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[Phys-L] Re: open/closed pipes



Open/closed pipes may seem like a simple question, but there are some details that make life interesting.

Consider a clarinet and a saxophone, which look almost alike. A clarinet behaves like it's closed at one end (the mouthpiece end) and open at the other. When you go to the "upper register" (either accidentally by over-blowing or intentionally by opening a hole), the instrument will go up by an octave and a fifth. This tripling of frequency is exactly what you would expect for the next note above the fundamental for a closed-at-one-end pipe.

For the sax, you only go up an octave. This doubling of frequency is exactly what you would expect for the next note above the fundamental for a open-at-both-ends pipe. The difference is not due to the material, but rather the shape. The clarinet has a straight bore, and acts like a true closed-at one-end pipe. The sax is conical, and this widening leads to harmonics like an open tube, even though one end is closed.

The trumpet is like the sax, but complicated slightly by the continuous flare and the shape of the mouthpiece. It IS closed at the mouthpiece end (where there is a pressure anti-node) and open at the bell end (a pressure node). It BEHAVES like it is open at both ends (due to the flare). That is why you may have seen it listed as both! The bell is not just to "get the sound out". (Generally the true fundamental of a trumpet is never played (and is badly out of tune). The notes you hear are the n=2,3,4... modes, but not n=1).


Tim F


Instructor of Physics
Barton County Community College
Great Bend, KS
620-792-9320
"The only thing necessary for the triumph of evil is for good men to do nothing." Edmund Burke



-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-L@list1.ucc.nau.edu]On
Behalf Of Anthony Lapinski
Sent: Monday, April 25, 2005 11:03 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: open/closed pipes


I'm looking for a complete list of all musical instruments divided into
open and closed pipes. I've searched the web, but have only found bits and
pieces here and there. I've even seen a trumpet listed both as an open AND
a closed pipe! Does anyone have a simple list or a good website that
categorizes instruments into these two types?
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