Sonar Equation & Transmission Loss Calculator
Underwater acoustic engineering: Solve active and passive sonar equations, transmission loss (TL), seawater chemical absorption, and acoustic Figure of Merit (FOM).
Sonar Architecture & Ocean Acoustics
Acoustic Detection Range
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Frequently Asked Questions
What is the Figure of Merit (FOM) in sonar operations?
The Figure of Merit (FOM) represents the maximum transmission loss (in dB) that a sonar system can tolerate while still maintaining the minimum signal-to-noise ratio needed for target detection (SNR = DT). For passive sonar, FOM = SL - (NL - DI) - DT. A sonar with FOM = 85 dB can detect targets through 85 dB of ocean attenuation.
Why does underwater sound absorption increase dramatically with frequency?
Unlike electromagnetic waves which travel poorly in conductive seawater, acoustic waves propagate thousands of kilometers. However, chemical relaxation of boric acid molecules (below 1 kHz) and magnesium sulfate (MgSO₄) ions (between 10 and 100 kHz) absorbs acoustic energy. At 1 kHz, seawater absorbs ~0.06 dB/km; at 50 kHz, absorption surges to >15 dB/km, severely limiting active high-frequency sonars to short ranges.
What is the SOFAR channel (Deep Sound Channel) in ocean acoustics?
The Sound Fixing and Ranging (SOFAR) channel is a horizontal layer in the ocean (typically 800 to 1,200 meters deep) where the speed of sound reaches a global minimum due to the competing effects of decreasing temperature from above and increasing hydrostatic pressure from below. Sound waves trapped in this acoustic waveguide refract continuously toward the axis without interacting with the reflective ocean surface or lossy seabed, traveling thousands of nautical miles.