Background Noise (噪声和混响:NL & RL)

The masking background noise level above which the signal echo level must be detected is the summation of:

A) SELF NOISE
B) AMBIENT NOISE      (A+B = Noise Level, NL)
C) REVERBERATION   (Reverberation level, RL)
 

A) Noise Level (NL): Self Noise
Self noise is comprised of: Self noise may reach a receiving hydrophone via a variety of paths. For example noise from the ship's engine, transmission and propeller may reach the hydrophone via; Self noise generally increases systematically, with the speed of the vessel and it is usually accounted for via empirical measurements of self noise versus vessel speed. (see Urick Chapter 11).

B) Noise Level (NL): Ambient Noise
The definition of ambient is, "surrounding on all sides, encompassing."
Sources of ambient noise include:

The principle components of noise in the deep water ambient noise spectrum are turbulence, shipping noise, surface agitation and thermal noise. Each of these noise components dominate at different frequencies (see Coates 1990, diagram 6.1). Below 10Hz oceanic turbulence dominates the noise spectrum. Between 10Hz and 100Hz shipping noise is the principle spectral component. Surface agitation is prevalent between 100Hz and 100kHz and at very high frequencies (>100kHz) thermal noise originating from molecular motion is dominant. The surface agitation band is of principle importance for the majority of sonar systems.

Temporal Variability In Ambient Noise
Below 100kHz there is considerable temporal (time) variability in the noise spectrum. This variability relates to:

Depth Variations In Ambient Noise
In general ambient noise in the deep oceans decreases with depth since the principle source of noise is from the surface. Sound channels are an exception to this rule. Here ambient noise levels are approximately constant with depth.

Angular Distribution In Ambient Noise
Normally we assume that ambient noise is isotropic (equal in all directions). This is the assumption we employ in the sonar equations. We then reduce the effective noise level by the directivity of the hydrophone (NL-DI). However, in deep water the sea surface may be considered as the principle sourses of ambient noise. In these areas the ambient noise level is not truly isotropic. Conversely, in shallow ambient noise is essentially isotropic due to reflections from the sea floor.

Noise Level Calculation
We have already stated that within the operational frequency band of most sonar systems the ambient noise level (in dB re. 1mPa) is dominated by surface agitation (waves). The magnitude of the ambient noise due to waves "rolls off" (decreases) linearly with frequency.

There are essentially two stages to the calculation of NL:
1) Estimate the ambient noise level (Nf) at the operational frequency of the sonar (f in Hz). This is done using the following equation:

Nf = N1 - 17 log(f x10-3)     (dB)

Here N1 is the spectrum level at 1kHz. The value of N1 is dependent on the wind strength and can be obtained using standard graphs or equations (see Coates 1990, pg 91).

2) Finally a correction must be made for the bandwidth of the receiving hydrophone. The term bandwidth here refers to the frequency band over which the hydrophone is sensitive. The broader the bandwidth of the hydrophone the greater the range of frequencies that the hydrophone can hear. Thus, a hydrophone with a broad bandwidth will hear more noise that a hydrophone with a narrow bandwidth. The bandwidth (Df in Hz) correction is given by the following equation:

NL = Nf + 10 log(Df x 10-3) - DI (dB re. 1mPa)

Notice also that a correction has been made here (using the isotropic assumption) for the directivity index of the receiving hydrophone (DI).

C) Reverberation Level (RL)
The term reverberation refers to the sum total of the scattered sound energy. If a sound pulse is emitted into a room (e.g. a church) the sound can be heard reverberating around, exponentially decaying in intensity with time. You will remember from our previous discussions of target strength that scattering (as opposed to specular reflection) is prevalent when the length scale (or texture) of reflector is small compared to the wavelength of the sound pulse.

Reverberations in the oceans may be caused due to:

Thus, reverberation limited conditions (requiring the reverberation limited sonar equation) occur when trying to sense a target close to the sea bed, surface or in a medium containing a large amount of particulate matter or bubbles.

Volume reverberation level:

Thus, in reverberation limited conditions is important to design a sonar system which is directive, has a short pulse length and uses a low source intensity. Increasing the source level above a critical level merely increases reverberation and therefore does not improve echo detection.

Fortunately, volume reverberation is generally low (-100dB to -80dB) compared to typical target strengths (-25 to +25dB). However, larger (-70 to -60dB) reverberation levels can occur in the deep scattering layer (DSL).

The Deep Scattering Layer  (DSL)
The deep scattering layer is a diffuse band of densely packed biological material including:

The magnitude of the backscatter from the DSL is related to the density of the biomass. The depth of the DSL migrates vertically with a diurnal period. The depth of the DSL is typically <200m by night and often > 1000m by day.