Mic Orientation - Home Theater Forum and Systems - HomeTheaterShack.com

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post #1 of 6 Old 10-07-15, 09:15 PM Thread Starter
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Mic Orientation

I just received a UMIK-1 Mic and want to use it with REW to smooth out my low-end response from my dual subs and mains. I've done a lot of reading including the REW Help file, the MiniDSP site instructions, and the AVSForum REW guide. One thing that's noticeably absent or even conflicting are instructions for mic orientation. This seems like a fundamental step in obtaining measurements.

MiniDSP says it can be used either way but it's less sensitive off axis.

AVSForum members are adamant that the mic be vertically oriented and pointed at the ceiling.

It would make the most sense to me to point it at the front of the room since that's the way a listener would be facing and where most of the sound is coming from. But I'm not an expert on this stuff (yet).

What is the best practise? Is there any info on the where, how and why of microphone placement that I've overlooked?

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post #2 of 6 Old 10-07-15, 09:49 PM
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Re: Mic Orientation

The datasheet for the UMIK-1 specifies that it's polar pattern is "omnidirectional". This means that nominally it doesn't care which direction it is aimed, and it should have pretty even response in any orientation. Most reference mics are omni's, so this is not at all uncommon.

They are most often located at or near your listening position (or positions), and aimed loosely "towards" your speakers. The rationale behind aiming at the ceiling, I think, is so it can hear side/rear channels. You can try it both aimed forward and aimed up and see if you get different results... though my money is on seeing minimal differences between the two orientations.
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post #3 of 6 Old 10-08-15, 08:26 AM
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Re: Mic Orientation

Welcome to HTS, VirtualRain! Glad you joined us!

It's my understanding - so please correct me if needed - to point the mic at the ceiling for room correction measurements, and at a particular speaker driver for near-field frequency response measurements. Some enthusiasts angle the mic a little (10-20°?) toward the front stage to capture the signal where most of a movie soundtrack (or music) originates.

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post #4 of 6 Old 10-08-15, 08:56 PM
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Re: Mic Orientation

VirtualRain wrote: View Post
I just received a UMIK-1 Mic and want to use it with REW to smooth out my low-end response from my dual subs and mains.
As long as you’re only interested in the lower end of the frequency spectrum, orientation doesn’t matter, as any measurement mic is omnidirectional in that range.

However, full-range measurements of the main speakers is another matter:

VirtualRain wrote: View Post
AVSForum members are adamant that the mic be vertically oriented and pointed at the ceiling.
All I can say about that is, I doubt any of those “adamant” people have any professional experience. If they did they’d know better. Zero-degree orientation for full-range measurements has been the professional standard since the dawn of room measurement equipment forty + years ago. If it’s changed, manufacturers of measurement equipment and mics such as B&K and Goldline (to name a couple) didn’t get the message. Their user manuals and white papers still reference 0° orientation for free field measurements.

Omnidirectional mics (such as used for measurements) become increasingly directional at higher frequencies; typically the transition starts between 2-4 kHz (depending on the size of the mic’s element – smaller elements retain omnidirectional properties higher up the frequency spectrum). Here is a graph showing the response of a measurement mic that was popular before the advent of the USB mics. As you can see, the difference in response at 10 kHz is at least 4 dB, and about 7 dB at 20 kHz.

Dayton EMM-6 On and Off Axis Frequency Response

Courtesy of HiFi Zine

As you can see, if you want to use a mic with 90° orientation, a calibration file is needed to offset the loss in high-end response. Cross Spectrum offers calibration files for both 0° and 90° use, I think I recently read that miniDSP is now offering both for the UMIK (previously they only offered 0°).

As to which orientation is preferable, Herb Singleton of Cross Spectrum Labs and REW author John Mulcahy have long recommended 0° orientation for in-room frequency response measurements of main-channel speakers (no delineation for near field or normal listening positions), and 90° for acoustics measurements (acoustic measurements don’t require a calibration file).

I’ve long held that 90° orientation (utilizing the correct calibration file) used for full-range measurements can be problematic in some rooms, as seen in this case. That said, many if not most people have no issues with their 90° measurements (assuming they do a comparison). But if you measure both ways and get a significant discrepancy, the 0° measurement is the one to trust.

BTW, some of those AVS stalwarts have participated in other discussions on this topic and have seen the graphs in the thread linked above, yet they cling to their position.

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post #5 of 6 Old 10-09-15, 12:42 AM
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Re: Mic Orientation

Wayne P. has given you a very thorough answer.

Just a couple of additional thoughts, not exactly on topic but for the sake of further thoroughness. Beg pardon, Wayne, if you covered these in your linked references, I did not read through them all.
  • The intended purpose has to be considered. For LF, as you intend, it makes absolutely no difference.
  • For Room EQ - with Dirac Live, for instance - the UMIK-1's slight directionality at high frequencies does make a difference. For room correction, definitely point at the ceiling. I experimented with the mic pointing down, hanging by its cable (it seemed convenient) - with less HF pickup from the ceiling due to the mic's HF attenuation at 180°, Dirac Live compensated to give more HF boost and the result was annoyingly bright. Pointing at the ceiling had the opposite result, a nicely controlled high end that was just right, more what I would have expected from the chosen target curve.
  • Measurements or EQ of a surround system require the mic to be pointed up for even coverage of all speakers.
  • For speaker measurements, point directly at the speaker under test, as Wayne P. says, the way it has been done for time immemorial.
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post #6 of 6 Old 10-09-15, 01:48 AM Thread Starter
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Re: Mic Orientation

Thanks guys. I guess it makes sense that different types of measurements call for different orientations (or it doesn't matter for low frequency measurements).
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