Robert Poole, age 10, of Cape Elizabeth, Maine, for his question:

What determines the pitch in a whistle?

The notes from a piping whistle are vibrating pulses of energy fanning outward in all directions. The pitch of the sound is heard as notes, high and low. The difference between high and low pitch is determined by the number of sound vibrations per second.

The pulsing vibrations of sound energy come in assorted sizes called wave lengths. A wave length is the measure from the peak of one vibration to the peak of the next one. Strike the A note above middle C on a piano or pipe this same note on your whistle. The length of each wave length In this particular stream of sound is about 22 feet. It reaches your ear with 440 pulsing vibrations per second. The pitch of this sound is determined by Vibrating frequency of the sound wave the number of pulsing vibrations that you hear in a second of a high pitch sounds have higher frequencies and low pitch sounds have lower frequencies. The A in the octave above the one you just tried has a frequency of 880 cycles. Its vibrations per second are twice as fast as the A in the octave below it. A note in the octave below our first note has a frequency of only 220 cycles. The pitch of each note on a whistle or on any other musical instrument is determined by a strict ratio of sound vibrations per second.

The lowest note on a piano has a frequency of 27 and its highest note has a frequency of 4,000. The average human ear can hear a very wide range of pitch from frequencies of about 20 to 20,000. An odd thing tends to distort the pitch of a sound. Notice how a train whistle rises in pitch as it comes toward you and sinks in pitch as the train chugs away from you down the track. Actually, the sound of the whistle does not rise and fall as it seems to do. Remember, sound waves have to travel. Through ordinary air they fan out at about 1,100 feet per second. This distorts the whistle frequencies of a moving object. As the train approaches the sound waves are crowded closer. They close ranks, increase frequency, and the pitch of the whistle rises. As the train departs, the sound waves are stretched apart. They lengthen into lower frequencies and the pitch descends lower. This odd trick is known as the Doppler Effect.

One of the most interesting gadgets is a so called soundless whistle used to call home the dog. Actually, it is not really a soundless whistle. Its frequencies happen to be higher than 20,000, and hence above the upper limit of normal human hearing. This is known as the ultrasonic range. Dogs have keener hearing than we do, and most of them can hear pitch in this high frequency range. When you blow one of these "silent" whistles, your dog hears and comes romping home for his dinner.