Introduction to SAT II Physics - FreeExamPapers

When the string on a violin, the surface of a bell, or the paper cone in a stereo speaker
oscillates rapidly, it creates pulses of high air pressure, or compressions, with low
pressure spaces in between, called rarefactions. These compressions and rarefactions are
the equivalent of crests and troughs in transverse waves: the distance between two
compressions or two rarefactions is a wavelength.
Pulses of high pressure propagate through the air much like the pulses of the slinky
illustrated above, and when they reach our ears we perceive them as sound. Air acts as
the medium for sound waves, just as string is the medium for waves of displacement on a
string. The figure below is an approximation of sound waves in a flute—each dark area
below indicates compression and represents something in the order of 10 24 air molecules.
Loudness, Frequency, Wavelength, and Wave Speed
Many of the concepts describing waves are related to more familiar terms describing
sound. For example, the square of the amplitude of a sound wave is called its loudness,
or volume. Loudness is usually measured in decibels. The decibel is a peculiar unit
measured on a logarithmic scale. You won’t need to know how to calculate decibels, but it
may be useful to know what they are.
The frequency of a sound wave is often called its pitch. Humans can hear sounds with
frequencies as low as about 90 Hz and up to about 15,000 Hz, but many animals can hear
sounds with much higher frequencies. The term wavelength remains the same for sound
waves. Just as in a stretched string, sound waves in air travel at a certain speed. This
speed is around 343 m/s under normal circumstances, but it varies with the temperature
and pressure of the air. You don’t need to memorize this number: if a question involving
the speed of sound comes up on the SAT II, that quantity will be given to you.
Superposition
Suppose that two experimenters, holding opposite ends of a stretched string, each shake
their end of the string, sending wave crests toward each other. What will happen in the
middle of the string, where the two waves meet? Mathematically, you can calculate the
displacement in the center by simply adding up the displacements from each of the two
waves. This is called the principle of superposition: two or more waves in the same
place are superimposed upon one another, meaning that they are all added together.
Because of superposition, the two experimenters can each send traveling waves down the
string, and each wave will arrive at the opposite end of the string undistorted by the
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