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Re: wind instrument +- filtering of broadband noise /long/



At 15:08 12/9/00 -0500, you wrote:
Thanks to all who have contributed to this topic. It has been very
interesting. If anyone has the patience to reinforce my improving
understanding, I would greatly appreciate it.

I do the probably common demonstration in which a large lighted laboratory
gas burner, e.g. Fisher burner, is placed on the floor and a long cardboard
carpet roll tube is lowered over it until a very loud low pitched sound is
produced. Is it true that even though this clearly IS a hissing source,
there is more going on than "a resonant filter applied to a broadband noise
source" (from J.D., I believe)? For example, I picture the air in the
tube and the gas in the burner flame coupling so that they both vibrate at
the resonant frequency of the tube. Is this a reasonable model? Is this
"mode locking"? How do I explain the large increase in sound intensity to
my students?

Thanks for any help here.

Tom McDonald
Harbor Springs High School
Harbor Springs, Michigan

It was a great pleasure to me to find that Xerox Corp's TextBridge
was more than equal to the chore of decoding to text, a couple of
pages of Physics, Starling & Woodall [1950] expounding experimental
acoustics.
The pages were scanned at 600 lpi, with a total of two letters
miscoded!
The excerpt is rather too long for convenience, but I hope you will
enjoy its period charm.
(The rotating mirror refers to a visualization method for flame
oscillations)

------------------------------------------------------------------

CHAP. 36] FREE AND FORCED VIBRATIONS 873

Oscillations of metal rods and plates can be maintained by holding a lump
of solid carbon dioxide against them (Waller, 1933). Solid carbon dioxide,
which is made commercially on a large scale, sublimes at about 780 C. The
rapid evolution of gaseous carbon dioxide when the metal makes contact with
the solid carbon dioxide exerts a considerable force on the metal object,
driving it away. The sublimation then ceases until the vibration of the
object again produces contact, and so on.
Another example of oscillations maintained by thermal energy is provided by
the singing flame. A small gas-flame is inserted into a fairly wide,
vertical tube. In favourable conditions a note is heard when the flame
reaches a certain position. if the tube is of glass and examination is
made with the aid of a rotating mirror, as described later (Chapter 38),
the "singing" flame is found to be fluctuating violently. It has been
shown that the flame burns most strongly, and hence supplies heat to the
air at the greatest rate, when the air surrounding it is in a state of
maximum compression, and therefore just on the point of expanding. This
state of affairs can be likened to giving a pendulum bob a push whenever it
is at one end of its swing, so as to maintain its oscillations. The flame
is periodic because there is a reaction on the pressure of the gas
supplying it, as well as the variation in air-supply; and it is found that
no singing can take place unless the length of the gas-supply tube is
correct in relation to the natural frequency of vibration of the gas-filled
vertical tube. The flame is most effective when placed at the position, in
the vertical tube, of greatest pressure-change~that is, when it is at a
node. It will not work when placed near the open end, since this is an
antinode for vibrations of the air-column. It may be remarked that the
singing flame emits a very pure note, almost free from overtones.

The howling tube consists of a wide tube (usually metal) containing a piece
of metal gauze placed across the tube about a quarter of the way up. This
gauze is made red-hot by a flame or other means, and then 1eft. A very loud
note is soon heard, continuing until the gauze has cooled. This time the
note is very strident on account of the presence of strong overtones. The
transfer of heat from the gauze, which accounts for the energy of the
oscillations, takes place at its greatest rate just before the compression
is greatest, for the air is then streaming upwards with its greatest
velocity towards the node at the centre of the tube. This supply of heat
assists the expansion of the air which takes place in the next phase of the
cycle, and the transfer stops, or at /over

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874 PHYSICS [CHAP. 36

least drops, during this part of the motion as hot air is being driven down
again over the gauze.
A stretched horizontal wire heated by an alternating current can be set
into vibration if its natural frequency is equal to that of the supply. The
rate of supply of heat to the wire is a maximum twice in each cycle of the
electrical supply, and the temperature of the wire therefore also
fluctuates at this double frequency. Owing to thermal expansion, the wire
is thus slightly longer twice per cycle. and is therefore able to vibrate
in its fundamental mode.

/snip/

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brian whatcott <inet@intellisys.net> Altus OK
Eureka!