We were animated about this in the BETA phase. There is a section located in fas.org that explains DEMOdulated noise and how it's derived from BB sources and NOT NB. Think of it this way...NB is a set of descrete frequencies that are seperated while BB is a ton of noise produced by the contact.
Typical broadband sources are noise from either the propeller/shaft, flow noise and some propulsion systems (example: steam). Noise from the propeller and shaft is generally at low frequency, meaning less than 1000 Hz. Furthermore, the rate of rotation can amplitude modulate the noise, which 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. For 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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