PHYS 201 | Dispersion 7 - DEMO: Dispersion in Sound Waves

The dispersion of sound in air and in solids is described and observed.
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[@llamafrhd]

This KZbin channel is a goldmine!

[@meddelavegaz5153]

Merci

[@upant11]

Amaazzing

[@lucasalexandreschirbel6623]

Whenever you hit the longer slinky, there are going to be two waves right? One travelling upwards and one downwards towards the microphone. Could it be that the initial weird sound we hear is the first direct wave arrival of the wave that originates closer to the microphone and then we hear the wave which has travelled upwards and was reflected back down as the lower pitched sound?

[@rektangel333]

Sorry to bother again but if we took this equation from 4:20 and we know that the spring is from steel (whichs speed of sound is 5790 m/s) and from experiments i got the highest frequency 7000kHz and the lower one 1000kHz the speed of the higher one is 333550 m/s and the lower one is 6753,452 m/s isnt that just a bit big difference???

[@kevinwaddell8720]

Is it just me but does he remind you of a young version of Carl Sagan via looks and sound of his voice?

[@Prof-Hafner]

I was watching a lot of Sagan when I made them, so I might’ve subconsciously started impersonating.

[@rektangel333]

Can i ask what is the k at 4:20?

[@Prof-Hafner]

That is the "wave number". It is simply a different mathematical way to treat the wavelength and often used in spectroscopy and wave theory. k = (2 * pi) / wavelength

[@rektangel333]

@@Prof-Hafner thanks!

[@blower05]

I saw someone explained that the ID and OD difference contribute the dispersion. He reasoned that ID have shorter acoustic path than OD's so caused the dispersion. This is the link: kzbin.info/www/bejne/i6HTdoCXbJypb6s I wonder if leaving the same length of the slinky but varying the ID and OD, shall there be any change?

[@Prof-Hafner]

The shape of the slinky may affect the sound (due to OD and ID), but the basic change is pitch is almost certainly due to dispersion within the solid material. I say this because you get the exact same kind of sound in other objects. Ben Burtt famously used this effect for the "pew pew" blaster sound. He used long steel cables rather than a slinky. I can't find him doing it, but here is someone else ( kzbin.info/www/bejne/nnm8k4RmatZsqpY ). You can also hear pretty much the same sound on a frozen lake. When the ice cracks it generates impulses that disperse as they travel through the ice: silentlistening.wordpress.com/2008/05/09/dispersion-of-sound-waves-in-ice-sheets/ I looked into this carefully before making that video and there are relatively recent scientific investigations into the effect of the slinky shape. If I could get the NSF to fund it I would study it for sure! :)

[@blower05]

​@@Prof-Hafner Thanks! I also noted the acoustic dispersion about the frozen pond which is pretty awesome example to prove some materials are acoustic dispersive, somehow analogous to the light wave counterpart. This also recalled me that I also found some strange sound effect in a just mixed hot cup of cappuccino when stroked by a spoon at the bottom. This sound may also be described and explained by the dispersion. However, this sort of strange sound will be gone after a moment of settling of the cappuccino.

[@blower05]

@@Prof-Hafner Luckily, I can find another person do the same thing as I did before for the cappuccino: kzbin.info/www/bejne/h4GYnqh4nb97ntE