Properties of Sound

Properties of Sound: Some of the important properties/characteristics of sound are as given below:

Intensity: While propagating from source, intensity of sound decreases as distance from source increases, because sound gets distributed in all direction as they travel. It is inversely proportional to the distance from the source.

It can be defined as the average rate of transmission of energy in a given direction measured over one square meter of area at right angles to the direction. It’s unit is watts per square meter (w/m²). Intensity of sound is usually represented in decibels (db) with reference to Threshold of Hearing.

Threshold of Hearing: The ear is very sensitive to sound intensity. It can sense very low sound intensity (10^-3 w/m²). However, sensitivity of ear is not same for all frequencies.

The minimum sound intensity which an ear can hear is called Threshold of Hearing. It varies with frequency usually at 1000 Hz frequency is taken as standard (10^-12 w/m²).

Threshold of Pain: The upper limit of hearing by human ear without causing pain is called Threshold of Pain. This also varies with frequency.

The power Ratio between Threshold of hearing and pain is very large i.e., 10¹². Its high to low power ratio is very large, Decibel (db) scale is generally used. In this method, when Threshold of Pain is 120 db above.

Loudness: This is subjective quantity which can not be measured like Intensity. Intensities of tones at 1 KHz are taken as standard levels for comparison. It is observed that two tones of different frequencies with same intensity will not cause equal loudness. PHON is the unit of equal loudness. The Threshold of Hearing is taken as ZERO Phon.

Hence, Threshold of pain is equal to 120 phon. Unit for loudness is Sone.

Pitch – Frequency Relation: Pitch is decided by frequency of sound wave. When frequency is constant, intensity can change Pitch. The unit of Pitch is Mel which is defined as below.

1000Hz tone having a loudness of Sone produces a pitch of 1000 mels.

The relation between pitch and frequency is Non-linear. A frequency of 1 KHz causes a pitch of 1000 mels. But a frequency of 2 KHz causes a pitch of 1500 mels only. Similarly 400 Hz frequency causes 500 mels pitch.

Harmonics: Overtone and timber

Harmonic: An integer ratio between two frequencies is called Harmonic. 2 KHz is second harmonic of 1 KHz. Similarly 4 KHz is fourth harmonic of 1 KHz.

Harmonic should not be considered equal to octave. In Above example,

1 KHz and 2 KHz are of one Octave.

4 KHz and 1 KHz are of two Octave.

Harmonics are Integers multiples of fundamental frequency where as Octave can be in fraction.

Over Tone: All frequencies greater than fundamental including harmonic are termed as overtone. Music is Harmonic blending of fundamental frequency and Harmonics.

Timber: The quality of sound as heard by the ear is called Timber. Timber depends on the proportion in which Harmonics are present. We identify different tones produced by Persons or Instruments by Timber.