First of all let's get rid of those uninteresting already defined spectrums, mainly of light, by doing this: In general, a frequency spectrum is a range of frequencies that have been analysed to reveal the characteristics of each frequency (amplitude and sometimes phase as well). Any decent amplifier of today has equalizing features that are actually a way of subclassing the entire frequency spectrum of the sound coming out of your speaker into smaller spectrums and change their characteristics seperately (altering bass+treble, or having eg. 16 different bands - WinAmp feutures 10 bands)
Spectrum analyzers are being used frequently by electronic engineers and can determine behaviour of signals. They output frequency spectrums of the signals they recieve.

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