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Adding Decibels

Adding Decibels

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Adding Decibels

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  1. Adding Decibels

  2. The Passive Sonar Equation Will the sensor detect the red submarine?

  3. Signal to Noise Ratio Signal The higher the SNR, the more likely you are to hear (detect) the signal. Noise (quiet) Noise (Loud

  4. SL TL Source Level and Transmission Loss

  5. IN IN IN Receiver IN IN Detector IN IN Sources of Noise • Shipping Noise • Wind and Weather • Marine Life – Biologic Activity • Self Noise • Flow of Water • Machinery Omni directional Noise Isotropic Noise

  6. IN IN IN Receiver IN IN Detector IN IN 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

  7. The Passive Sonar Equation

  8. 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/NThreshold - (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.

  9. 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 submarine’s 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.

  10. 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

  11. IN IN IN Receiver IN IN Detector IN IN Noise Level and Directivity Index OR IS