The Physical Formation of Sound in Birds
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You might assume that in order for a parrot to be able to imitate the human voice-to use a person's same spoken words, stresses and pronunciation-they must possess a larynx whose structure is similar to a human's. However, the structure of the human larynx bears no resemblance to these creatures' physical structures. The larynx, vocal cords, tongue, lips, palate and teeth that humans use in speech are completely different in birds, and some do not exist at all. But even though all birds lack these structures, still these species can reproduce phrases spoken by humans-and in the same tones. If we consider that a person without a tongue is unable to speak or that we lose our voice if the vocal cords are damaged, it's also worth considering that parrots, budgerigars, and mynahs, members of the crow family, have completely different physical characteristics which nevertheless enable them to talk in the same way as humans.
There are other differences between the systems that humans and birds use to produce vocal sounds. We produce most sounds d by expelling air from the lungs through the larynx. Different sounds are created, according to the degree of vibration of the vocal cords. The position of the tongue and lips and the flow of air through the mouth or nasal cavity are only a few of the many other factors affecting sound production. The pharynx, found in humans, lets the tongue divide the vocal tract above the larynx into two cavities with their own distinct resonances. Where these resonances occur, the overtones of the frequencies (or number of vibrations) from the vocal cords are amplified. Formants (from the Latin formare: to shape, or form) are resonant frequencies of the vocal tract, the natural shapes that air assumes in the vocal passage. When you make a consonant, for example, this has an effect on the formants of the neighbouring vowels, raising or lowering formants as the vowel sound gets closer to the corsonant. Experiments have shown that two formants are sufficient in order to differentiate speech sounds from each other. 5
Birds have no larynx similar to a human's, but do have a special vocal organ, known as the syrinx, that enables them to produce sounds. In birds, air from the lungs passes through this organ. In a sense, the bird's syrinx is the equivalent of our human larynx. One of the principal differences is that in humans, our vocal cords are positioned closer to the windpipe. So far, the fact that the bird's syrinx is deep inside the body has prevented scientists from obtaining a complete answer as to how birds produce sound. Scientists have filmed birds using infra-red and x-ray cameras, and have made close studies of their song and speech by means of fiber-optic microscopes inserted in their throats. Yet we still cannot explain the physical process by which birds produce song and imitate sounds.
 The two-branched syrinx, located deep in the breast of a bird, is situated where the two bronchia divide in the windpipe. In this complex system, the vocal organ's muscles and inner membranes affect the production of mid-tones. By contraction of the pectoral and stomach muscles, air is directed from the air sacs to the bronchia and the syrinx, where the air molecules vibrate as they pass through narrow passages. Vibrations of the tympana membrane also affect the frequency of the sound. The pressure of the air sacs in the clavicle in turn affects the tympana membrane. The syrinx muscles also affect the flow of air and consequently, the quality of sound. |
Within the bird's breast, its vocal organ is like a branched instrument, located at where its voice box meets the two bronchial tubes. As shown on the adjacent page, one branch of the syrinx opens into one bronchus and the second branch into the other; and either one of these two bronchi can produce sound. Some birds can use either both sides of their voice organ simultaneously, or one of the two independently and, by this means, can produce two separate tones of the same frequency, at the same time. They can sing a high note with one side, while producing a low note with the other. And since the bird's vocal organ is situated at the juncture of the two bronchial tubes, it can produce sound from two different sources. This even allows the bird to produce two different notes simultaneously, and even to sing a duet with itself. To a great extent, sounds produced here are subsequently combined, giving birds the potential of creating rich melodies. While humans use only about 2% of the air they inhale to produce sound, birds have the ability to use it all.
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