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Recently I was recording a PASS device (Personal Alert Safety System) that is used in a firefighters SCBA (self-contained breathing apparatus). Using motion it alerts when a firefighter may be in trouble. It emits a very loud and high pitched series of sounds.

Most of my recordings are clean except for in a couple parts of the devices alerts.

[soundcloud]auddity/pass-device-distortion[/soundcloud]

http://soundcloud.com/auddity/pass-device-distortion

I'm curious if more experienced ears can recognize the cause of this distortion. Is it my microphone or maybe the pre-amp on my recorder?

Signal flow is a Rode NTG-2 into a Marantz PMD661. I also recorded it with a Sony M10, and that came out clean.

I'm guessing that the mics condenser couldn't handle the frequency at such a high SPL.

Other notes: I was positioned apx 10-12 feet away from the PASS device (M10 was about 8-10ft away). The Marantz input never peaked. It's meter went no higher then -6db. Also the sample linked was recorded in a live burn building, with metal walls concrete floor and wood barriers hanging about.

Would like to know your thoughts. Thank You.

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  • Well I love the sound of it!
    – Andrew Spitz
    Sep 20, 2011 at 7:01
  • @Andrew It is pretty cool :) There's a series of tones during its alert cycle that I swear is some of the first sounds you hear of the USS Kelvin in the new Star Trek. All day on set I had the theme music in my head from just hearing those series of beeps.
    – Auddity
    Sep 22, 2011 at 1:04

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yeah, sounds like the mic not being able to handle it. actually an interesting sounding distortion to my ears, though that's probably no consolation to you. :)

I've actually had similar results with high spl recordings and shotguns. I feel like even the much more expensive MKH60 doesn't give me very good results when the dbs get up above 120 or so.

here's a purely unscientific and speculative guess as to why this is:

Shotgun mics use interference tubes to enhance directionality. These work by allowing direct sound through the front of the tube unimpeded, and simultaneously rerouting off-axis sounds through the slots in the side of the tube - causing partial phase cancellation and reducing the apparent level of the re-routed off axis sounds in relation to the directly recorded on axis sounds.

when you use a shotgun in a high-spl situation, the rerouting of the air may turn into actual acoustic distortion that occurs before the sounds hits the capsule. This is probably doubly true with sine-wavy sources played at super high levels in reverberant spaces like yours.

this would also explain why your non-shotgun mic caught a clean recording.

again that's just a guess, but if I were you I'd bring an omni to your next high spl adventure. :)

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  • @Rene That's interesting, I wouldn't of thought of it like that. About it being in a highly reverberant space: I had recorded the PASS device in a small room, it was a small storage room (and deader acoustically), I was closer to the device as well. While I still had the same distortion, it was alot less then what I got inside the live burn building. I wonder if it happend to be comb-filtering and I was in the right spot to pick up constructive interference in that frequency range.
    – Auddity
    Sep 22, 2011 at 1:00
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Personally I think it's cool, and in this case a touch of distortion's going to help it sound loud, which is the intended effect right?

In terms of what might be happening technically, the analog stage can clip before the signal even gets in to the digital realm so those meters aren't necessarily telling the truth.. could it be that?

Maybe try recording with an inline attenuator and see if that helps

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It looks/sounds very likely that the Rode ran out of power-rail headroom. It is highly unlikely that there was distortion "in the air". This could only occur if the air particles were accelerated beyond the local speed of sound effectively creating mini sonic booms.

Interesting concept though, does a shotgun body design either a) increase the air pressure modulation beyond 200 Pascals? b) increase the local temperature to cause increase in speed of sound?

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  • This makes me want to go back to school and study physics. Unfortunatly they don't offer classes on 'acoustical physics'. I would love a class or program based on just that!
    – Auddity
    Sep 22, 2011 at 1:14
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Several years ago I had to fly to LA to do a last-minute VO session in a talent's hotel room. We grabbed the Marantz PMD-661 with a Rode NT-2 and thought we were good. The first session was rushed and the VO talent was so high-strung that we didn't do a thorough level-check. Thought we'd keep a close eye on the meters and all would be ok.

Wrong.

As with your experience, the meters never topped -6 but we ended up with some distortion. It works well for your file, but it nearly wrecked our VO. And only 'nearly' because the intended delivery was 48 kbps OGG for the web, rendering the entire thing a mess anyway. And the client was entirely pleased. :)

Live and learn.

I attributed it to the pre-amps in the Marantz, seeing as how I'd never had condenser distortion off the NT-2 prior to that, with strict VO usage. But it's funny that the meters never topped -6. I'd have to guess now that it was the Rode.

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  • You didn't have the level control set to ALC did you? The only time I ran into a bit of distortion was when I had that setting on. Was fustrating because I had changed that setting on my presets on accident.
    – Auddity
    Sep 22, 2011 at 0:45
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I don't think it's the mic...

How loud was this alarm? A personal alert safety system?

Rode "specs" it at 131 dB. So I think it's safe to say the mic could handle it unless you placed the mic 2 millimeters from the source.

Was the mic placed near a bald wall or other hard surfaces?

I personally think it's the first input stage of your pre-amp not able to handle it.

I have this problem on some pre-amps when recording drums - the level looks fine but it's distorting due to the first stage not being able to handle the sound level.

I say if you really want to know, put a pad (shure makes some good ones) and go back and put it in line before the pre-amp and see what you get.

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