Editing Acoustics (section)

=== Wave propagation: frequency ===
{{further|Sound#Frequency}}

Physicists and acoustic engineers tend to discuss sound pressure levels in terms of frequencies, partly because this is how our [[ears]] interpret sound. What we experience as "higher pitched" or "lower pitched" sounds are pressure vibrations having a higher or lower number of cycles per second. In a common technique of acoustic measurement, acoustic signals are sampled in time, and then presented in more meaningful forms such as octave bands or time frequency plots. Both of these popular methods are used to analyze sound and better understand the acoustic phenomenon.

The entire spectrum can be divided into three sections:  audio, ultrasonic, and infrasonic.  The audio range falls between 20 [[Hertz|Hz]] and 20,000&nbsp;Hz.  This range is important because its frequencies can be detected by the human ear.  This range has a number of applications, including speech communication and music.  The ultrasonic range refers to the very high frequencies: 20,000&nbsp;Hz and higher.  This range has shorter wavelengths which allow better resolution in imaging technologies.  Medical applications such as [[Medical ultrasonography|ultrasonography]] and elastography rely on the ultrasonic frequency range.  On the other end of the spectrum, the lowest frequencies are known as the infrasonic range.  These frequencies can be used to study geological phenomena such as earthquakes.

Analytic instruments such as the [[spectrum analyzer]] facilitate visualization and measurement of acoustic signals and their properties.  The [[spectrogram]] produced by such an instrument is a graphical display of the time varying pressure level and frequency profiles which give a specific acoustic signal its defining character.