The pure musical tone produced by striking a tuning fork is said
to have a certain pure pitch or frequency. That frequency is the
number of times that the sound waves compress and dilate in a
second, or, for water waves and other types of vibration, the
number of times the wave rises or falls in a second. Frequencies
are given in hertz, abbreviated Hz, a unit of measurement named
in honor of Heinrich Hertz, a German physicist who in 1887 first
produced electromagnetic waves. One hertz is equal to one cycle
of rise and fall per second. The time between the crests is the
wave period; it is equal to the reciprocal of the wave
frequency. Human beings can detect sounds having frequencies
between 20 and 10,000 Hz. A seismic P wave can refract out of
the rock surface into the atmosphere, and if the frequencies are
in the audible range, the wave can be heard as a rumble as it
travels by the ear. Most earthquake waves have frequencies lower
than 20 Hz, and are usually felt by human beings rather than
felt.