Unit of Sound - SI, CGS and Other Sound Units

In this article, we define the unit of loudness. It also explains the concept of sound measurement, si and cgs unit of frequency and the unit of measuring the intensity of sound,

What is the sound?

A sound is a form of energy that comes from the vibration of particles. Vibration is a to and fro motion of a body that is also called oscillation. Irregular vibrations with irregular wave patterns are called noise whereas, the regular periodic vibration with regular wave patterns is called music.

Propagation of sound is the travelling of sound waves. For the propagation of sound waves, a medium is essential. When sound waves are transmitted from the medium such as solid, liquid, or gas then propagation takes place. Sound waves do not pass through the vacuum. There must be the presence of matter for the waves to pass through it.

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Propagation of sound in human ears:

Sound is collected by the funnel-shaped outer ear. The waves are transmitted through the ear canal to the eardrum. Vibrations are produced at the eardrum. In the middle ear, the vibrations are increased with the help of three bones. Then the sound wave is passed to the inner ear. There is a coiled part present in the inner ear called the cochlea. It is connected to the middle part of the ear at one side and the auditory nerve of the brain at the other side. It helps to hear the sound using the liquid present in it. Sound is sensible to the liquid containing nerve cells. Due to this, the brain commands signals as sound and then we hear.

What is the unit of sound?

Sound measurement: Sound is measured in units called decibels. Decibel or dB is used as a unit to measure the sound intensity in tribute to Alexander Graham Bell. He is the inventor of the telephone and audiometer. The word decibel has come from the early 20^th century. In the field of telecommunication, scientists introduced the unit of sound energy.

Sound measured by decibel is unique from other measurements. Ten bels constitute one decibel. Therefore, the decibel measurement is more intense. They give data in logarithmic scale while other devices measure in linear quantity. When two quantities are measured on a logarithmic scale, then the ratio of those quantities gives dB or decibel range.

Then, decibel L=10log I/I₀

Here I is the intensity of the sound wave and I₀ is the reference intensity.

From this, it clears that with the increase in decibel value, the intensity of the wave is also raised with a constant multiplicative factor. Both the factors are proportional to each other. The following table indicates the relation between the intensity of the sound wave and the decibel value.

The decibel range distinguishes noise from the audible range. But for humans, it is not possible to hear sound having all frequencies. A scale dBA is used to measure the loudness corresponding to the threshold of human hearing. The dBA unit is preferred over dB as dBA measurement takes consideration of different frequencies of the human ear.

Characteristics of sound:

There are three main characteristics of sound:

FigureRepresentation of characteristics of sound wave

• Amplitude (A): When sound is produced, the maximum displacement of vibrating particles from their average position is called the amplitude of a sound wave. It represents the height of the wave. The amplitude of a wave is relative to the loudness. Loudness is the sound energy released per second. It connects the threshold intensity of normal hearing and the intensity of any given sound.

Loudness ∝ (Amplitude)²

So, the sound wave with a larger amplitude will have more loudness and the smaller amplitude has weak sound. SI unit of amplitude is meter.

• Frequency (υ): Sound wave has a frequency which is referred to as pitch. When a wave passes through the medium, how often the particles of the medium vibrate refers to the frequency of that wave.

Frequency= Number of oscillation/Total time

Sound waves are related to atmospheric pressure. That is, when sound waves travel in the air medium, there will be a periodic change in atmospheric pressure. The frequency of sound waves is also defined as the number of pressure variations per second. The CGS unit of frequency is Hertz or Hz. The CGS system of measurement is smaller than the SI unit. Based on frequency, the sound is divided into infrasound and ultrasound. Infrasound is any sound having a frequency below 20Hz and ultrasound has a frequency above 20 kHz. Below is a table representing the list of the frequency range of common examples:

• Time-period (T): In a sound wave, the time taken by the particles to complete one vibration cycle is called the time period of the wave.

Time period=Time/Number of vibration cycle

Frequency and time period are inversely proportional to each other.

ϑ=1/T

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Difference between sound and noise:

Sound and noise are classified as follows:

Phon unit:

For a given sound, a unit measures its loudness range. The unit is called a phon unit. The sound pressure level of 1 kHz pure pitch is considered as having the same loudness. It is the loudness range in phons of a sound.

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