Sound propagates through water in the form of alternating compressions and rarefactions in the medium. These disturbances are periodic, taking the form of longitudinal waves having different frequencies, which can be expressed in sinusoidal form.
Water has higher viscosity, heat capacity and conductivity than air. These features make the study of sound waves in water more complex than in air, especially when the physical parameters of the water are rapidly changing, as with oceans. The speed of sound in air has an approximate value of 1,126 ft/s for most practical problems.
Such estimations are not possible for sound propagating in water, as speed is highly affected by the temperature, density, type of dissolved impurities and hydrostatic pressure of the water. These complex interactions have yet to be fully understood as of 2015, and empirical formulas derived through trial and error are often all scientists can rely on when calculating this speed.
The typical frequency range of underwater acoustics is also different from that of air. Frequencies in water are typically between the range of 10 Hz and 1 MHz. Frequencies lower than 10 Hz do not excite the aquatic medium sufficiently to carry the wave, whereas frequencies higher than 1 MHz decay too quickly.