Explaining the sound of scraping

[digiology]

Hi,

I'm new to acoustics in general but I have this instrument which involves lightly scraping the end of aluminum pipes off porcelain glazed tiles (a sort of glassy surface). One tile is on the ground and the other is lifted slightly off the ground for a different timre. I put a video up to show it :

http://www.youtube.com/watch?v=-LP62rnCNAk

I have to be able to explain why this sounds the way it does for my acoustics class, while you can see from the video that the sound of the pipes ring through and is roughly the same pitch and timbre as when the pipes are struck normally (like with a mallet). This sound should be easy to explain (the pipes resonate to a frequency whose wavelength is double the pipe's length) but how do I explain the overtones that are brought out by the scraping? How do I explain the actual scraping sound and the role the tiles material and size plays in this?
Obviously friction is setting it in motion to resonate but the more unusual sounds this produces seem harder to explain. The sound of scraping materials (friction) seems to be undiscussed in my acoustics book.


If you could point me to resources (websites or books) that could help me or answer this directly I would be so grateful. Go easy on me, I'm new to this : )

 

[Strong]

If your friction is acting like a person making "singing water glasses" by rubbing the rim with a wet finger, then I would say it is a form of "stick-slip friction". If you search on these words, then you'll find an explanation. I am not a musician, but I would guess that the overtones from your tubes are no different than from a wind or string instrument. Your tubes have both acoutic modes (as you indicated) and also vibration modes (cirmferential and longitudinal bending). One or more of the vibration modes may also be radiating sound. If their frequency is close to an acoustic mode, then "beating" can occur causing amplitude variation. Your finger may also be modulating the amplitude of the tones that are produced, and causing a complex or distinctive sound. Another effect is damping where your finger is altering the free vibration of the tube by absorbing energy. There are many possible effects (some may be dominant), but some experimentation may be needed to isolate or define the actual effects. You can search on "wind chimes", since there is a lot of info on this topic. The tube properties affect the vibration modes, but not the acoustic modes.

Walt