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Sound
physics
Media

Steady-state waves

Spectral analysis

The Fourier theorem

Fundamental to the analysis of any musical tone is the spectral analysis, or Fourier analysis, of a steady-state wave. According to the Fourier theorem, a steady-state wave is composed of a series of sinusoidal components whose frequencies are those of the fundamental and its harmonics, each component having the proper amplitude and phase. The sequence of components that form this complex wave is called its spectrum.

The synthesis of a complex wave from its spectral components is illustrated by the sawtooth wave in Figure 9. The wave to be synthesized is shown by the graph at the upper middle, with its fundamental to the left and right. Adding the second through fourth harmonics, as shown on the left below the fundamental, results in the sawtooth shapes shown on the right.

The sound spectrograph

A sound that changes in time, such as a spoken word or a bird call, can be more completely described by examining how the Fourier spectrum changes with time. In a graph called the sound spectrograph, frequency of the complex sound is plotted versus time, with the more intense frequency components shown in the third dimension or more simply as a darker point on a two-dimensional graph. The so-called voiceprint is an example of a sound spectrograph. At one time it was believed that people have voiceprints that are as unique as their fingerprints, so that individuals could be identified by their voiceprints, but the technology of the voiceprint has never been developed. In certain bird atlases, sound spectrographs of bird calls are included with other information, allowing identification of each bird by its call.

Generation by musical instruments

The steady-state tone of any musical instrument can also be analyzed and its Fourier spectrum constructed. The amplitudes of the various spectral components partially determine the tone quality, or timbre, of the instrument.

Bore configuration and harmonicity

The bore shapes of musical instruments, which have developed over the centuries, have rather interesting effects. Cylindrical and conical bores can produce resonances that are harmonics of the fundamental frequencies, but bores that flare faster than a cone create nonharmonic overtones and thus produce raucous tones rather than good musical sounds. A fact discovered by early musical instrument builders, this is the reason why the musical instruments that have developed over the past millennium of Western history are limited to those with either cylindrical or conical bores. In general, a rapidly flaring bell is added to the end of the instrument to reduce the impedance mismatch as the sound emerges from the instrument, thus increasing the ability of the instrument to radiate sound.

The presence of any given harmonic in the spectrum of a particular musical instrument depends on the nature of the vibrating system. For example, if the system functions acoustically as an open tube or a vibrating string, all harmonics will likely be present in the wave. Examples of this are the flute, the recorder, and the violin. On the other hand, the clarinet functions acoustically as a closed tube, because it is cylindrical in shape and has a reed end. Therefore, as explained above in Standing waves: In air columns, the odd harmonics are emphasized in the clarinet spectrum—particularly at low frequencies. Other wind instruments function acoustically as open tubes for a variety of reasons. The addition of a mouthpiece and a bell to a tube, either cylindrical or conical, results in all harmonics being possible, as in both the trumpet (cylindrical) and cornet (conical) family of brasses. Even after fixing a reed to one end of a conical tube—as in the oboe, bassoon, and saxophone families—the instruments still function acoustically as open tubes, producing all harmonics. The sawtooth wave, having all harmonics, therefore sounds more like a trumpet or a saxophone than like a clarinet.

Other effects on tone

Because many musical instrument families have similar spectra, there must be other factors that affect their tone quality and by which their tones can be distinguished. Attack transients, such as the way in which a string is bowed, a trumpet tongued, or a piano key struck, and decay transients, such as the way the sound of a plucked string dies away, are very important in many instruments, particularly those that are struck or plucked. Vibrato (a periodic slow change in pitch) and tremolo (a periodic slow change in amplitude) also aid the analysis of steady-state sounds.

Inharmonicities, or deviations of the frequencies of the harmonics from the exact multiples of the fundamental, are very important in tuned percussion instruments. For example, because of the inherent stiffness of piano strings, the overtones of the piano have slight inharmonicities. Indeed, the frequency of the 16th harmonic as played on the piano is about one-half step higher than the exact frequency of the harmonic.

Variations in air pressure

Basic to flutes and recorders, an edge tone is a stream of air that strikes a sharp edge, where it creates pressure changes in the air column that propagate down the tube. Reflections of these pressure variations then force the air stream back and forth across the edge, reinforcing the vibration at the resonant frequency of the tube. The time required to set up this steady-state oscillation is called the transient time of the instrument. The human ear is extremely sensitive to transients in musical tones, and such transients are crucial to the identification of various musical instruments whose spectra are similar.

In musical instruments the pressure variations generated by edge tones, a reed, or the lips set up standing waves in the air column that in turn drive the air stream, reed, or lips. Thus, contrary to common belief, the vibrations of the air column drive the reed or the lips open and closed; the reed or lips do not drive the air column. In the clarinet, for example, air is forced through the reed, creating a pulse of air that travels down the tube. Simultaneously, the reed is pulled closed by pressure of the lips and by rapid air flow out of the reed. After one reflection off the end of the tube, the pulse reflects as a rarefaction, holding the reed shut, but after the second reflection the pulse returns as a compression, forcing the reed open so that the process is repeated.

The human voice

Groups of emphasized harmonics, known as formants, play a crucial role in the vowel sounds produced by the human voice. Vocal formants arise from resonances in the vocal column. The vocal column is about 17.5 centimetres (7 inches) long, on the average, with its lower end at the vocal folds and its upper end at the lips. Like a reed or like lips at the mouthpiece of a wind instrument, the vocal folds function acoustically as a closed end, so that the vocal column is a closed-tube resonator with resonant frequencies of about 500, 1,500, 2,500, and 3,500 hertz, and so on. The vibration frequency of the vocal folds, determined by the folds’ tension, determines the frequency of the vocal sound. When a sound is produced, all harmonics are present in the spectrum, but those near the resonant frequencies of the vocal column are increased in amplitude. These emphasized frequency regions are the vocal formants. By changing the shape of the throat, mouth, and lips, the frequencies of the formants are varied, creating the different vowel sounds.

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