Vocalization

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Many animals communicate with acoustic signals. Crickets rub a hind leg along a row of protruding spikes; rattlesnakes shake a set of beads in their tail. But vocalization is by far the most common mechanism of acoustic communication. Found only among vertebrates, vocalization involves forcing air through a tube across one or more thin membranes located in a tube between the lungs and the mouth. These membranes vibrate, producing rapid fluctuations in air pressure in the outgoing air stream that can be detected as sound by a receiver that is tuned to the appropriate frequencies.

Vocalization results from the interaction of four forces acting on each membrane. The first force is air pressure, which is generated by expelling air from a flexible sac, such as a lung. The second force is the elasticity of the membrane, which returns the membrane to its original position after it is disturbed. The third force, which is muscular, forces the membrane into the airflow. The fourth force is called Bernoulli's force, which sucks the membrane into the airflow.

To produce a vocal sound, air is forced through the tube by the contraction of the air sac. Simultaneously, a muscle thrusts the membrane intothe airflow, causing air pressure to build up. Eventually, this air pressure forces the membrane out of the airflow, rapidly releasing air from the tube. The rapid flow of air past the membrane generates Bernoulli's force, which pulls the membrane back into the airflow.

This cycle repeats until the muscles forcing the membrane into the airflow relax and the elasticity of the membrane pulls it out of the airflow. The rate at which the cycle repeats is equivalent to the frequency of the sound produced and is dependent on the degree of muscular contraction blocking the airflow.

Mammals, anurans, and birds each have a unique mechanism of vocalization. All possess a tube called a trachea, which connects the bronchi (tubes leading to the lungs) to the mouth and nose. In mammals, a set of two membranes called the glottis, or vocal cords, partially blocks the airflow through the trachea when a pair of muscles contracts. The diaphragm, an abdominal muscle underneath the lungs and above the intestines, contracts to expand the lungs and relaxes to expel air. Thus, the rate of airflow during vocalization, which can occur only during exhalation, is not under direct muscular control.

The signal produced is periodic and nonsinusoidal, meaning that the rate of airflow varies regularly, but not continuously, around the mean. This produces harmonics, which are sounds at higher multiples of the fundamental frequency. These harmonics can be altered by downstream cavities, such as the mouth, to produce different tones, such as the vowels.

Frogs and toads have two consecutive sets of membranes: the one closer to the lungs acting as the vocal cords, and the farther one referred to as the glottis. The vocal cords control the frequency of the signal, whereas the glottis provides periodic amplitude modulation (AM, as in a radio signal). Expelled air is captured in a throat sac and returned to the lungs to be reused, thereby reducing the amount of work required of the diaphragm. The throat sac also serves as a resonant coupler, its vibrations transferring the signal to the outside air.

Birds have two separate membranes: one between each bronchus and the trachea, together referred to as the syrinx. Each membrane functions independently, allowing birds to produce two different sounds simultaneously. Muscles cause the membranes, which line the side of the syrinx, to buckle, allowing Bernoulli's force to pull them into the airflow.

In addition to lungs, birds possess air sacs throughout the body that are connected to the bronchi and subject to muscular contraction. Thus, exhalation is under direct muscular control, allowing for finely calibrated amplitude modulation. Furthermore, many species employ a labium to control the aperture of the syrinx, and therefore amplitude, onset, and offset of the signal. Finally, the degree of tension in the membranes affects both frequency and amplitude, which are often correlated in bird calls.

Although mammals, anurans, and birds differ in the way they control the frequency and amplitude of their acoustic signals, they share an ability to exploit air as a medium for the rapid transmission of information. This is especially important for animals that live in visually cluttered habitats such as trees, or those that communicate over long distances. Sounds do not require localization by the receiver in order to be recognized, whereasvisual signals must be noticed first. Therefore, acoustic signals serve well as alarm calls and to attract attention from potential competitors or mates.

Bernoulli's force, also known as Bernoulli's principle, draws its name from its discoverer, Daniel Bernoulli (1700-1782). The principle relates fluid velocity and pressure. Among many other applications, the principle provides an explanation for aircraft lift. The wing shape and the angle of its tip relative to the flow of air are configured so that the air flowing over the wing moves faster than the air under the wing. The pressure difference between the two makes the lift possible.

Acoustic Signals; Communication.

Hopp S. L., M. J. Owren, and C. S. Evans. Animal Acoustic Communication: Sound Analysis and Research Methods. Berlin: Springer-Verlag, 1997.

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