoeps mics (in the repair shop, not in the field) found them t=
o go way, way down so that the slightest boom movement would create LF nois=
e if not high-passed. Of course in the concert hall that LF response is pre=
cious.

The Mitra maker brags about LF response, but his array isn't directional in=
 the low end any more than the shotgun technology, maybe even less--the sam=
e laws of physics apply re the size of the array and the ability to focus l=
ow frequencies.

-Dan







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