consists of machinery noise (main engines, auxiliary machinery,
gears) and hydrodynamic noise (flow past the hull, appendages +
cavities; blade rate tones; propeller singing; cavitation);
• all machinery on a ship radiates sound through the hull into the water;
• noise increases with ship size, power, load, and speed;
• ships with propellers are generally louder than ships with jet
propulsion;
• propeller cavitation is the loudest component for speeds greater than
the cavitation inception speed;
• tones at low frequencies correspond to propeller blade rate and
harmonics: f [Hz] = number of blades x rpm/60 ;
• small vessels have small propellers turning at higher speeds resulting
in higher frequency blade-rate tones;
• at low frequency (< a few 100 Hz), the ship spectrum also has tonal
components from engines and gears; these spectral lines form the
acoustic signature of the ship and allow acoustic identification;
• for f >100Hz the spectrum falls as -20log10
f.