Characteristics of Sound:
A pleasing sound produced by regular and periodic vibrations is called musical sound while an unwanted sound due to irregular and non-periodic vibration is called noise.
Musical notes may differ from one another in three characteristics:
(1) Pitch- Pitch is that characteristic of sound by which the ear distinguishes between a shrill and grave sound. To be more precise it is the characteristics of sound by which the ear is able to assign it a place in the musical scale.
When a string is plucked it produces a sound of a definite pitch. If we increase the tension, the pitch becomes higher. Similarly, the pitch rises as the length of the string is reduced. We know that the frequency of sound also increases with the increase in tension and decrease in length. Hence we conclude that the principal physical quantity on which the sensation of pitch depends is the frequency. Other factors include the intensity of sound.
(2) Loudness- Loudness of sound is the magnitude of auditory sensation. The physical quantity on which loudness depends is intensity. Sometimes back it was believed that sensation for sound loudness S can be expressed by the following law, known as Weber-Fechner law.
| S = k log I/I0 |
Where I0 is the intensity of reference level and k is a constant. However, this law is not really valid and is expected to die a natural death. Instead, nowadays the following relation is used.
| S = A (I/I0)0.3 |
Where A is a constant. If sone is defined as the unit of loudness, then the above relation takes the form-
| S = (1/16) (I/I0)0.3 |
Which shows that S = 1 sone when I/I0 = 104 (at 1000 Hz).
(3) Quality- Quality is that characteristic that enables us to distinguish two notes of the same pitch and loudness produced by two different musical instruments. The difference in quality arises because of the presence of higher overtones. Each musical instrument produces its own characteristic overtones.
In order to preserve all the essential characteristics of sound, an acoustic device like radio, gramophone, telephone, etc. must reproduce faithfully the amplitudes of all the frequencies in the right proportions- this property of an acoustic device is known as fidelity.
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