Sound under water travels at a speed that is five times greater than in the air. One might expect that hearing under water is easier but this is not so. The volume does not depend on the speed of sound; rather, it is dependent on the amplitude of sound waves and on the perceptive capabilities of the audial organs. There are two methods of perceiving sound waves; the first being air conductivity (outer audial opening, eardrum or tympanum, audial bones of the middle ear) and the second being bone conductivity (the vibration of the bones of the skull). Air conductivity is prevalent in the air, whereas bone conductivity is prevalent under water. This peculiarity is due to the fact that the acoustic resistance of water is close to that of human tissues and the loss of energy for the transition of sound waves into skeletal bones is less under water than it is in the air. Air conductivity under water disappears because the outer audial opening is filled with water and there are no conditions for normal vibration of the eardrum. It has experimentally been proven that bone conductivity is weaker than air conductivity by 40%. Consequently, hearing under water is impeded. The distance within which sound can be heard depends on tonality rather than on the volume of sound. Sounds of greater tonality can be heard at greater distances than those of lower tonality. Sounds that are being emitted under water are usually inaudible above the surface of the water and vice versa.

In order to perceive sounds that are emitted under water, one must be immersed into the water at least partially. If the water reaches as high as a person’s knees, he or she will be able to hear underwater sounds that have previously been inaudible. Such sounds are best heard when one is fully submerged.

Relying on one’s hearing, it is extremely difficult to orientate oneself under water. In the air, sound reaches one of the ears .00003 seconds earlier than the other. This fact allows the source to be identified within an error of 3?. Because of the high speed of sound under water, it is perceived by both ears virtually simultaneously and the orientation error may reach up to 180?. Bad orientation under water is also due to the prevalent bone conductivity. Sufficient audial orientation is possible to be acquired only after systematic training. After training ceases, however, this ability disappears.

The diving suit isolates the human ear from the surrounding water medium. That is why sound waves penetrate the helmet and the layer of air but reach the eardrum partly absorbed and scattered. In this case, sound perception through air conductivity is insignificant.

However, while diving without a helmet, which is possible in warm water, sound is perceived just like in the air. If the rubber helmet fits tightly, sound is well perceived because of bone conductivity – sound waves are transmitted through the bones of the human skull. With no helmet, a diver can hear very well, with a rubber helmet – fairly well, and with a metal one – very bad.

Various types of telephones are used in diving. However, different sources of sound are used for communication between divers wearing close fitting helmets. To create sound signals, either special “clappers” are used, or divers knock with metal tools at the gas bottles. 

Sound is a periodic motion of pressure change transmitted through a gas (air), a liquid (water), or a solid (rock). Since liquid is a denser medium than gas, more energy is required to disturb its equilibrium. Once this disturbance takes place, sound travels farther and faster in the denser medium. Several aspects of underwater sound are of interest to the working diver.

During diving operations, there may be two or more distinct contiguous layers of water at different temperatures; these layers are known as thermoclines. The colder a layer of water, the greater its density; as the difference in density between layers increases, less sound energy is transmitted between them. This means that a sound heard 164 feet (50 meters) from its source within one layer may be inaudible a few meters from its source if the diver is in another layer.

In shallow water or in enclosed spaces, reflections and reverberations from the air/water and object/water interfaces will produce anomalies in the sound field, i.e., echoes, dead spots, and sound nodes. When a diver is swimming in shallow water, among coral heads, or in enclosed spaces, periodic losses in acoustic communication signals and disruption of signals from acoustic navigation beacons are to be expected. The problem becomes more pronounced as the frequency of the signal increases.

The use of open-circuit scuba affects sound reception by producing high noise levels at the diver's head and by creating a screen of bubbles that reduces the effective sound pressure level (SPL). If several divers are working in the same area, the noise and bubbles will affect communication signals more for some divers than for others, depending on the position of the divers in relation to the communicator and to each other.

A neoprene wet suit is an effective barrier to sound at frequencies above 1000 Hz, and it becomes more of a barrier as frequency increases. This problem can be overcome by exposing a small area of the head either by cutting holes 0.79 to 1.18 in. (2 to 3 cm) at the temples or above the ears of the hood.

The human ear is an extremely sensitive pressure detector in air, but it is less efficient in water. A sound must therefore be more intense in water (+20 dB to 60 dB, SPL) to be heard. Hearing under water is very similar to trying to hear with a conductive hearing loss under surface conditions: a smaller shift in pressure is required to hear sounds at the extreme high and low frequencies, because the ear is not as sensitive at these frequencies. The SPL necessary for effective communication and navigation is a function of the maximum distance between the diver and the source (-3 dB SPL for every doubling of the distance between the source and the measurement point), the frequency of the signal, the ambient noise level and frequency spectrum, type of head covering, experience with diver-communication equipment, and the diver's stress level.

The use of sound as a navigation aid or as a means of locating an object in the environment depends primarily on the difference in the time of arrival of the sound at the two ears as a function of the azimuth of the source. Recent experiments have shown that auditory localization cues are sufficient to allow relatively precise sound localization under water. Moreover, it has been demonstrated that under controlled conditions divers are able to localize and navigate to sound beacons (Hollien and Hicks 1983). This research and practical experience have shown that not every diver is able to localize and navigate to sound beacons under all conditions. In general, successful sound localization and navigation depend on clearly audible pulsed signals of short duration that have frequency components below 1500 Hz and above 35,000 Hz and are pulsed with a fast rise/decay time.

Sound is transmitted through water as a series of pressure waves. High intensity sound is transmitted by correspondingly high intensity pressure waves. A diver may be affected by a high intensity pressure wave that is transmitted from the surrounding water to the open spaces within the body (ears, sinuses, lungs). The pressure wave may create increased pressure within these open spaces, which could result in injury.

The sources of high intensity sound or pressure waves include underwater explosions and, in some cases, sonar. Low intensity sonars such as depth finders and fish finders do not produce pressure waves of an intensity dangerous to a diver. However, some military anti-submarine sonar-equipped ships do pulse high intensity pressure waves dangerous to a diver. It is prudent to suspend diving operations if a high-powered sonar transponder is being operated in the area. When using a diver-held pinger system, it is advisable for the diver to wear the standard 1/4 inch (0.64 cm) neoprene hood for ear protection. Experiments have shown that such a hood offers adequate protection when the ultrasonic pulses are of 4 ms duration, are repeated once per second for acoustic source levels up to 100 watts, and are at head-to-source distances as short as 4 inches (10 cm).