Digital Sound & Music: Concepts, Applications, & Science, Chapter 2, last updated 6/25/2013
graphed over time in the same way that a sound wave‟s air pressure amplitude can be graphed
over time. The spring‟s position increases as the spring stretches downward, and it goes to
negative values as it bounces upwards. The speed of the spring‟s motion slows down as it
reaches its maximum extension, and then it speeds up again as it bounces upwards. This slowing
down and speeding up as the spring bounces up and down can be modeled by the curve of a sine
wave. In the ideal model, with no friction, the bouncing would go on forever. In reality,
however, friction causes a damping effect such that the spring eventually comes to rest. We‟ll
discuss damping more in a later chapter.
Now consider how sound travels from one location to another. The first molecules bump
into the molecules beside them, and they bump into the next ones, and so forth as time goes on.
It‟s something like a chain reaction of cars bumping into one another in a pile-up wreck. They
don‟t all hit each other simultaneously. The first hits the second, the second hits the third, and so
on. In the case of sound waves, this passing along of the change in air pressure is called sound
wave propagation. The movement of the air molecules is different from the chain reaction pile
up of cars, however, in that the molecules vibrate back and forth. When the molecules vibrate in
the direction opposite of their original direction, the drop in air pressure amplitude is propagated
through space in the same way that the increase was propagated.
Be careful not to confuse the speed at which a sound wave propagates and the rate at
which the air pressure amplitude changes from highest to lowest. The speed at which the sound
is transmitted from the source of the sound to the listener of the sound is the speed of sound.
The rate at which the air pressure changes at a given point in space – i.e., vibrates back and forth
– is the frequency of the sound wave. You may understand this better through the following
analogy. Imagine that you‟re watching someone turn a flashlight on and off, repeatedly, at a
certain fixed rate in order to communicate a sequence of numbers to you in binary code. The
image of this person is transmitted to your eyes at the speed of light, analogous to the speed of
sound. The rate at which the person is turning the flashlight on and off is the frequency of the
communication, analogous to the frequency of a sound wave.
The above description of a sound wave implies that there must be a medium through
which the changing pressure propagates. We‟ve described sound traveling through air, but
sound also can travel through liquids and solids. The speed
at which the change in pressure propagates is the speed of
sound. The speed of sound is different depending upon the
medium in which sound is transmitted. It also varies by
temperature and density. The speed of sound in air is
approximately 1130 ft/s (or 344 m/s). Table 2.1 shows the
approximate speed in other media.
Medium Speed of sound in m/s Speed of sound in ft/s
air (20 C, which is 68 F) 344 1130
water (just above 0 C,
which is 32 F)
steel 5100 16,700
lead 1210 3970
glass approximately 4000
(depending on type of glass)
Table 2.1 The speed of sound in various media