Page "Bandwidth (signal processing)" Paragraph 9

In some contexts, the signal bandwidth in hertz refers to the frequency range in which the signal's spectral density is nonzero or above a small threshold value.

That definition is used in calculations of the lowest sampling rate that will satisfy the sampling theorem.

Because this range of non-zero amplitude may be very broad or infinite, this definition is typically relaxed so that the bandwidth is defined as the range of frequencies in which the signal's spectral density is above a certain threshold relative to its maximum.

Most commonly, bandwidth refers to the 3-dB bandwidth, that is, the frequency range within which the spectral density ( in W / Hz or V < sup > 2 </ sup >/ Hz ) is above half its maximum value ( or the spectral amplitude, in V or V / Hz, is more than 70. 7 % of its maximum ); that is, above − 3 dB relative to the peak.