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Author Topic: Measurement mic..  (Read 1211 times)

Al Rettich

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Measurement mic..
« on: December 10, 2018, 11:11:36 am »

Recently, I was mixing a show, where along with our artist there was two others.  I noticed something.  All three of us had measurement mics on stands.  Mine pointed towards the stage, another one pointed straight up towards the sky.  The third was right above the engineers head, also pointed straight towards the sky.  I didn’t get the chance to ask, but thought I’d come here and ask.  What do you obtain by pointing the microphone straight into the sky?
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Taylor Hall

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Re: Measurement mic..
« Reply #1 on: December 10, 2018, 11:47:08 am »

It might depend on the type of mic being used and what it's intended to measure (free field, diffuse, etc). What type of venue was this at? Open field? Closed hall? Highly reverberant environment?

I know a lot of home theater guys prefer the straight up measurement technique, but I think that's mostly when measuring HF response as those towers can cause a big buildup of HF energy that would skew results when the mic is placed in such close proximity to the source.
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Dave Garoutte

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Re: Measurement mic..
« Reply #2 on: December 10, 2018, 12:56:23 pm »

Measurement mics are omni-directional, so technically it doesn't matter where it's pointed.
But up seems to be fairly standard when measuring room response.
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Frank Koenig

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Re: Measurement mic..
« Reply #3 on: December 10, 2018, 02:44:29 pm »

Measurement mics are designed to be omnis, of course. Typically they become slightly directional in the top octave due to the nonzero extent of the diaphragm and the presence of the microphone body. The guidance I've received is to point the mic toward the source of interest when, for example, doing speaker SPL-at-a-point measurements. I understand the rationale for pointing the mic up for room acoustic measurements as it will make its HF polar response symmetrical in the horizontal plane, but how much of a difference does this actually make in a reverberant room?

I avoid pointing the mic up because I don't want dirt falling on the diaphragm. While I don't know how much of a problem this really is, I do know how much the mic cost :-[

--Frank
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Steven A. White

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Re: Measurement mic..
« Reply #4 on: December 25, 2018, 11:09:41 am »

^^^ Frank, I like the way you think.  I do about the same.  When measuring my HT stuff in the den, I will point it up - but in venues I point at the PA/Stage.

One thing for sure, mic placement and utilization for analysis is an art.
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Russell Ault

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Re: Measurement mic..
« Reply #5 on: January 02, 2019, 05:57:17 pm »

It might depend on the type of mic being used and what it's intended to measure (free field, diffuse, etc).

From the Rational Acoustics' (brilliant) Smaart Gear Choices Guide (p.9):
Quote
Intuitively, one might expect “omnidirectional” to mean a microphone that is equally sensitive in all directions across the full audio spectrum, but in practice even the very best omnidirectional measurement microphones exhibit some directional characteristics. This is especially true at higher frequencies, where the diameter of the diaphragm starts to become significant with respect to wavelength.

[...]

The Beyerdynamic MM1 is a Diffuse field omni (sometimes called a Random Incidence mic) optimized for smooth off-axis response. This type of microphone will exhibit some build-up at high frequencies when pointed directly at a discrete sound source such as a loudspeaker. The iSEMcon EMX-7150 is a Free field omni, designed to be pointed directly at whatever you’re measuring. It rolls off in the high end at 90° off axis.

[...]

For best HF accuracy:
  • Use a free field microphone on-axis for predominantly direct sound measurements of loudspeakers and other discrete sound sources.
  • Use a diffuse field response microphone, typically pointed straight up, for diffuse reverb measurements or background noise surveys, where no single sound source dominates the noise field. Consult the manufacturer’s documentation for specific recommendations regarding the optimal angle of incidence for direct sound (free field) measurements.
  • [...]

To add to this, because the difference between a diffuse and free field microphone will only show up in the top octave or so, it's probably not a huge deal one way or another for the kind of work we're doing, and the same can be said for pointing up vs. point at the source.

Moreover, you can typically take one type of mic and use it for the other application by using an appropriate correction curve (which is why many calibrated microphones include two files that either talk about "diffuse" and "free field" or mention angles). This is especially handy since many cheap omnis (think RTA-420 and ECM8000) are actually diffuse field microphones.

-Russ
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Re: Measurement mic..
« Reply #5 on: January 02, 2019, 05:57:17 pm »


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