Sound production

There are four common mechanisms of producing sounds. Essentially, any movement or action which causes air molecules to vibrate or makes pressure waves in air produces sound. Most animals make sound using special structures, but a few simply take advantage of a resonant substrate by slapping, tapping, or drumming on it. Specific sound production mechanisms include:

Vibrating a drum-like membrane. Vibrating membranes are used by cicadas to produce their mate attraction calls. The membrane, or tymbal, is moved held in place by rigid exoskeletal structures and moved by a muscle attached directly to the membrane.
File and scraper. In crickets, grasshoppers, some ants, and a variety of other insects, one body part is equipped with a scraper (imagine something like a guitar pick protruding a little from the animal), and an adjacent body part has a rough or file-like structure. The animal makes a chirp or whir by moving the two body parts against each other, dragging the scraper across the file. The entire structure (file and scraper) is called a stridulatory organ. Frequency (pitch) changes depending on how fast the two structures are moved. Because insects are poikilotherms (cold-blooded) their stridulations may vary in pitch depending on the temperature.
Vibrating a membrane in an air flow--Like the reed in a saxophone or clarinet, the larynx of frogs, toads and mammals and the syrinx of birds feature a membrane which vibrates as air from the lungs is pushed past it. The rate of vibration, and hence the frequency (pitch) of the sound can be regulated by muscles which tighten or loosen the membrane.
Hitting a substrate.

Beaver slap their tail on the surface of the water when alarmed. Because sound travels much faster in water than air, this is a very effective warning signal.
Some termites, ****, bash their head against substrate when alarmed, producing sound and substrate vibrations. Literally millions of termites in a colony may do this synchronously, producing a sound that is audible for several meters around the nest.
Woodpeckers, such as the flicker, Caleptes cafer, knock on hollow trees (or buildings that resonate like hollow trees) as a way of attracting mates.
Drumming during courtship sequences in spiders.

Regulating pitch and amplitude

Each of the first three mechanisms (tymbal, stridulatory organ, larynx/syrinx) offer the animal a way of varying pitch, making it easier to produce calls which are distinctive for a species, sex, or individual . In all cases, amplitude (loudness) also usually can be varied, by varying the amount of energy put into making the sound.

Body size is an important limitation on the amplitude that can be produced.While small animals, such as cicadas and frogs, sometimes produce enormous sounds, Production of very loud sounds is limited by the amount of force (energy) an animal can generate behind the sound. In an absolute sense, strength is a function of body size, so very loud sounds are reserved for very large animals. However, special adaptations help to surmount this limitation. One is using a resonant structure to amplify the sound. This can be external to the animal, like a cricket calling from inside a hollow stem, or internal to the animal, such as the hyloid cartilage of the howler monkey.

Vocal sacs, such as those found on many frogs and on howler monkeys, allow larger volumes of air to be impelled through the larynx. While these alone do not amplify the sound, they allow the production of sounds of longer duration than would be produced by air from the lungs alone.

Size imposes something of a limitation on pitch as well. Although low frequency vibrations can be produced simply by moving the sound generator slowly, the mass of air needed to move to make an effective low frequency sound is large, and a correspondingly large amount of energy is required to move that air. Also, low frequency sound generators are often thicker, heavier, or longer than high frequency generators (think of the lower pitched strings on a piano or guitar). The size limitations associated with low frequency sounds are even more important at the receiver's end; choice of pitch for communication is driven both by what can be produced and what can be perceived.

Wiley R. H., Richards D. G. 1978. Physical constraints on acoustic communication in atmosphere - implications for evolution of animal vocalizations. Behavioral Ecology And Sociobiology 3: (1) 69-94