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Why do sounds have certain properties?
Underwater sounds generated by marine animals and people have characteristics that depend on the purposes for which they will be used. It is important to understand why specific sounds or signals are chosen for specific tasks. For example, why do the echolocation signals generated by dolphins and killer whale differ from the songs of humpback whale? Why do the signals used to locate submarines differ from the signals used for diver communications? Two important characteristics of a signal are its frequency and how long it lasts (duration).
The distance that the sound needs to travel is one of the key considerations in determining the frequency of the signal. Low frequency signals are absorbed less rapidly in the ocean than high frequency signals. They can therefore travel longer distances and still be detected. On the other hand, the background ambient noise level is greater at low frequencies than at high frequencies. High frequency signals can therefore be detected at lower sound levels than low frequency signals. The competing effects of absorption and ambient noise mean that there is a best frequency that will maximize the chances that the signal will be heard over the ambient noise. This best frequency depends on the distance over which the sound travels. Echosounders used to measure water depth in the deep ocean, which is 4000-5000 m deep, typically use frequencies of about 12,000 Hz. Echosounders used in shallower water typically use much higher frequencies, often 30,000 Hz or more. Signals designed to travel thousands of kilometers have to use very low frequencies. The Acoustic Thermometry of Ocean Climate (ATOC) project transmitted signals at 75 Hz, for example.
Signals used to determine the structure of the earth below the seafloor, such as seismic profiling systems used to locate oil and gas reserves, have to use very low frequencies because high frequency sounds are absorbed even more rapidly in the sea floor than in the ocean.
For signals used to echolocate, find fish, and image the seafloor, as well as for many other purposes, another consideration in selecting the frequency is the size of the target. The wavelength of the sound must be small compared to the size of the target to provide detailed images. High frequency signals, which have smaller wavelengths, therefore provide higher resolution and can image the detailed structure of the target better than low frequency signals. The echolocation signals of bottlenose dolphin, for example, have frequencies of 110-130 kHz (110,000-130,000 Hz), with wavelengths of about 1 cm (0.5 inches). Fish finders often operate at frequencies of 26 kHz, with a wavelength of about 6 cm (2.3 inches). Side scan sonars use frequencies as high as 500 kHz, with a wavelength of 3 mm (0.1 inches), to generate detailed images of objects on the seafloor, but the sonar then needs to be towed near the seafloor very close to the object.
Another important signal characteristic is how long the signal lasts. Some signals, such as those used for echolocation, are very short. The echolocation pulses transmitted by bottlenose dolphins are only 50-80 microseconds long, for example. Other signals, such as the songs of humpback whales, can last for hours.
Echolocation pulses, as well as the pulses of active sonars, are short for a number of reasons. Both echolocation and active sonars work by transmitting a pulse and then listening for echoes, whether from a fish or a submarine. It is impossible to hear echoes at the same time that a transmission is occurring, which means that the transmitted pulse must be short, to allow the echoes to be heard. Further, precise measurements of the round-trip travel time from the source to the target and back are needed to accurately determine the distance to the target. Short pulses provide more precise travel time measurements than long pulses. Finally, short pulses make it possible to separate closely spaced echoes, which occur when multiple targets are present.
The songs of humpback whales, on the other hand, serve as mating calls. Only male humpback whales sing. It is advantageous to a male to sing long songs when seeking a mate, so that as many females as possible will hear the song. Humpback whale songs represent a basic form of communication. More generally, communication signals, such as those transmitted by divers, can last for a long time when a large amount of information needs to be transferred, just as it can take a long time to download a large file to a computer over the internet.
Sometimes specially-designed, complex signals with long durations are used to improve the signal-to-noise (SNR) ratio of the received signal, while still providing accurate travel time measurements and allowing closely spaced echoes to be separated.