Ambient noise :
The sound levels are given in underwater dB, which is described as “dB re 1 μPa”. The sounds under the sea are measured relative (re) to a pressure of 1 microPascal (μPa). A reference to measure sounds in air is 20 μPa.
The frequency range: 10 Hz to 1MHz
Frequencies under 10 Hz can be only detected deep into the seabed
20 to 500 Hz: distant ships, that are the main cause of ambient noise.
500 Hz to 100, 000 Hz: breaking waves, that are wind – dependent and they increase with wind speed
Above 100, 000 Hz: sound generated by the motion of water molecules (called “thermal noise”)
Hydrophone vs. Sonar
Hydrophone – measure sound in water, it monitors and records sound from all directions. It features a piezoelectric receive-only transducer.
SONAR – “Sound Navigation and Ranging”. A technique that is based on sound propagation and has several purposes.
The term “sonar” also designates the material used to produce and receive the sound, on frequencies that vary from infrasonic to ultrasonic. It can be either active or passive.
A hydrophone is an unavoidable component of all passive sonar devices, allowing the latter to listen, receive the echo from any underwater object and locate it. A sonar may use a multitude of hydrophone sensors, however, not all kinds of hydrophones are used in sonar.
