Musical sounds
Audible sounds are classified into two groups, namely musical sounds and noise. A musical sound is that in which the vibrations of the sounding body are periodic, follow each other regularly and rapidly, so as to produce a pleasing effect on the ear without any sudden change in loudness.
The waveform of musical sounds is smooth as shown in figure (a). Musical sounds can be produced using a flute, a veena, sitar, harmonium, etc. periodicity, regularity and continuity are their hallmarks. The waveform of noise, on the other hand, is irregular as shown in figure (b).
Characteristics of musical sounds
The three characteristics which differentiate musical sounds from one another are:- Loudness
- Pitch
- Quality or timbre
Loudness
It is that characteristic of a musical sound that determines the degree of sensation that the sound can produce in the ear. It depends on the intensity of the sound which is objective in nature and the sensitivity of the ear which is subjective in nature. The factors on which loudness depends and the unit of measurement have been described earlier in this section.
Pitch
It is that characteristic of a musical sound by which a shrill sound can be distinguished from a grave one, even though the two sounds may be of the same intensity. It is also defined as that characteristic of sound by which the ear assigns it a place on a musical scale. When a stretched string is plucked, a sound of a certain pitch sensation is produced. If the tension in the string is increased, the pitch (the shrillness) becomes higher. Increasing the tension, also increases the frequency of vibration. Therefore, pitch is intimately related to frequency. But frequency alone does not determine the pitch. Below 1000Hz, the pitch is slightly higher than the frequency and above 1000Hz the position is reversed. The loudness of sound also affects the pitch up to 1000Hz. An increase in loudness causes a decrease in pitch. From about 1000 to 3000Hz, the pitch is independent of loudness, while above 3000Hz an increase in loudness causes an increase in pitch.
The voice of a woman or a child is shrill but that of a man is hoarse (i.e., flat or grave). The sound emitted by a cuckoo is shrill while that by a crow or a lion is hoarse. The buzzing of a mosquito, though low in intensity has a high pitch. The pitch of the sound changes when the source or the observer or both are in relative motion.Quality or timbre
It is that characteristic of a musical sound which enables us to distinguish between the sounds produced by two different musical instruments or two different persons although their pitch and loudness may be same. It is because of this characteristic that we are able to recognize the voice of a known person over the telephone or to distinguish between the sounds produced by different musical instruments in an orchestra.
The quality depends primarily on the waveform of the sound. The waveform of the sound emitted by a tuning fork is a simple sine curve. But most bodies vibrate in a very complex manner. Besides the fundamental frequency f, they consist of additional frequencies of 2f, 3f, 4f, etc. These additional tones are called overtones or harmonics. According to Helmholtz, the quality of a musical note is determined by the number, relative frequency and intensities of the component simple tones of which a complex vibration is composed of.One can produce the same tone with the same frequency and intensity as one's favorite singer, but the singer's voice has a better quality because of a different combination of the overtones in it.
