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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.

[Richard Crowley]

wrote ...
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,

Dunno exactly what you are trying to explain? You have two equations
and about a dozen unknowns. I was never very good at solving for
more than one unknown.

Not clear whether your "timbre difference" is due to the tile
being raised, being at an angle, or because you weren't
scraping the same (with a different hand, etc.) etc, etc.

Also not clear what the purpose of using two different lengths
of pipe are? Since you scraped the flat tile only with the shorter
pipe (and vice-versa) we don't know whether ANY differences
were due to the mode/method of scraping vs. simply the length
of the pipe, etc.

Not that you have to answer these (or dozens of other) questions
here, but they would all seem to have some bearing on what
you are hearing.

[Kevin McMurtrie]

In article
,
wrote:

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.

The friction is acting as an energy source that induces resonation.
It's how most musical instruments work.

Resonation in the real world is never easy to explain. It's not just
the end-to-end wave in the pipe. Standing waves can exist in
transverse, longitudinal, and torsional forms. They can interact in
stable or chaotic manners. Look up a "double pendulum".

[Ian Iveson]

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.

Friction is a complicated thing. The simplest idea is of a
force proportional to, and in the opposite direction to,
velocity. That would make it a simple damping force, which
isn't what is responsible for the sound. Try the same
experiment with treacle.

So something else is going on. Find out how a violin bow
works. Note that in the right hands there is hardly any
scraping noise when played clean. So it may not just be
about a noise and a resonant filter. Perhaps the excitation
process becomes synchronous somehow?

Try tapping the tile, and compare the sound, in the same two
positions.

Compare the sounds of spring and plate reverb if you have
access to such effects.

Look for some info on how violin makers fine tune the shape
of the front and back before assembly. Get some dry sand and
try it.

If you have access to a bow, try it on the plate and the
pipe.

Lastly, are you sure you are doing it right? Is it supposed
to just scrape, or can you get it to ring, like running a
damp finger round the rim of a wineglass?

Ian