Re: What causes the wind to whistle?

Dear Brian,

Background: To answer this question, let us first understand the origin of sound waves. Sound waves occur in fluids and plasmas and are generated by variations in pressure. These variations in pressure lead to the generation of what are called longitudinal waves (waves in which the medium oscillates in the direction of the transmission of waves). The longitudinal waves themselves have minute variations in pressure throughout their zone of propagation. These pressure variations are the ones that are picked up by our ears and sensed by our brains. Thus, sound is a sensation created in the human brain in response to pressure fluctuations in the medium (usually air).

These pressure variations are longitudinal waves which can have a large set of frequencies and wavelengths. The distance between any two waves is called the wavelength and the time interval between waves passing is called the frequency. The brain associates a certain musical pitch with each frequency; the higher the frequency, the higher the pitch. But the human brain cannot perceive all possible frequencies - there are upper and lower limits to this. The waves that have audible frequencies are termed as sound waves. For more details on this, look up Resnick and Halliday's High School Physics which, I think, has good details on sound waves.

The information given in the previous paragraphs is a sufficient enough background for answering out question.

Question A. What causes the wind to whistle? How does this relate to the playing of any instruments?
Answer: When the wind blows, it passes through various obstructions. In the process of passing through these obstructions, a disturbance is created in air that leads to the generation of pressure variations. Any disturbance in a medium even if the medium is moving, would lead to some kind of pressure variation within it. These variations lead to longitudinal waves, which, if in the audible frequency range, would be heard by us. Note that in some cases this pressure variation could be generated by the wind being obstructed by your ears, which is heard out quite loud. Also, if you take a long stick and whip it in the air you would hear a sound similar to the wind blowing. In an open place like the desert (a largely obstructionless place) you can usually either hear the wind passing your ears or the sound generated by the same passing over dunes. (The sound of the whip cracking, on the other hand is because of a shock wave created by the whip end traveling faster than the speed of sound: if you are interested in wave propagation in case of supersonic objects, do write to me).
In general, this does relate to playing of instruments such as the flute but is not related to the playing of string instruments like the guitar. In case of the string instruments the primary source is the standing transverse wave created on the strings while in case of the flute it is the standing wave in air. In case of the wind, it usually would not be a standing wave. Standing waves are composed of even number of wave components traveling in opposite directions in a way such that they don't have any particulate motion within them.

Question B. What could you do to make the wind whistle in an architectural space?
Answer: Theoretically it should be possible to do it. If you have a high speed fan blowing air through a set of rigid rods or similar obstruction, it would generate the effect of wind blowing. In fact, any kind of obstruction that would make the wind pressure to change rather abruptly would create sound that would be close enough to that created by wind. The abruptness of change in this sound would in turn effect the variation in the pressure change thereby changing the frequency. There are many ways to achieve this kind of a setup - all you need is a kind of adjustable obstruction to sound.

--Arjun Kakkar
a_kakkar@vsnl.com