The frequency spectrum of a typical AM or FM modulated radio signal, as it would be seen on a spectrum analyzer.  The vertical axis is signal strength or voltage, the horizontal axis is frequency. It consists of a strong carrier wave (C) at the frequency fC of the radio transmitter.  The information (modulation) carried by the signal is contained in two narrow frequency bands above and below the carrier frequency, called sidebands (SB).  The sidebands are symmetrical about the carrier.  In an audio radio broadcast the sidebands carry the audio signal that represents the sound waves.  The portions of the sidebands close to the carrier contain the low audio frequencies, while the portions further from the carrier contain the high audio frequencies.  Therefore the sidebands tend to fall off farther from the carrier,  and also close to the carrier at points which represent bass frequencies below 20 Hz, the lower human hearing limit.
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Chetvorno