The Thermal-Noise Limit in the Detection of Underwater Acoustic Signals

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Abstract
The criteria for an “optimum” hydrophone receiving system for underwater acoustic signals are considered, and the equilibrium thermal‐noise pressurespectrum expression is shown. Experimental ambient sea‐noise spectrum levels are in excess of the thermal noise at all frequencies below 25 kc. Extrapolation of experimental curves (−5 db/octave) shows intersection with thermal‐noise spectrum (+6 db/octave) between 50 and 200 kc, depending on sea state. (For a definition of “sea state,” see Instructions to Marine Meteorological Observers (U. S. Department of Commerce Weather Bureau, U. S. Government Printing Office, Washington, D. C., June, 1941), seventh edition, circular M.) The ambient‐noise level is expressed as a function of the equivalent temperature. Formulas are then derived for the required plane‐wave signal level for which electrical signal‐to‐noise ratio is unity. The operating noise factor is defined in terms of the electroacoustical efficiency of the linear reversible hydrophone, the amplifiernoise figure, and the equivalent ambient‐noise temperature for the sea condition and frequency under consideration.

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