Università degli Studi di Pavia

Centro Interdisciplinare di Bioacustica e Ricerche Ambientali

Via Taramelli 24 - 27100 Pavia - Italy
e-mail : cibra@unipv.it

Underwater Bioacoustics
A short introductory chapter on underwater bioacoustics, instrumentation and methodologies.

Underwater acoustics
Underwater acoustics is a branch of acoustic science which investigates the propagation of sound underwater. Underwater acoustics is strictly related with the development of the SONAR (Sound Orientation Navigation And Ranging) which allows to detect underwater obstacles and to measure the water depth to make navigation safer.
SONARs can be either active or passive. Active sonars emit sounds and listen for the echoes reflected by underwater objects. The most advanced ones allows to map the seabead features and to discriminate among different objects, for example to find mines.
Passive sonars simply listen for the sounds emitted underwater by ships, submarines, ...and whales as well.
The major developments in passive sonars have been initially accomplished for military purposes, i.e. to detect submarines and ships by means of the noise emitted by their engines and propellers.
Once-secret military hydrophones that were used to listen for submarines are now providing acoustic data for more peaceful purposes. Scientists are listening to whales, tracking fish, and even measuring global temperature changes using sound (ATOC - Acoustic Thermometry of Ocean Climate). As sound waves move slightly faster in warm water than in  cold water, by measuring the time it takes sound to travel underwater between two points, scientists can figure out the average temperature along that path.

Bioacoustics is a branch of zoology, strictly related to ethology, that investigates sound production and reception in animals, including ma, and how they communicate each other by means of sound. Bioacoustics also concerns the organs of hearing and the sound producing apparata as well as the physiological processes by which sounds are produced and received for communication as well as for echolocation purposes. Finally, it attempts to understand relationships between the features of the sounds an animal produces and the nature of the environment in which they are used and the functions they are designed to serve. Its development dates effectively from about 1950, when practical recording and analyzing methods became readily available to the scientific community.

Underwater Bioacoustics
Underwater bioacoustics studies the acoustic behaviour of acquatic animals and the acoustic features of the underwater environment in which they emit sounds. In the underwater environment acoustic communication plays a crucial role: the high propagation speed (about 1500 m/sec, five times than in air) and the low attenuation with distance allow an effective acoustic transmission of signals. Many texts of acoustics, electroacoustics and bioacoustics may be consulted to get a better knowledge of underwater acoustics; among them: Urick (1983), Au (1993), Richardson et al. (1995).
Many of the organisms which live underwater, or near the interface between air and water, produce sounds: invertebrates, amphibians, reptiles, fishes, mammals. In the marine environment, biological acoustic sources are mainly represented by invertebrates ("snapping shrimps", for example), fishes, and marine mammals (cetaceans and pinnipeds).
The study of underwater acoustic communication is an important branch of bioacoustics. It concerns not only the characteristics of the acoustic signals by which organisms communicate and echolocate but also the structure of the organs for production and reception of signals, the acoustic characteristics of the environment through which  signals propagate, as well as the development of suitable instruments to detect and analyze sounds.
Many research projects focus on acoustics and underwater bioacoustics in relation to the problem of conservation of the fauna and to the environmental impact of human activities. Scientific goals associated with monitoring and management of the marine environment can benefit from non intrusive study methods such as the detection of the presence of certain species on the basis of their species specific acoustic signals. The ability to detect and recognize the acoustic signature of each species allows the identification from a distance and, in certain cases, the survey, tracking, and census of the animals, even at  a large distance or when lacking visual observation.
Also, acoustics might be used to develop both active and passive deterrent devices to warn echolocating cetaceans on the presence of fishing nets and thus to prevent their incidental take.
Since bioacoustic research involves the detection, the systematic classification, and the analysis of animal acoustic emissions, zoological sound libraries are of primary importance. Sound libraries are national and  international institutions whose main task is to collect, file, and preserve the recordings of animal voices and to make them available to the scientific community as well as to educational activities.

Sound detection

Hydrophones are specially designed microphones that transform sounds propagating underwater into electrical signals. Regarded as single transducers, hydrophones are usually omnidirectional and may cover a wide range of frequencies, from a few Hz to more than 100 kHz. In the marine environment, different and more complex hydrophonic systems are used. They often consist of multiple transducers (array of hydrophones) in order to be more directional and sensitive.
By using a towed dipole array, made by just two hydrophones spaced few meters, it is possible to assess the direction of incoming sounds by measuring the time delay of a same signal on the two transducers (TDA - Time Difference of Arrival). The TDA is then used to determine a conical surface on which the sound source is located; to resolve the real bearing it is required to repeat the measure after some change in the ship course. In this way it is possible to locate the animal where conic surfaces intersects.
To get a real source position to be plotted on a GIS the position of the ship must continuously updated and recorded with a GPS receiver.

Example of acoustic tracking of a sperm whale with a dipole array

As far as the application in underwater bioacoustics is concerned, there are two main kinds of utilization of the transducers: stationary mono/multi-hydrophonic configurations by which to control selected areas, and towed hydrophonic arrays to continuously detect sound during navigation.
Hydrophones may be connected to a receiving equipment for recording and sound analysis by means of wires or a radio transmitter. For some applications they may be packaged with a recorder and batteries to operate autonomously for periods of time extending from few hours to months. These devices can be deployed on the sea bottom (or even attached on an animal if they are compact enough) and then retrieved for analysing the recordings.

Arrays used for military purposes, i.e. to detect submarines at great distances, may be very powerful and expensive. They are often made by hundreds of sensors and they are connected to very powerful processing systems able to perform beamforming in real-time. Beamforming is a technique based on the processing of signals detected by a series of sensors to produce a plot of received energy vs arrival direction; in other words, a beamformer allows to show the energy of received sound in a degree vs time plot. Thus it is possible to separate sources at different angles and to listen to a specific direction only. Arrays for studying marine mammals' sounds are often based on a small series of sensors to keep low the complexity, the weight and the cost of the array. A small array can be deployed and retrieved by hand from a small platform such as a motorsailing boat.

The dipole towed array of hydrophones developed by CIBRA and ALENIA

Sonobuoys are expendable devices developed and used to detect submarines by either listening for the sounds produced by propellers and machinery (passive detection) or by emitting a sonar "ping" to be reflected by the surface of a submarine (active detection). They are basically consist in a hydrophone, an amplifier, a VHF radio transmitter and a battery to provide energy for a selected number of hours; all the electronics are located in a cylindrical enclosure with a floater. They may be dropped by aircrafts or deployed by ships and their radio signals can be received within few miles.
Passive sonobuoys can be effectively used to detect and record cetacean sounds too.  There are several sonobuoy types, among them, the Q57 type uses a calibrated hydrophone for audio frequencies in the 10Hz to 20kHz range.

Deployement of sonobuoys from the IT Navy RV Magnaghi (SOLMAR SIRENA 99)

Display of signals received from a sonobuoy (IT Navy RV Magnaghi, SOLMAR SIRENA 99)

Bottom deployed recorders
This type of recorder is a very special piece of equipment that can be deployed on the sea bottom to record sounds for a certain period, continuously or according to a predefined scheduled, and then, at the end of the task, it can be retrieved. The equipments needs to be waterproof and pressure resistent; also, it needs long life batteries to provide power to the recording electronics. At the end of the task it can be retrieved in different ways; some models have an acoustic release system to be activated by a specific acoustic command sent from the surface. Once released the equipment goes to the surface, pulled by floaters, to be recovered.

Recording tags
This is another very special piece of recording equipment to be attached on the back of an animal. The most sophisticated one, named D-TAG, has been developed by the WHOI team to record, other than sounds, the depth, the orientation and the movements of the swimming animal to reconstruct all its movements underwater along with the sounds received and emitted by an animal. These features are very important to document the behaviour of a diving animal and in particular to record behavioural reactions to the playback of sounds and noise in Controlled Exposure Experiments (CEE).
The D-TAG is not invasive as it is attached with suction caps; a timed release system detaches the tag after a given number of hours and then the tag goes to the surface and emits a radio-signal to drive the recovery operations.

Instruments developed by the Centro Interdisciplinare di Bioacustica:

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Page written by G.Pavan, August 2005