Unit of Sound - SI, CGS and Other Sound Units

Edited By Vishal kumar | Updated on Jul 02, 2025 04:45 PM IST

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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What is the sound?
What is the unit of sound?
Characteristics of sound:
Difference between sound and noise:

Unit of Sound - SI, CGS and Other Sound Units

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Propagation of sound in human ears:

Structure of the human ear

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.

Type of sound	Intensity	Sound measurement units (Decibels)
Bus	10^-3	90
Near jet engine	1	120
Loud telephone bell	10^-5	70
Private office	10^-7	50
Silence	10^-12	0

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.

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Characteristics of sound:

There are three main characteristics of sound:

Representation of characteristics of sound wave

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.

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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:

Examples	Lower frequency (Hz)	Upper frequency (Hz)
Pigeon	1	20
Fish	800	1000
Guitar	80	1200
Bats	20	120x10³
Dogs	50	45 x10³
Dolphins	0.25	200x10³
Radio waves	300x10⁹	3x10³
Humans	20	20x10³

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:

Sound	Noise
Sound is the regular vibrations or oscillations of particles moving from one medium to another.	Noise is an irregular vibration of particles.
It is audible and has regular patterns of the wave.	It is not in the audible range and has an irregular pattern.
Sound is not harmful to the body.	Noise is harmful to the body as it is incompressible and irrelevant.

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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NCERT Physics Notes:

Frequently Asked Questions (FAQs)

1. What is the SI unit of noise?

dB or decibel is the unit of noise also.

2. What is the unit of frequency of sound?

The frequency of sound is measured in hertz or Hz.

3. Is phon unit an SI unit?

No, phon is not an SI unit.

4. What is the SI unit of the loudness of sound?

The unit of loudness of sound/ si unit of loudness/ loudness of sound is measured in units of sone.

5. What is pitch or shrillness?

The frequency of an emitted sound is called the pitch. The frequency of vibration is responsible for the pitch or shrillness.

6. A unit or an unit?

It is a unit.

7. How does frequency affect the perception of sound intensity?

Human ears are not equally sensitive to all frequencies. We are most sensitive to frequencies between 2000-5000 Hz. As a result, sounds of equal intensity but different frequencies may be perceived as having different loudness levels.

8. What is the relationship between sound intensity and distance from the source?

Sound intensity decreases with the square of the distance from the source in an ideal, free-field environment. This is known as the inverse square law. If you double the distance from a point source, the intensity decreases to one-fourth of its original value.

9. How is sound pressure level different from sound intensity level?

Sound pressure level (SPL) measures the local pressure deviation from atmospheric pressure caused by a sound wave, while sound intensity level (SIL) measures the sound power per unit area. Both are typically expressed in decibels but use different reference values.

10. How is sound power related to sound intensity?

Sound power is the total acoustic energy emitted by a sound source per unit time, measured in watts. Sound intensity is the sound power per unit area, measured in watts per square meter. The total sound power can be calculated by integrating the intensity over a surface enclosing the source.

11. What is the difference between sound intensity and loudness?

Sound intensity is an objective, physical measure of sound energy, while loudness is a subjective perception of sound strength. Loudness depends not only on intensity but also on frequency, duration, and individual hearing sensitivity.

12. What is sound intensity mapping and how is it used?

Sound intensity mapping involves measuring sound intensity at multiple points in space to create a visual representation of sound distribution. This technique is used for identifying noise sources, analyzing sound fields, and optimizing acoustic environments.

13. How does sound intensity measurement differ for continuous versus impulse sounds?

Continuous sounds are typically measured using time-averaged intensity, while impulse sounds require measurement of peak intensity or sound exposure level. The choice of measurement technique and duration can significantly affect the reported intensity values for these different types of sounds.

14. How does sound intensity measurement differ in free field versus diffuse field conditions?

In a free field, sound intensity decreases with distance according to the inverse square law. In a diffuse field (like a reverberant room), sound intensity is theoretically constant throughout the space due to multiple reflections, making measurements more complex.

15. How is sound intensity level calculated using decibels?

Sound intensity level (SIL) in decibels is calculated using the formula: SIL = 10 log₁₀(I/I₀), where I is the measured intensity and I₀ is the reference intensity (10⁻¹² W/m²).

16. What is the significance of the critical distance in sound intensity measurements?

The critical distance is the distance from a sound source in a room where the direct sound intensity equals the reverberant sound intensity. Beyond this distance, the sound field is dominated by reflections rather than direct sound, affecting intensity measurements.

17. How does air density affect sound intensity measurements?

Air density affects the speed of sound and the acoustic impedance of the medium. Changes in air density (due to temperature, humidity, or altitude) can influence sound intensity measurements, particularly over long distances or in extreme environments.

18. What is acoustic intensity?

Acoustic intensity is the amount of sound energy flowing through a unit area perpendicular to the direction of sound propagation per unit time. It is measured in watts per square meter (W/m²) and is a vector quantity, having both magnitude and direction.

19. How does sound intensity relate to amplitude in a sound wave?

Sound intensity is proportional to the square of the amplitude of the sound wave. Doubling the amplitude of a sound wave results in a four-fold increase in sound intensity.

20. What is meant by the threshold of hearing and how is it expressed in different units?

The threshold of hearing is the minimum sound intensity that can be perceived by the human ear. It is typically expressed as 0 dB (SPL), which corresponds to a sound pressure of 20 μPa or a sound intensity of 10⁻¹² W/m² at 1000 Hz.

21. What is the difference between sound intensity and sound energy density?

Sound intensity is the rate of energy flow through a unit area (W/m²), while sound energy density is the amount of sound energy contained in a unit volume of the medium (J/m³). Energy density is related to intensity by the speed of sound in the medium.

22. What is the SI unit of sound intensity?

The SI unit of sound intensity is watts per square meter (W/m²). This unit measures the amount of sound energy passing through a unit area per second.

23. Why is the decibel (dB) scale used to measure sound levels instead of watts per square meter?

The decibel scale is used because it better represents how humans perceive sound intensity. Our ears respond logarithmically to sound intensity changes, and the decibel scale is logarithmic, making it more practical for describing the wide range of sound intensities we can hear.

24. How does the bel relate to the decibel?

One bel is equal to 10 decibels. The decibel is more commonly used because it provides a more convenient scale for typical sound levels we encounter in daily life.

25. What is the CGS unit for sound intensity?

The CGS unit for sound intensity is erg per square centimeter per second (erg/cm²/s). This unit is less commonly used than the SI unit (W/m²) in modern physics.

26. What is the reference intensity (I₀) used in calculating sound intensity levels in decibels?

The reference intensity (I₀) is 10⁻¹² W/m², which corresponds to the threshold of human hearing at 1000 Hz. This value is used as the baseline (0 dB) in the decibel scale for sound intensity levels.

27. How does the phon unit relate to decibels?

The phon is a unit of perceived loudness level, equal to the sound pressure level in decibels of a 1000 Hz tone. At 1000 Hz, the phon scale matches the decibel scale, but at other frequencies, they may differ due to the varying sensitivity of human hearing.

28. What is the sone unit and how does it differ from the phon?

The sone is a linear unit of perceived loudness, where a doubling of the sone value represents a doubling of perceived loudness. One sone is defined as the loudness of a 40-phon (1000 Hz) tone. Unlike the logarithmic phon scale, the sone scale is designed to be more intuitive for describing loudness differences.

29. Why is the pascal (Pa) relevant in sound measurements?

The pascal is the SI unit of pressure, including sound pressure. Sound waves create pressure variations in the air, and these variations are measured in pascals. Sound pressure level in decibels is calculated using a reference pressure of 20 μPa, which corresponds to the typical threshold of human hearing.

30. How does the neper unit relate to decibels in sound measurements?

The neper is a logarithmic unit that can be used to express ratios of field quantities (like sound pressure) or power quantities (like sound intensity). One neper is approximately equal to 8.686 decibels. While decibels are more common in acoustics, nepers are sometimes used in theoretical work.

31. What is the significance of the 3 dB rule in sound intensity measurements?

The 3 dB rule states that an increase of 3 dB in sound intensity level corresponds to a doubling of sound intensity. This rule is useful for quick estimations of sound intensity changes and understanding the logarithmic nature of the decibel scale.

32. How does sound intensity level change when multiple identical sound sources are combined?

When multiple identical, incoherent sound sources are combined, the total sound intensity level increases by 10 log₁₀(n) decibels, where n is the number of sources. For example, two identical sources result in a 3 dB increase, while ten sources result in a 10 dB increase.

33. How does temperature affect sound intensity measurements?

Temperature affects the speed of sound in air, which in turn influences sound intensity measurements. Higher temperatures increase the speed of sound, which can affect how sound waves propagate and interact with the environment, potentially altering intensity measurements over long distances.

34. What is the relationship between sound intensity and particle velocity in a sound wave?

In a plane wave, sound intensity is the product of sound pressure and particle velocity. The particle velocity is the speed at which air particles oscillate due to the sound wave, not to be confused with the speed of sound itself.

35. How does humidity affect sound intensity measurements?

Humidity affects sound intensity measurements by changing the absorption of sound in air. Higher humidity generally leads to greater absorption, especially at higher frequencies, which can result in lower measured intensities over long distances.

36. What is acoustic impedance and how does it relate to sound intensity?

Acoustic impedance is the resistance of a medium to the propagation of sound waves. It is the product of the medium's density and the speed of sound in that medium. The intensity of a sound wave transmitted between two media depends on their acoustic impedance mismatch.

37. How does the concept of sound intensity apply to standing waves?

In standing waves, the sound intensity varies with position, being zero at nodes and maximum at antinodes. The time-averaged intensity at any point is proportional to the square of the amplitude of the standing wave at that point.

38. What is the difference between instantaneous and time-averaged sound intensity?

Instantaneous sound intensity is the intensity at a specific moment in time, which fluctuates rapidly in a sound wave. Time-averaged sound intensity is the average intensity over a period of time, typically much longer than the period of the sound wave, and is the value usually measured and reported.

39. How does sound intensity relate to sound power level?

Sound power level (SWL) is a measure of the total acoustic power emitted by a source, while sound intensity level (SIL) is the power per unit area at a specific point. SWL can be calculated by integrating SIL over a surface enclosing the source, assuming a free field.

40. How does the concept of acoustic near field and far field relate to sound intensity measurements?

In the acoustic near field (close to the source), sound intensity patterns can be complex and may not follow the inverse square law. In the far field, sound intensity behaves more predictably, decreasing with distance according to the inverse square law. This distinction is important for accurate measurements and interpretations.

41. What is the relationship between sound intensity and sound pressure in a plane wave?

In a plane wave, sound intensity (I) is related to sound pressure (p) by the equation I = p²/(ρc), where ρ is the density of the medium and c is the speed of sound in the medium. This relationship assumes a free field and far field conditions.

42. How does atmospheric attenuation affect sound intensity over long distances?

Atmospheric attenuation causes sound intensity to decrease more rapidly than predicted by the inverse square law alone. This effect is more pronounced at higher frequencies and is influenced by factors such as temperature, humidity, and atmospheric composition.

43. What is the significance of the 6 dB rule in relation to sound intensity and distance?

The 6 dB rule states that sound intensity level decreases by 6 dB for each doubling of distance from a point source in a free field. This rule is derived from the inverse square law and is useful for quick estimations of sound level changes with distance.

44. What is the role of sound intensity in determining the directivity of a sound source?

The directivity of a sound source is determined by measuring sound intensity at various angles around the source. This creates a directivity pattern, showing how sound intensity varies with direction, which is crucial for understanding and characterizing sound sources.

45. How does sound intensity relate to the concept of acoustic radiation efficiency?

Acoustic radiation efficiency is the ratio of the actual sound power radiated by a source to the theoretical maximum power it could radiate. It relates to how effectively a source converts its input energy into sound energy, which directly affects the sound intensity produced.

46. What is the significance of sound intensity in the context of noise control engineering?

In noise control engineering, understanding and measuring sound intensity is crucial for identifying noise sources, assessing their impact, and designing effective noise reduction strategies. It helps in pinpointing the direction and strength of sound energy flow, which is essential for targeted noise control measures.

47. How does the concept of sound intensity apply to the characterization of musical instruments?

Sound intensity measurements help characterize the power output and directivity patterns of musical instruments. This information is valuable for understanding how instruments project sound, designing performance spaces, and creating realistic sound reproductions.

48. What is the relationship between sound intensity and the perception of pitch?

While sound intensity primarily affects the perception of loudness, it can also influence pitch perception, especially at extreme intensities. Very loud sounds may be perceived as slightly higher in pitch, a phenomenon known as the intensity-pitch effect.

49. How does sound intensity relate to the concept of acoustic streaming?

Acoustic streaming is a steady fluid flow induced by high-intensity sound waves. The intensity of the sound wave directly influences the strength of this streaming effect, which can be significant in applications like ultrasonic cleaning or acoustic levitation.

50. What is the role of sound intensity in determining the acoustic absorption coefficient of materials?

The acoustic absorption coefficient of a material is determined by measuring the ratio of absorbed to incident sound intensity. This measurement is crucial for characterizing acoustic materials and designing spaces with specific acoustic properties.

51. How does sound intensity measurement contribute to the assessment of hearing protection devices?

Sound intensity measurements are used to evaluate the effectiveness of hearing protection devices by comparing the intensity levels with and without the protection in place. This helps in determining the noise reduction rating (NRR) of these devices.

52. What is the significance of sound intensity in underwater acoustics?

In underwater acoustics, sound intensity measurements are crucial for various applications, including sonar systems, marine life studies, and underwater communication. The different acoustic properties of water compared to air (like higher density and sound speed) affect how sound intensity is measured and interpreted underwater.

53. How does sound intensity relate to the concept of acoustic cloaking?

Acoustic cloaking aims to redirect sound waves around an object, making it "invisible" to sound. This involves manipulating the sound intensity field around the object, often using metamaterials designed to guide sound waves in specific patterns.

54. What is the role of sound intensity in architectural acoustics?

In architectural acoustics, understanding and controlling sound intensity is crucial for designing spaces with desired acoustic properties. It affects how sound propagates within a room, influences speech intelligibility, and determines the overall acoustic comfort of a space.

55. How does sound intensity measurement contribute to the study of psychoacoustics?

In psychoacoustics, precise measurements of sound intensity are essential for studying how humans perceive and respond to sound. This includes research on loudness perception, masking effects, and the relationship between physical sound properties and subjective auditory experiences.

56. What is the significance of sound intensity in the context of environmental noise assessment?

In environmental noise assessment, sound intensity measurements are used to evaluate noise pollution levels, assess compliance with regulations, and determine the impact of noise sources on surrounding areas. This information is crucial for urban planning, noise mitigation strategies, and public health policies.