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Curran919

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Reply with quote  #1 
Literally, here's a short youtube video:


Notice how when he hits the gong at 00:12, the perceived sound intensity increases over the next 3-4 seconds.

At first, I thought of it like this: If you had an accelerometer on the gong, you would see the maximum amplitude directly after the (final) strike and it would then undergo free vibration. However, given the low modal damping designed into the gong (0.3-0.5% apparently, contrary to most western drums), the decaying plate vibration would essentially act as a steady-state excitation of the room acoustics, which would experience "forced vibration". If it excited an acoustic resonance, then the acoustic energy could build up over a few seconds, before the structural amplitude decreased and the sound followed.

However, apparently that is not the case. Check out this PHd thesis (what a topic...). Apparently those aforementioned damping values only apply to a gong in a vacuum. Otherwise, the system is actually incredibly non-linear, with vibration amplitudes > gong thickness, and large proportion of air mass loading to gong mass.

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...the most salient feature appears to be a ”pitch glide” effect... the resonant frequencies of the structure depend on the amplitude of vibration. After the strike, the amplitude grows quickly... during the instants of rapid growth and decay, the nonlinear phenomena account for the perceived pitch glide effect.


The pitch glide in the video almost sounds like a Shepard Tone for a bit, as the pitch is increasing. The acoustic guys know that your A-weighted curve has a maximum at ~1000Hz, meaning we are most sensitive to 1000 Hz sound, and less so to the 50-150Hz Gong sounds. Ostensibly, when the gong starts out at low frequencies, we don't hear it as much, but as the pitch shifts, even though the plate vibration amplitude and acoustic wave amplitude is decreasing, the effective loudness of the sound increases.
RustyCas

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Reply with quote  #2 
Very interesting. You know you’re a vibration geek, right? 😉

As vibration analysts, “hearing” is one of our most valuable tools. I’m always surprised at how easily I hear problems, often before I even get to a machine, and at how you develop a memory of what certain problems sound like. Using headphones while I collect data has been a real game changer for me. I can’t imagine working without headphones now. The 2140 w/ Bluetooth and a pair of OSHA approved noise-isolating earbuds makes it extremely easy, and comfortable even for all day use.

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electricpete

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That is an interesting one all right.  

My very first thought after reading your words and before watching the video: maybe there are two modes with very close frequencies excited… resulting in a beat, and for whatever reason it starts in the phase relationship where the beat is a minimum.   That’s the only linear explanation that came to my mind. Your first explanation was also linear (exciting a room resonance over time).

And linear is where we spend most (all?) of our time thinking about in analyzing vibrations.  Open the door to non-linearity and who knows what might be hiding on the other side.  Obviously some pretty weird stuff.

Pitch glide – maybe.  I think he was trying to demonstrate the pitch glide effect starting around 1:15. 

The picture that comes to my mind is many unorganized independent vibration waves rebounding back and forth across the gong and maybe the non-linearity enables them to slowly organize together somehow.  Who knows.

Interesting that the guy striking the gong appeared to have a very specific place where he wanted to hit it, but it wasn't in the center.  

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You know you’re a vibration geek, right?

Reminds me of something that happened to me awhile back.    Every day off I spend a little time with my morning coffee on my back porch practicing a sort of meditation by relaxing and watching the world go by  - with the objective to leave my hurried day to day thinking behind and get in touch with my inner self... or something like that. 

….I was watching the wind blowing in the trees for awhile, soaking in the rhythms.  Suddenly I realized – the tiny twigs had a tendency to jiggle quickly while the big trunks had a tendency to sway slowly, and the in-between branches tended to move at a rate in between.  Each of those was swaying roughly at their own resonant frequency, excited by the broadband wind.  (!)

 My conclusion: you can take the engineer out of the office, but you can’t take the office out of the engineer.

RustyCas

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Reply with quote  #4 
Pete, I try that same sort of meditation, but I fall asleep so fast I don’t know if it works or not.
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John from PA

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Reply with quote  #5 
Quote:
Originally Posted by RustyCas
Pete, I try that same sort of meditation, but I fall asleep so fast I don’t know if it works or not.


Go to https://www.researchgate.net/publication/271943820_Study_of_vibration_and_sound_characteristics_of_a_copper_gong and enjoy your nap.[biggrin]
ivibr8

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Reply with quote  #6 
So modes 3, 4 and 5 are close together and dominant in SPL.
Does this imply the close modes of the copper clappers are clean and coherent?

I hope the copper clappers don't get stolen or the cops will get involved with the caper


 
OLi

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Reply with quote  #7 
It is similar as blades in circular blad sawmill but there it is the other way around and some funny patterns are cut in them to avoid cracking.... but the behavour is strange. When cutting frozen timber it is even worse.
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Curran919

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Reply with quote  #8 


yikes, that was a bottom-of-the-barrel type of article...

I was really hoping that it was written by an undergrad, but i looked the primary author up on linkedin, and he had been a professor already ten years when that article came out.

It doesn't even mention linearity... [frown]

 

Quote:

My very first thought after reading your words and before watching the video: maybe there are two modes with very close frequencies excited… resulting in a beat, and for whatever reason it starts in the phase relationship where the beat is a minimum.   That’s the only linear explanation that came to my mind. Your first explanation was also linear (exciting a room resonance over time).



of course the beat frequency is the first thing that comes to mind for most people. I had a much longer discussion about that in my company. I don't think it is the driving factor here, as you reallz don't hear the periodic nature of what the beat frequency would be. The volume just increases for 4 seconds, then decreases for the duration of the ringdown. I wouldn't put tuning the gong to have semi-repeated roots which generate a long beat frequency out of the realm of possiblities. After all, church bells are tuned to have the second mode as a major fifth of the first node so that they are the first and second overtones of a phantom fundamental that we hear, yet doesnt actually exist.

An interesting quote came out of the reddit post on this, which gives a bit of insight into what is going on:

Quote:
I believe he is making the bloom speak by silently priming the outside of the nipple: a very difficult task apparently.


I wasn't able to find much explaining this phenomenon, but it definitely is gong-speak. He does hit a number of times when he excites the gong, and 'blooming' sure does sound like a great way to describe the observation we are discussing, so perhaps it only comes around because he hits multiple times... If he is exciting different modes with each hit, and they are almost the same frequency, then this could support the beat frequency, but then how could he so precisely time the phase of a 100 Hz mode to get the bloom and not the opposite?

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