![]() ![]() The time period shown is about half a second. The attack is almost instantaneous, but the decay envelope is very long. This shows the sound envelope of striking a cymbal with a stick. The ear is sensitive to these attack and decay rates and may be able to use them to identify the instrument producing the sound. The decay is long and gradual by comparison. The plucking action gives it a sudden attack characterized by a rapid rise to its peak amplitude. The illustration above shows the attack and decay of a plucked guitar string. The primary contributers to the quality or timbre of the sound of a musical instrument are harmonic content, attack and decay, and vibrato/tremolo. Quality demo: factory whistle, soprano, piano Their distinctly different quality is attributed to vocal formants, frequency ranges where the harmonics are enhanced. The recognition of different vowel sounds of the human voice is largely accomplished by analysis of the harmonic content by the inner ear. It is common practice to characterize a sound waveform by the spectrum of harmonics necessary to reproduce the observed waveform. While there is some efficiency in characterizing such sources in terms of their overtones, it is always possible to characterize a periodic waveform in terms of harmonics - such an analysis is called Fourier analysis. Some musical sound sources have overtones which are not harmonics of the fundamental. For sustained tones, the most important of these is the harmonic content, the number and relative intensity of the upper harmonics present in the sound. The primary contributers to the quality or timbre of the sound of a musical instrument are harmonic content, attack and decay, and vibrato. It is suggested that it takes about a 4 dB change in mid or high harmonics to be perceived as a change in timbre, whereas about 10 dB of change in one of the lower harmonics is required. Some investigators report that it takes a duration of about 60 ms to recognize the timbre of a tone, and that any tone shorter than about 4 ms is perceived as an atonal click. Timbre is mainly determined by the harmonic content of a sound and the dynamic characteristics of the sound such as vibrato and the attack-decay envelope of the sound. Timbre is then a general term for the distinguishable characteristics of a tone. Sound "quality" or "timbre" describes those characteristics of sound which allow the ear to distinguish sounds which have the same pitch and loudness. Sounds may be generally characterized by pitch, loudness, and quality. Horner, "Synthesis of trumpet tones using a fixed wavetable and a centroid-controlled second order filter," Proceedings of the 1994 International Computer Music Conference, used by permission of the authors. Three-dimensional plot of a trumpet tone showing the amplitude envelope of the first 20 harmonics. Peak intensities occur in the frequency bands centred on 200, 315 and 630 Hz, with a fundamental about 80 Hz. Third-octave spectrum of the Salvatore Mundi bell, Salzburg. Line spectrum of the partials of a viola string, omitting their time variation. The amplitude of each partial is inversely proportional to the partial number. The first 32 partials of a HARMONIC SERIES shown as a line spectrum. Spectrum may also refer to a RANGE of frequencies, as in the audio spectrum (see AUDIO FREQUENCY ) or the RADIO SPECTRUM. The spectrum of a sound may be altered electronically by FILTER ing or EQUALIZATION. The spectral envelope refers to the contour or shape of the spectrum, particularly when it shows the maximum strength of each frequency component during the sound. ![]() The auditory system is designed to balance the simultaneous resolution of detail in both domains, as expressed by the LAW OF UNCERTAINTY. Īnalyzing the spectrum of a sound is a way of understanding its behaviour in the frequency domain, as opposed to its behaviour in the time domain, according to its WAVEFORM or ENVELOPE. See: diagrams under BROAD BAND NOISE, FOURIER ANALYSIS, INFRASONIC, SPECTROGRAPH, WHITE NOISE. A continuous spectrum, on the other hand, shows frequencies continuously distributed over the audible range. A partial spectrum is also known as a line spectrum, where discrete frequencies are present. The spectrum of a sound may be determined by a SOUND ANALYSER or by FOURIER ANALYSIS and is distributed over the audible range (20 to 20,000 Hz). A continuous spectrum consists of NOISE components. Ī PARTIAL spectrum consists of discrete frequencies known as OVERTONE s, HARMONIC s or INHARMONIC s. The spectrum of a sound is a primary determinant of its perceived TIMBRE. Three-dimensional displays of a spectrum add the time variation on the third axis (see below). The FREQUENCY content of a sound or audio SIGNAL, often displayed as a graphic representation of amplitude (or INTENSITY LEVEL ) against frequency. ![]()
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