JEE Main & Advanced Physics Wave Mechanics Musical Sound

A musical sound consist of a series of harmonic waves following each other rapidly at regular interval of time without a sudden change in amplitude.

(1) Noise : A noise consists of a series of waves following each other at irregular intervals of time with sudden changes in amplitude.

(2) Pitch : The pitch of a sound is the characteristic which distinguishes between a shrill (or sharp) sound and a grave (or flat) sound.

(i) A sound of high pitch is said to be shrill and it's frequency is high.

(ii) A sound of low pitch is said to be grave and it's frequency is low.

(iii) The pitch of female voice is higher than the pitch of male voice.

(iv) The pitch of sound produced by roaring of lion is lower where as the pitch of sound produced by mosquito whisper is high.

(3) Quality (or timbre) : A musical instrument vibrates with many frequencies at the same time. The quality of any musical sound is determined by the number of overtones and their relative intensities.

(i) The quality of sound enables us to distinguish between two sounds having same intensity and pitch.

(ii) The sounds of different instruments (such as Tabla and Mridang) are said to differ in quality.

(iii) Due to quality of sound one can recognise the voice of his friend without seeing him.

(4) Loudness : Characteristic of sound, on account of which the sound appears to be intense or slow.

(i) The loudness that we sense is related to the intensity of sound though it is not directly proportional.

(ii) The loudness depends on intensity as well as upon the sensitiveness of ear.

(iii) Our perception of loudness is better co-related with the sound level measured in decible (dB) and defined as follows \[\beta =10{{\log }_{10}}\,\left( \frac{I}{{{I}_{0}}} \right)\]; where \[{{l}_{0}}=\] The minimum intensity that can be heard called threshold of hearing \[={{10}^{-12}}W/{{m}^{2}}\] at 1 KHz.

(iv) At the threshold of hearing \[\beta =0\]. At the threshold of pain \[\beta =10{{\log }_{10}}\frac{1}{{{10}^{-2}}}=120\,dB.\]

(v) When the intensity doubles, the intensity level changes by 3 dB.

(vi) When the intensity increases 10 times the level increases by 10 dB.      

Different sound intensity level  

Interval : The ratio of the frequencies of the two notes is called the interval between them e.g. interval between two notes of frequencies 256 Hz and 512 Hz is 1 : 2. Different interval  

Musical scale : It consists of a series of notes of successively increasing frequency, having constant intervals. The note of the lowest frequency is called the key note.

These are many kinds of musical scales. The most commonly used scale is called major diatonic scale. It is formed by introducing six more notes between a given note and it's octave, so that these are eight notes in all. Major diatonic scale

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