Navigation and Seamanship (1+1)

3.1.7. Sound in water

Sound waves can travel through a material medium such as a gas, a liquid or a solid, with particular speed ,but not a vacuum . If we count the number of peaks which pass a given point during one second (s) we get the frequency of sound. The frequency thus gives the number of waves or periods per second. A period is also called a cycle and frequency is sometimes expressed as cycles per second (c/s). One thousand cycles per second is one kilocycle per second (1 kc/s). The unit ‘cycle per second’ is now internationally called Hertz (Hz: 1 kc = 1 kHz) after a famous physicist of the late 19^th century. Each tone corresponds to a certain frequency. The human ear can detect sounds of frequencies from about 20 Hz to 20 kHz ( 20 Hz to 20,000 Hz) . Those above the normal range of the human ear are termed ultrasonic or supersonic frequencies.

Sound velocity is the distance the sound waves travel per second, which thus corresponds to the frequency multiplied by the wavelength. The sound velocity depends on certain physical properties of the material through which the sound travels, namely elasticity and density. In air, the sound velocity is approximately 322 m per second (m/s) at 0°C, and it increases slightly with higher temperature. In water, sound velocity is about, 1,500 m/s at 15°C( roughly 1.5 km/sec (just under 1 mile/sec) or about 4 times faster that sound travel through air) and here also the velocity increases at higher temperatures as well as with increasing salinity. The wavelength of the sound waves can be calculated in a very simple way when we know the sound velocity of the medium and the frequency of the sound.