Please reference these definitions...
Broadband sources create acoustic energy over a wide range of frequencies, similar to a thermal source in electro-optics. Typical broadband sources are noise from either the propeller/shaft, flow noise and some propulsion systems. Noise from the propeller and shaft is generally at low frequency, meaning less than 1000 Hz. The rate of rotation can amplitude modulate the noise, and this propeller blade rate tonal noise can be detected and demodulated to measure the shaft or propeller blade rate.
Narrowband sources radiate within a small band about a particular frequencies, or class of frequencies. Typical sources are the various pieces of machinery found in every ship, including example, pumps, motors, electrical generation equipment and propulsion systems. When specifying narrowband sources, it is important to also specify the frequency at which it occurs.
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