[[email protected]]

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.

[Scott Dorsey]

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, 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.

It's the same mechanism that makes bowed strings work. Ask your local
library for a copy of "The Physics of Music" published by Scientific
American. It's a bunch of reprints of old SciAm articles, and the one
you want is by Carleen Hutchins of the Violin Octet fame.

Basically, it's a matter of alternately slipping and grabbing the surface,
and it's a broadband excitation that requires a resonant system (like
a string or a crystal glass) to give a good loud tone.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[[email protected]]

Ok, I see, so the surface of the tile is sliding against it but
sometimes 'grabs' an edge to pulling it only to be released as its the
force of me moving the pipe exceeds the friction holding the pipe in
place. I presume that its then slightly muted by the fact that the
pipe is rubbed against the tile while vibrating as it touches the tile
after 'excitation' (as you put it).

Can you explain what you mean by Broadband excitation?

[Scott Dorsey]

wrote:
Ok, I see, so the surface of the tile is sliding against it but
sometimes 'grabs' an edge to pulling it only to be released as its the
force of me moving the pipe exceeds the friction holding the pipe in
place. I presume that its then slightly muted by the fact that the
pipe is rubbed against the tile while vibrating as it touches the tile
after 'excitation' (as you put it).

Right.

Can you explain what you mean by Broadband excitation?

It's not just sticking and letting go at a constant rate, it is producing
vibration of lots of different frequencies. So it would sound like noise.
BUT, when you use noise to shake a resonant system, you get something
like a pure tone with a few harmonics. As a result, violins give you a
nice pure-sounding note, but it's NOT the same tone quality you get if you
pluck the string.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[[email protected]]

Ok, that makes sense, the vibration's frequency is changing
constantly, so is there an obvious link between the current frequency
of vibration as a note resonates (you'll notice sometimes it doesn't
resonate and therefore doesn't give any definite pitch). I'm getting
the impression that the pipes are 'tuned' to certain frequencies of
vibration and only result in resonance when one of these frequencies
of vibration is exerted on the pipe at particular points in time.
Perhaps I read in to what your saying though, hopefully you can clear
that up : )

When you say that using noise to shake a resonant system will cause a
pure tone with harmonics, how are the higher pitches (later in the vid
I think) explained? Although these higher frequencies must be related
to the normal lower frequency (the 'fundamental' I suppose) its
actually perceived as a pitch difference (as if the fundamental was
missing or now of a higher frequency). This is unlike a violin with
the difference is simply timbre is it not?


You've certainly helped me allot already with thinking about how it
sounds, cheers.

[Scott Dorsey]

wrote:
Ok, that makes sense, the vibration's frequency is changing
constantly, so is there an obvious link between the current frequency
of vibration as a note resonates (you'll notice sometimes it doesn't
resonate and therefore doesn't give any definite pitch). I'm getting
the impression that the pipes are 'tuned' to certain frequencies of
vibration and only result in resonance when one of these frequencies
of vibration is exerted on the pipe at particular points in time.
Perhaps I read in to what your saying though, hopefully you can clear
that up : )

Right. Same mechanism that a violin string uses.

When you say that using noise to shake a resonant system will cause a
pure tone with harmonics, how are the higher pitches (later in the vid
I think) explained? Although these higher frequencies must be related
to the normal lower frequency (the 'fundamental' I suppose) its
actually perceived as a pitch difference (as if the fundamental was
missing or now of a higher frequency). This is unlike a violin with
the difference is simply timbre is it not?

Well, in the case of a pipe, it has multiple resonant modes, remember.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[[email protected]]

I see, do you know what resonant modes these are or where I'd find
information on them? I presume each possible frequency is associated
with each mode?

[Scott Dorsey]

wrote:
I see, do you know what resonant modes these are or where I'd find
information on them? I presume each possible frequency is associated
with each mode?

You can find THAT out with a different article in the same Physics
of Music book that I suggested. It's a good introduction to the subject.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[[email protected]]

I've been scouring the web for the last two days on this information
and have found fairly little. I found that book at http://asadl.org/
but I've not been able to download it, I believe that a subscription
is required and no other sites seem to have it.

[Scott Fraser]

Well, in the case of a pipe, it has multiple resonant modes, remember.
--scott

And in this case the pipe is behaving like a vibraphone bar, not like
an organ pipe, so the overtone series contains many irrational
partials.

Scott Fraser

[Scott Dorsey]

wrote:
I've been scouring the web for the last two days on this information
and have found fairly little. I found that book at http://asadl.org/
but I've not been able to download it, I believe that a subscription
is required and no other sites seem to have it.

It's a book. It's not a thing you download. You go to the library and
you ask for a copy. Any good college library should have it, and any
library in the country can get one by interlibrary loan.

The web is probably the worst place in the world to look for actual
technical information.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Scott Dorsey]

Scott Fraser wrote:
Well, in the case of a pipe, it has multiple resonant modes, remember.
--scott

And in this case the pipe is behaving like a vibraphone bar, not like
an organ pipe, so the overtone series contains many irrational
partials.

And that, in short, is why all instruments don't sound the same when they
are playing the same note.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[hank alrich]

Scott Dorsey wrote:

Scott Fraser wrote:
Well, in the case of a pipe, it has multiple resonant modes, remember.
--scott

And in this case the pipe is behaving like a vibraphone bar, not like
an organ pipe, so the overtone series contains many irrational
partials.

And that, in short, is why all instruments don't sound the same when they
are playing the same note.

They're not actually playing the same note; only the pitch remains.

--
ha
Iraq is Arabic for Vietnam