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.