low tech mic matching

[antichef]

I have been using a pair of SM81s for stereo recording for some time - I got them in different places and they are of very different ages - they even look different. I had heard that Shure says you can count on any two SM81s being a stereo match, but I wanted to see for myself. Following the advice on this board, I rigged up a pink noise generator and recorded the noise with both mics onto to different tracks, then shifted phase on one track.

Here's a crappy cell phone picture of the two mics:



The one on top is beat up and yellowed (although you can't tell because of the quality of the pic, and the one on bottom is in good shape, physically. The one on top also has a different exterior design, indicating its age.



I used pink noise ( http://en.wikipedia.org/wiki/Pink_noise ) instead of white noise for the test, because I think it resembles the sounds we work with in terms of frequency energy, and it's easier on the ears/monitors/etc.

I use Logic Express 8. I created an audio track and used one insert -- Utility -> Test Oscillator -> Mono -- that has a "Pink Noise" option - I clicked that option, then set up the two SM81 mics right next to each other pointing straight at the bigger cone in one of my cheap monitors (I disconnected the other monitor so it would be quiet). My monitors are Edirol MA-15D -- I use them a lot because they're baby resistant and sound pretty good (I also have some KRKs that I pull out when the baby's not around, but she's always around ) . They *don't* have a good bass response, and so my test isn't valid to show the actual frequency response of the SM81. No big deal, since I'm only trying to show matching between the mics, or lack thereof. The bass fall-off of the monitor would tend to conceal a matching problem in the very low frequencies, but I'm not too worried about that, based on the results you see everywhere else. I hear the SM81s aren't famous for a really strong bass response, anyway.

I associated each SM81 with its own audio track, and turned off the output ("No Output") so that I wouldn't get feedback. Then I recorded the pink noise sound through the mics on to the tracks (I did this with some other mics, too).

After that, I re-established outputs ("Out 1-2") for the recorded tracks and inserted the "Gain" plugin ( Utility -> Gain -> Mono), which, in it's default setting doesn't change the sound at all. It does include a phase inversion option, and that's why I used it. Next, I turned on track automation, and on one of the tracks, I picked the Phase Invert option for automation. I then enabled the EQ analyzer on the output so I could see as well as hear what's going on.

Here's a screenshot of the analyzer when the phase of the two tracks was normal -- tracks 4 and 5 are the SM81s (all other tracks are muted, but you can't see that in the screenshot):



You can see the rough resemblance to the pink noise slope, but with a big hole at the low end -- again, the monitor is probably the most responsible for this.

And here's a screen shot of the analyzer when the phase of track 4 is inverted:



You see a little smidge of mismatched sound, but you can also see how very much of the waveform is canceled out.

Here's an mp3 of that same thing, with the track automation - first the phase is together, then inverted, then back together -- warning - this is staticy when the phase is together, and I'm not sure how loud it'll sound to you -- you can hear how much of the sound is canceled when track 4's phase gets inverted:

http://www.recursor.net/sm81pinknoise.mp3


Cool, huh?

[antichef]

for comparison's sake, here's the same exercise with a pair of AT4033s that I got together (used) at the same time and place, but with no representation that they were matched. They look identical (so I'll spare you the crappy picture), so I was kinda hoping they were matched, but witnesseth:

here's a screenshot of them with the same phase (tracks 2 and 3, and you can see the same bass response issue as before -- low end monitors...):



and again out of phase:



and here's the corresponding mp3:

http://recursor.net/at4033pinknoise.mp3

not nearly as close as the SM81s. Truth be told, I've used these two for stereo recording before, and didn't notice a problem. That could be a function of my untrained ear and sub-fantastic monitoring setup, though.

[rayc]

Nice piece of work matey.
I take it you're reasonably happy with the SMs.
That's a big cancellation with the inversion section.
I didn't listen to the other pair as the point was all about the SMs.
I was thinking about an easy way of doing it _ I might be able to approach it with waverepair - it has a noise sampling component in which the ignal (usually the run in on an LP) is highlighted in a wave file. The prog takes a "fingerprint" & then highlight the section you want that noise removed from & click remove noise or decrackel depending on the rob. I'll have to think about depth 7 sensitivity of the removal but it might work. I only have a type of mic taht I have multiples of - a trio of Sony ECM 19bs. They are pretty good fun to play with so I'll stuff around & see if any two match to any degree.
Thanks for kick starting my head.

[antichef]

here's a kind of epilogue for this post -- I saved this pink noise DAW project, and as I've had the chance I've tested almost all my mic pairs to see how they matched -- I have a pair of SM7b mics that I got new from the same place a different times (both were demos and below market price) - they didn't test out to match too well, and it occurred to me that I liked one of them better than the other, so I marked it with some embossed tape and used it primarily. I was subconsciously bummed about it.

Just now, I, um, thought to examine the switches on the backs of the mics - the one I like was set to be flat, and the other was set with the bass cut and the mid boost - I set that one to flat and re-tested - nice match!! Doh!! My respect for the Shure mics grows over time.

[superspit]

very nice testing procedure there mate!...well done!
guess what?...i was seriously going to ask you about them switches!...yeh! seriously!
cheers,
ss.

[sweetbeats]

So if I were to do this with my Studio Projects B3's, would I just mount them as close together as possible and as much equidistant from the monitor (I thought it might be a better test if I removed my sub and one monitor speaker from the monitor setup signal path)? I have a multi-mic bar and shock mounts for each.