Adding Decibels

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Adding Decibels
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The Passive Sonar Equation
Will the sensor detect the red submarine?
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Signal to Noise Ratio
Signal
The higher the SNR, the more likely you are to
hear (detect) the signal.
Noise (quiet)
Noise (Loud
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Source Level and Transmission Loss
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Sources of Noise

• Shipping Noise
• Wind and Weather
• Marine Life Biologic Activity
• Self Noise
• Flow of Water
• Machinery

Omni directional Noise
Isotropic Noise
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Directivity Factor

• Some detectors are only able to provide a voltage
  proportional to all incident sound from all
  directions. (non-directional ND)
• Other detectors use more sophisticated signal
  processing and form beams thereby providing a
  voltage proportional to sound incident from a
  particular direction. (directional D)

IS
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The Passive Sonar Equation
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Figure of Merit

• Often a detection threshold is established such
  that a trained operator should be able to detect
  targets with that LS/N half of the time he hears
  them. Called Recognition Differential. (RD)
• Passive sonar equation is then solved for TL
  allowable at that threshold. Called Figure of
  Merit. (FOM)
• TLallowable Figure of Merit SL- LS/N
  Threshold - (NL-DI)
• Since TL logically depends on range, this could
  provide an estimate of range at which a target is
  likely to be detected. Called Range of the
  Day. (ROD)
• Any LS/N above the Recognition Differential is
  termed Signal Excess. (SE) Signal Excess
  allows detection of targets beyond the Range of
  the Day.

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Example

• A hostile submarine with a Source Level, SL 130
  dB re 1 mPa is near a friendly submarine in a
  part of the ocean where the Noise Level from all
  sources, NL 70 dB re 1 mPa. The directivity
  factor is 3000 for the friendly submarines
  sonar. If the Recognition Differential for the
  friendly submarine is 20 dB, what is the Figure
  of Merit?
• If the actual Transmission Loss is 50 dB, what is
  the Signal Excess.

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Signal to Noise Level
But we will be measuring the signal intensity
level at the receiver/detector, IR (in the
frequency band of the detector) This is
different from the signal intensity level leaving
the target, IS (in the frequency band of the
detector)
OR
Fraction of emitted intensity reaching receiver
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Noise Level and Directivity Index
OR
IS