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Old September 24th, 2012, 12:19 PM   #1
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Ringing noise while setting down a glass

I was boom op for a restaurant scene where an actor set his glass down on a table somewhat forcefully.

For some reason I got a LOT of ringing noise in this recording, as if he was ringing a bell instead of setting a glass on a cloth-covered wood table.

I'm curious if anyone else has encountered this problem and what to do about it if it arises in the future. Attached are the wav file and a picture of the set scene.

My thoughts:

- I ended up accidentally having the headphone volume turned up very loud while filming this scene. I was booming from a position just a few feet from the actors, microphone under the grey blanket on the light stand by the table in the middle. I wonder if it's possible that the ringing was caused by feedback between the mics and my HD25 II sealed headphones. I've seen that a little bit in the past when my boom has been near my headphones, but never anywhere near this bad.

- Looking at the picture now a day later, I think I have a better theory: The table was on apple boxes, and I wonder if the metal table stand interacting with the resonant cavity of the apple boxes could have held that long sustain on the ringing.
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Ringing noise while setting down a glass-img_1537.jpg  
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File Type: wav glass_ringing.wav (384.3 KB, 68 views)
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Old September 24th, 2012, 08:54 PM   #2
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Re: Ringing noise while setting down a glass

I didn't hear any similar resonance in the voices, it's only the glass sound; so I would tend to rule out feedback from your cans.

And, no math here, but I rather doubt it has anything to do with the wooden boxes. (It would take some tricky tests to confirm or refute that source.)

Actually, I suspect it's one of two things: the steel column supporting the table, or the goblet itself. Both should be relatively easy to test, if you still have access to those two items.

The steel column is, after all, a sort of tubular bell, albeit damped at both ends. But it still might have some resonance if properly excited. Tap it with your knuckle, or with something a little harder. Does it resonate at the same frequency that we hear on this test file? Alternatively, rap on the table top with something hard (other than a goblet)... a knife handle, your knuckle, etc. See if you can get the same resonance.

Or what about the goblet? Remember how you can wet your finger and rub it along the rim, and get a resonant tone (a la a glass harmonica)? Get one of those goblets, fill it to approximately the same level, wet your finger, and see if you can make it sing? If so, compare the pitch to the note we hear on the test file.

Gosh, I just love mysteries... ;-)
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Old September 24th, 2012, 10:49 PM   #3
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Re: Ringing noise while setting down a glass

Tom, definitely sounds like the glass is resonating. It's very similar to when I "play" glasses. You fill the glasses with various levels of water to tune them and then you run your finger around the rim to make the glass resonate.

Here's a guy playing one of my favorite pieces, Bach's Taccata and Fugue on glasses.

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Old September 25th, 2012, 01:03 PM   #4
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Re: Ringing noise while setting down a glass

That's the glass itself. Take that into an audio program and you can probably see the sine wave from it. It'll look pretty much like what you get from a tuning fork.
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Old September 26th, 2012, 07:24 AM   #5
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Re: Ringing noise while setting down a glass

I'd love to see a frame grab of the frame where the goblet hits the table. I'm curious about how he's holding the goblet, and whether his hand would damp out the vibrations. I certainly don't rule out the goblet as a possible source of the "ringing" but I'm not ready to rule out the table's steel support column, either.

BTW you can pretty well squelch it with a deep notch at 390 Hz, it seems to be almost entirely one frequency. There is a much quieter resonance up around 700 Hz, it's not very audible until you get rid of the 390 Hz tone first.
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Old September 26th, 2012, 10:20 AM   #6
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Re: Ringing noise while setting down a glass

If the camera didn't have a headphone plugged in, it could be feedback from the speaker in teh camera (if it had one).

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Old October 17th, 2012, 12:25 PM   #7
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Re: Ringing noise while setting down a glass

Later we did some foley work with those same goblets, and I got to hear the same ringing when they were set down on a normal heavy desk (not on apple boxes).

So I think the folks who pointed to the glass itself win the challenge! It seems that the ringing noise occurred when there was no liquid in the glass, but didn't happen when there was significant amounts of water. Lesson learned.
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Old October 17th, 2012, 12:27 PM   #8
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Re: Ringing noise while setting down a glass

Later we did some foley work with those same goblets, and I got to hear the same ringing when they were set down on a normal heavy desk (not on apple boxes).

So I think the folks who pointed to the glass itself win the challenge! It seems that the ringing noise occurred when there was no liquid in the glass, but didn't happen when there was significant amounts of water. Having the actor hold the glass with their fingers towards the top would probably also solve it. Lesson learned.
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Old October 17th, 2012, 03:32 PM   #9
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Re: Ringing noise while setting down a glass

Interesting report.

I would try the test myself, but I don't seem to have any goblets... just Welch's grape jelly glasses.
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