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Matched Microphone Pairs - What to expect

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Matched Microphone Pairs - What to expect

Postby Bob Bickerton » Thu May 18, 2017 12:15 am

This follows on from my recent thread to do with purchasing a pair of MKH8090 microphones. Supply delays indicated a matched pair would be sometime away, so I have now received a pair of (factory-unmatched) microphones. Given John Willet's advice that capsules are glued, rather than screwed, into the MKH8000 series, Hugh recommended I test the matching of the mics when received.

I haven't done this test before, so I want to ensure my methodology is correct. I mounted each pair of microphones with their capsules as close as possible and (obviously) with the same orientation. These were situated 300mm distant from a PMC TB2a speaker. I used onboard UAD Apollo preamps, played a 1kHz test tone through the speaker and set the input level to -18dBFS for both channels.

I then recorded a 20Hz to 20kHz test tone which was played through the speaker.

Files were then bounced with one channel of the pair phase reversed using the Logic gain/phase reverse utility plug-in. No other processing was used.

I tested all my 'paired' mics including MKH8020s, MKH8040s, MKH8050s and KM183s (which were all 'factory matched') as well as a pair of TLM193s and of course the MKH8090s.

Interestingly none of the pairs fully cancelled on bounce and the test tone was still audible, though at a much reduced level, and that level varied, presumably due to how well matched the mics were at different frequencies.

My conclusion is that the 8090s are well, if not better, matched in the lows and mids, possibly less so in the highs, but I'm curious to know what I 'should' be hearing from these tests.

I've uploaded the bounced files here: https://we.tl/IzFcC3b4RD. All files of 30secs, 44.1kHz/16bit.

As always much appreciated!

As an aside, I've at least learned my speakers are nowhere near 'flat'!

Bob
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Re: Matched Microphone Pairs - What to expect

Postby Tim Gillett » Thu May 18, 2017 2:10 am

I tried the first two in the list. Unless something wrong with my system or technique, amazingly good result. Complete cancellation at all frequencies.
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Re: Matched Microphone Pairs - What to expect

Postby Bob Bickerton » Thu May 18, 2017 2:49 am

That's comforting, but I'm not hearing complete cancellation!

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Re: Matched Microphone Pairs - What to expect

Postby Tim Gillett » Thu May 18, 2017 9:09 am

Bob, what do the left and right channels of the stereo files represent?
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Re: Matched Microphone Pairs - What to expect

Postby Bob Bickerton » Thu May 18, 2017 9:21 am

Ah ha! Just found the frequency plots in the manuals:

Image

Looking pretty close to me. Slight discrepancy around 10kHz, but otherwise fairly good.

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Re: Matched Microphone Pairs - What to expect

Postby Bob Bickerton » Thu May 18, 2017 9:26 am

Tim Gillett wrote:Bob, what do the left and right channels of the stereo files represent?

The source files were bounced into a stereo WAV but both were centred so shouldn't make any difference.

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Re: Matched Microphone Pairs - What to expect

Postby Tim Gillett » Thu May 18, 2017 9:53 am

Bob Bickerton wrote:
Tim Gillett wrote:Bob, what do the left and right channels of the stereo files represent?

The source files were bounced into a stereo WAV but both were centred so shouldn't make any difference.

Bob

So they're dual mono files representing the difference signal?
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Re: Matched Microphone Pairs - What to expect

Postby Hugh Robjohns » Thu May 18, 2017 10:37 am

Bob Bickerton wrote:Hugh recommended I test the matching of the mics when received. I haven't done this test before, so I want to ensure my methodology is correct.

It's something I normally do by ear simply because if you can't hear a discrepancy it probably doesn't matter, even if it can be measured! Sorry... this is going to be a wordy response:

My methodology requires a large and well-damped (ie dead as possible) studio space (or outdoors on a wind-free day) and a helpful assistant.

Rig the two mics horizontally, one above the other, as close as possible, and both facing the same (forward) direction. The stand should hold them at the mouth height of your assistant.

Route the two mics to separate channels (left/right), set nominal gain for a sensible speech level, and then fine-balance the gains to match when the assistant talks directly on axis a couple of feet from the mics.

I would do that by switching the monitoring to mono with a polarity reverse in one channel and then adjust one mic preamp's gain for the best null. You'll never get a completely perfect null, but 25-30dB should be easily achievable across the full bandwidth.

If at this point there is an obvious lack of cancellation at some specific frequency range that would suggest the on-axis frequency responses are poorly matched and this pair should be rejected as unmatched...

Assuming a good on-axis null was achieved you can now reset the monitoring to stereo, with no polarity reverse. With the assistant still talking you should have a solid, sharply focused phantom centre image.

I then get the assistant to walk slowly around the mics in a circle, maintaining a constant distance and keeping talking while facing the mics. The overall level will fall (and rise again) according to the polar patterns involved, but the phantom image should remain steady and sharply focused if the mics are well matched -- at least over the normal coincident stereo working angle of up to +/-90 degrees (relative to the on-axis direction).

It would be quite normal to find significant anomalies around the back of cardioids and hyper-cardioids, but we don't rely on those regions for stereo imaging so it's not something I'd worry about, other than if the outputs were excessively different, in which case I'd be checking that the two mics really do have the same pattern, rather than one being a cardioid and the other a hyper, or omni, etc... (easy to cock-up if testing switchable pattern mics).

If the polar patterns are different, or the off-axis frequency responses aren't closely matched, the phantom centre image will pull or flick away from the centre -- it's usually most obvious with sibilants and fricatives because most frequency response errors occur at the high end.

If the phantom centre image remains stable across the front half of the mic's pickup, and isn't too bad across the back, I'd accept the two mics as a workably matched pair. If the centre image flocks all over the place, I'd be looking to test alternative mic pairs.

I mounted each pair of microphones with their capsules as close as possible and (obviously) with the same orientation. These were situated 300mm distant from a PMC TB2a speaker.

My concern here would be the proximity of the speaker relative to the spacing of its two drivers. Small differences in height between the two mics are likely to producing significant comb-filtering and thus response differences an octave either side of the cross-over region. If you have to use a speaker, use a single-driver design, or move it further away -- ideally at least 2-feet if not more.

I then recorded a 20Hz to 20kHz test tone which was played through the speaker.

So you are only testing the on-axis response here... which is worth checking but won't tell you much about their ability to create stable images when used in a coincident or near-coincident array when the mics are angle relative to the sound source.

Interestingly none of the pairs fully cancelled on bounce...

I'm not at all surprised. The mics will be picking up different reflections from the floor/ceiling/walls to some extent which will dilute the cancellation, in addition to any inherent minor response differences. As I said earlier, as long as you have more than about 20dB of cancellation that's fine. It only takes a 16dB inter-channel level difference to move a phantom stereo image all the way to the side.

My conclusion is that the 8090s are well, if not better, matched in the lows and mids, possibly less so in the highs
.

Sounds plausible to me! ;-)

As an aside, I've at least learned my speakers are nowhere near 'flat'!

Of course not! But it also demonstrates how tricky it is to measure speaker responses accurately. It only takes the measurement mic to be somewhere other than the manufacturer's design axis (even by a few millimetres), and the room to have some standing wave issues (and what rooms don't?) to measure something completely different from the manufacturer's published responses.

H
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Re: Matched Microphone Pairs - What to expect

Postby John Willett » Thu May 18, 2017 5:23 pm

Bob Bickerton wrote:Ah ha! Just found the frequency plots in the manuals:

Image

Looking pretty close to me. Slight discrepancy around 10kHz, but otherwise fairly good.

Bob

I was just about to say - look at the plots.

These look pretty close to me and, actually, what a matched pair should look like.

Put one plot on top of the other - align the graphs - and look at them against the light.

You will then clearly see how they differ. :thumbup:

But I think you have been very lucky with these two. :thumbup:
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Re: Matched Microphone Pairs - What to expect

Postby Bob Bickerton » Fri May 19, 2017 9:59 am

Thanks everyone.

Hugh, I did the test you mentioned, not ideal as I didn't have an assistant on hand, but I recorded it too and I'm happy enough with the match. So they're keepers.

First outing tonight recording the local Cathedral organ. Actually, my plan was to use spaced omnis, which I did, but I put the 8090s up for comparison. I liked what I heard and will probably use a little of each.

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Re: Matched Microphone Pairs - What to expect

Postby Hugh Robjohns » Fri May 19, 2017 10:01 am

Bob Bickerton wrote:Hugh, I did the test you mentioned ... and I'm happy enough with the match. So they're keepers.


:thumbup: :D

I hear what John was saying about the construction technique employed in these mics, but frankly, if I'm paying the best part of £1k for a high-end mic like anything in the MKH8000 series, I expect matching between unrelated samples to be comfortably close enough for normal stereo applications without any trouble. Anything else would not rate as being of professional standard quality and to me!

I'd have been quite surprised and extremely disappointed if your individual examples had failed to provide a decent match.

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