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Exploring Sound



FM sound
A Frequency Modulated wave is a sine wave with a periodically varying instantaneous frequency and a constant amplitude. The average frequency is called the carrier frequency and the instantaneous frequency changes at the modulation frequency . The maximum excursion of the instantaneous frequency from the average is related to the modulation depth .

FM radio
If this were an FM radio transmission, the carrier frequency would be the station you tune to, and you would hear a pure audio tone at the modulation frequency, with a loudness derived from the modulation depth.


FM wave
A Frequency Modulated wave can also be considered as a sum of several waves with constant frequencies. The frequency modulated wave has frequency components at the carrier frequency and at sidebands, spaced above and below the carrier frequency at integer multiples of the modulation frequency.

The sidebands must have a particular balance of phases and amplitudes in order to add to the carrier wave and maintain a constant amplitude. Upper and lower sidebands of the same order are matched in amplitude. The odd order sidebands must be exactly out of phase, and the even order sidebands must be exactly in phase.
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