Unexplained audio rumble

D

David W. Richardson

Chapelgrove Films
I'm working on editing a scene for 'Tangled Web', and I've got an occasional audio rumble the source for which I can't figure out. It sounds for all the world like a boom operator's hands moving on a boom pole, with a mic with no shock mount. (Been there!) But we had no boom operator. Sound was recorded using a Zoom H2 with its built-in mics. The H2 was mounted on a mic stand with a boom arm. Granted, we did not use a shock mount. But the rumble isn't specific to movement in the room -- it occurs even when both actors are standing stock still. (There is no one else in the room at all, not even crew or director.)

We had a 2nd H2 located close by -- a foot or two further away from the actors -- and it doesn't seem to have picked up the same rumble. At least, not in the same sections of dialogue. The main H2 was probably 2 feet or so from the actors, and they were not facing directly at it.

For what it's worth, during times when people ARE walking in the room, we don't get any kind of rumble consistent with their movements -- even when two kids are RUNNING through the room, the audio is pretty much clean. So it doesn't seem like it would be vibration being conducted up the mic stand and boom arm. I just don't get it.

I know there's little I can do in post to remove the rumble from sections of actual dialogue -- at least, not without compromising the tonal quality of the dialogue. Right now I'm just trying to figure out where the heck the rumble could've come from.

Any ideas?
 
I've played that clip several times (and perhaps it is thru a low-quality built-in computer speaker) but I can't find any audio malfunction.
 
Really? Interesting. When editing I listen through some headphones that I know have excessive bass. Hmmm.....let me give it a listen on the ol' TV.
 
I hear that on the headphones. That could be some kind of air wave getting into the mic . Did you ave a wind scree on it??? You can use little base roll off and it will be fine but as you see on the tv or speakers u won't notice that. Since headphones have wider range of frequencies you hear more.
 
Well, I'm hearing two different types of noise

First, I hear what sounds like a bumping or perhaps 'rumbling' over the womans voice when she says "homework"

Followed by a raised hissing sound immediately after the word "homework"

Followed again by that bumping sound.
Very odd.

Have you tested out the unit to make sure its not an issue with the H2 itself?
The more I listen to is the less it sounds like something bumping the mic - it sounds too much like a pulsing sound... not as jarring as a bump.

Not much help, i know - just telling you what I"m hearing in it.
 
Luis Caffesse said:
I hear what sounds like a bumping or perhaps 'rumbling' over the womans voice when she says "homework"
Click to expand...

Yes, bumping.

Was it in a apartment building? Was there someone next door, out in the hall or upstairs or downstairs? It sounds a bit like footsteps or a heavy object being put down (or dropped) in another room or floor. Floors can transmit these kinds of low frequency impacts quite well up through a solid, non-shock mount equipped mic stand.

Also, there's a rhythmic pattern to the first four thumps - like a dummer might play on a bass drum. Got any drummer or drum machine owner neighbors?

As for removing it, it would take a fairly aggressive shelving filter cut below about 500Hz or so to make it inaudible, which might thin the voices too much. You could compensate by adding some peaking EQ above that, at maybe 800Hz or so. If you have good DAW skills you could isolate just the thump "events" and process only those spots.
 
Thanks for the input, folks.

Not an apartment, this is a house. No drummers for neighbors -- in fact, the neighbors were pretty quiet that day.

Everyone else in the house was located at the other end of the house, watching the scene unfold on a remote monitor. The only movement was when the kids ran out of the room -- a few moments before this dialoque clip. They immediately went to the monitoring room to watch as the rest of the scene played out. No significant movement in that room -- nobody dropped anything or bumped anything.

I did not put wind protection on the H2, since it was isolated in an interior room with no significant air movement. I no longer have that H2, so I can't test it for problems now.

I'll study the sound further and see what I can glean. I have little to no DAW skills, so that's over my head.

Thanks for the input! If you come up with any other ideas, let me know.
 
Is there anyway it would be some sort of electronic interference?
It almost sounds like a pulse...like something in the signal as oppossed to an actual sound picked up by the mic.

The H2 isn't wireless in any way, is it?
I looked at it on Samson's site and it seems to be a digital recorder.

How was it being powered?
Battery or AC?

I'm assuming battery....but just trying to think of anything else that could have caused it.
 
Luis Caffesse said:
Is there anyway it would be some sort of electronic interference?
It almost sounds like a pulse...like something in the signal as oppossed to an actual sound picked up by the mic.

The H2 isn't wireless in any way, is it?
I looked at it on Samson's site and it seems to be a digital recorder.

How was it being powered?
Battery or AC?

I'm assuming battery....but just trying to think of anything else that could have caused it.
Click to expand...

I can't imagine what kind of electrical pulse there could be. The H2's were powered by A/C adapters. The H2 isn't wireless in any way.
 
chapelgrovefilms said:
I'm working on editing a scene for 'Tangled Web', and I've got an occasional audio rumble the source for which I can't figure out. It sounds for all the world like a boom operator's hands moving on a boom pole, with a mic with no shock mount. (Been there!) But we had no boom operator. Sound was recorded using a Zoom H2 with its built-in mics. The H2 was mounted on a mic stand with a boom arm. Granted, we did not use a shock mount. But the rumble isn't specific to movement in the room -- it occurs even when both actors are standing stock still. (There is no one else in the room at all, not even crew or director.)

Any ideas?
Click to expand...

Hi David,

Yes, that's troubling. Interestingly, using a spectral analysis view on Audition, it doesn't look like a low-frequency problem (yes, I realize it sounds like a rumble). The spectrum analysis at 04.546 - 04.628 clearly shows significant repeating (harmonic) energy bands at 4Khz, 8Khz, 12Khz, 16Khz, 20Khz on both channels (this timeline should otherwise be silent). Then at 04.787 - 05.274 I see lots of energy from about 1K - 5K, with lessor harmonics as before.

I'm still guessing vibrations are your culprit. It would be interesting to test the pole to see if it resonates at 4Khz. I suppose it could be something obscure like an electronics problem with the low-cut filter in the H2 or a common electronics problem like the H2 internal switching power supply, but I doubt it. One thing is certain - your problem manifests itself on BOTH channels. And there-in lies the smoking gun! So if I was a gamling man, I might wager the boom mount to the mic stand wasn't tight, perhaps slipping because the audio engineer didn't tighten it enough at the hinge or some other loose mechanical fitting (just a long shot). Perhaps this will help you see the problem in a new light.

I'd mark the questionable H2 and see what happens on future shoots. Of course in a perfect world, someone with headphones should monitor and catch the rumble during shooting. But then there's the real world...

Good luck, Michael
 
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moonlitnite said:
Hi David,

Yes, that's troubling. Interestingly, using a spectral analysis view on Audition, it doesn't look like a low-frequency problem (yes, I realize it sounds like a rumble). The spectrum analysis at 04.546 - 04.628 clearly shows significant repeating (harmonic) energy bands at 4Khz, 8Khz, 12Khz, 16Khz, 20Khz on both channels (this timeline should otherwise be silent). Then at 04.787 - 05.274 I see lots of energy from about 1K - 5K, with lessor harmonics as before.

I'm still guessing vibrations are your culprit. It would be interesting to test the pole to see if it resonates at 4Khz. I suppose it could be something obscure like an electronics problem with the low-cut filter in the H2 or a common electronics problem like the H2 internal switching power supply, but I doubt it. One thing is certain - your problem manifests itself on BOTH channels. And there-in lies the smoking gun! So if I was a gamling man, I might wager the boom mount to the mic stand wasn't tight, perhaps slipping because the audio engineer didn't tighten it enough at the hinge or some other loose mechanical fitting (just a long shot). Perhaps this will help you see the problem in a new light.

I'd mark the questionable H2 and see what happens on future shoots. Of course in a perfect world, someone with headphones should monitor and catch the rumble during shooting. But then there's the real world...

Good luck, Michael
Click to expand...

Now that's an interesting idea.....slippage! The boom arm was tightened -- HARD -- by me. But the arm was extended so far, and the H2 weighed so much, that I had to put a counterweight on the opposite end of the boom just to keep it from sloooowwwly sinking down to the floor. I'll bet it still had some almost imperceptible slippage that caused the rumble. Good thinking, Michael!

Unfortunately, I no longer have the H2's. I have an H4, though, so that's something I'll keep an eye on in the future if I use that mic stand and boom arm again.

Thanks!
 
It doesn't sound like interference. That usually sounds like a hiss, buzz, or zap sound. This sounds like wind or bumps on the mic.

First piece of advice. Don't record your audio this way. You're lucky this was your only problem, but also it sounds very "roomy". You'd be better off with a short shotgun going right into your camera.

Second, you can probably get rid of it by using a low cut filter. Start at 40hz rolling off at 12db per octave. Don't cut around it, or you'll hear holes in your audio.
 
But the sound occurs in scenes where that actor isn't even on the set. As well as when he's in various different positions in the room -- some even farther away from the mic.

I really don't have a good explanation for the sound yet, so I'm still open to suggestions as to where it may have come from.
 
chapelgrovefilms said:
But the sound occurs in scenes where that actor isn't even on the set. As well as when he's in various different positions in the room -- some even farther away from the mic.

I really don't have a good explanation for the sound yet, so I'm still open to suggestions as to where it may have come from.
Click to expand...

Have you been able to duplicate it with your H4? Could it have been vibration from trucks or other traffic on the street outside coming in through the building structure? As an FYI, you can usually roll-off below about 80-100 Hz with a high pass filter when recording without affecting the speech - most human voices have little or no energy below 100 Hz.
 
To me it sounds like wind noise (I see you addressed this point in which way they were facing, but were people waving their hands?)

If not that then yeah, my second guess would be boom noise. As if the unit was moving on the pole, or the pole itself was "settling" (drooping) slightly... and the friction of the lock was causing the stuttering "wind noise".

It's not coming from an outside source... or the apartment next door... there's no doubt in my mind that the noise is originating right at the mics/recorder... either from boom movement or wind. If the pole is rock-steady (no bounce) and the actors aren't facing the unit, and the actors aren't waving their hands or anything else to cause wind... then I'd get rid of that thing and just run a mic directly to the cam... I think it must be one of those three things though.
 
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