A primer on frequency and sound

This section is a primer on the definition of frequency, our range of hearing, and how we shift frequency to bring waves of subsonic frequency into our range of hearing.

Frequency is the measure of how often something happens, or in the words of the Oxford dictionary, it is "the rate at which a vibration occurs that constitutes a wave." It is measured in repetitions per second, or Hertz (Hz). As shown below, the range of frequencies of seismic waves are much lower than those we can hear:

And this is a human ear. It picks up pressure waves in the air, but only in a certain range of frequencies (marked above) can we recognize those waves as sound.

What happens to sound when we change its frequency? To illustrate this question, we take a recording of a simple clap of hands, recorded with a microphone and slow down its frequency.

Hand Clap, slowed down

Making sounds from seismic data

By analogy to a hand clap, an earthquake emits waves in the solid parts of the earth that can be recorded by a seismometer:

The waves that emanate from earthquakes occur much lower ranges of frequency that what we can hear. In order to hear these waves, we must shift the frequencies (in our computer program).

Here is an example of an earthquake wave sped up into our range of hearing, and then several more examples of the same wave sped up several times, still within our range of hearing. Each bang is the same earthquake.

Earthquake, sped up
(Niigata-Chuetsu Earthquake, July 16, 2007, Magnitude 6.6)

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