Film-Tech Forum ARCHIVE: soundwaves

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Posted by Matias Bo (Member # 3000) on 05-04-2006, 05:19 AM:

I found this filmclip on the net where sound produces different patterns in sand (or salt or something)

http://www.stumfilm.dk/resonance-video.html

I dont really understand how this works... I mean, I know about soundwaves and how they move as rings in water but this is complicated patterns.

Can any of you smart guys explain this?

Matias

Posted by William Collins (Member # 3773) on 05-04-2006, 06:06 AM:

Just watched the video, thats incredible! i think that may have something more to do with the board itself and the way it vibrates with the sound, but thats just my thoughts! how did you come across that?

Posted by Matias Bo (Member # 3000) on 05-04-2006, 06:11 AM:

I found it on a "stupid videos of peoble doing stupid things" site. Not a scientific site at all... [Wink]

But it is a cool video.

Matias

Posted by David Stambaugh (Member # 1102) on 05-04-2006, 02:03 PM:

I gotta admit that's pretty cool.

Posted by Alex Rolfe (Member # 1648) on 05-04-2006, 07:00 PM:

When you excite the board at a particular frequency, you get nodes, just like with sound in a room. The sand sits still at points where the standing waves interfere destructively (think soft points in a room) and jumps off the points where the waves interfere constructively because the board is moving a lot (think lound points in a room).

You'd probably get much simpler patterns on a round board since all the edges would be a fixed distance from the center where it's driven. In the square, you get dead spots in different places at different frequencies. The pattern features get smaller as the frequencies increase because of the shorter wavelengths.

Posted by Kara J. McVay (Member # 1285) on 05-04-2006, 08:14 PM:

That was really cool, it woke the dogs up, they raised their heads and tilted them at the same time.

Posted by Monte L Fullmer (Member # 2797) on 05-05-2006, 02:47 AM:

quote: Kara J. McVay
they raised their heads and tilted them at the same time.

...replay the video and when a certain frequency arouses them, this would be a good frequency to control their barking instincts - same procedure as a training whistle.. Since their hearing is ten times as sensitive as ours and they can hear frequencies higher than we can..over 20Kz plus.

This certain frequency gives them instant migranes...and confuses them when they want to bark. And where, after awhile they know not to bark.

Posted by Dieter Depypere (Member # 3184) on 05-05-2006, 02:54 AM:

These are called "Chladni's sound patterns". How this exactly works - I don't know. Search in Google for "Chladni".

I saw this in a Physics lesson long ago. Students usually remember this because the sound exciting the board must be extremely loud.

Posted by Josh Jones (Member # 447) on 05-05-2006, 04:02 AM:

pretty cool

Posted by Tim Reed (Member # 137) on 05-05-2006, 04:48 AM:

Standing waves induced in the board, would be my guess. The sound waves are beating against the natural resonance of the board (and its slight variations throughout the media). If the board were cut smaller or larger, the patterns would likewise be different.

Posted by Kenneth Wuepper (Member # 1174) on 05-05-2006, 07:29 AM:

Yamaha manufactured an instrument they called the "electone". It was an analog oscillator/divider organ with a main speaker of foam polymer that was in the shape of a large kidney. Much like the thin wooden sound board of a piano having reinforcing resonance, the 'cone' was excited off center and the sound it produced was remarkably clear but contained many more harmonics than a paper coned round speaker with the same input.

I wonder if you could simulate sound in a room by cutting out the shape of the room and varying where you introduced the sound?

This video may even be of a commercial vibration testing device (shaker table)used for analyzing materials for their resonance.

Posted by Oliver Pasch (Member # 1335) on 05-10-2006, 03:24 PM:

[Eek!]

Hi Matias,

you'll find me pretty scared here just having realized what you've posted. You're not going to remove speakers behind the screens at Fisketorvet in order to have them lying backwards on the floor, covered with fine white danish sand?
I mean at least you should know damn well what it's like when a JBL 4675C8-II falls into the screen whilst the plane-crash-sequence in "Cast Away" (no Tom Hankses to be blamed here, rumourwise more a field service technician of a company conducted by the dark side of force that used - or tried - to certify the quality of some cinema systems, including an alignment of the radiation-angle of a loudspeaker in a screenframe, not careing about mechanical stability...). OK, it was (and still is) only the largest screen my companies operating, 2.39 something close to 25 Meters wide...pretty cheap adventure, wasn't it?

Could you please prevent me from these nightmares?

Best regards to the little mermaid!

Oliver [Wink]

Posted by Gerard S. Cohen (Member # 994) on 05-10-2006, 09:34 PM:

As the pitch rose on the video, my two Siamese cats leaped down off the furniture and beat it down the stairs in a panic.

I remember reading the results of an audio research project to determine the cause of the sonic excellence of old Cremona violins. Tradition supposed some quality in the varnish, but an audio scientist, using equipment making sound waves in resonating wood visible, determined that the back panel of Stradivarius instruments were not planed symetrically, but to thickness patterns quite unsymetrical. That way vibrations produced harmonious overtones, whereas cheap modern violins, with machine-cut and uniformly planed backs, give harsher notes.

He claimed he was able to reproduce some of the mellowness musicians find most desirable by carefully hand shaping the sounding board instrument backs according to the patterns he made visible.