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# Sound Intensity, Power and Pressure Levels

## Introduction to decibel, sound power, intensity and pressure.

### The Decibel

Decibel is a logarithmic unit used to describe ratios of physical values like - like power, sound pressure, voltage, intensity and more.

The decibel can be expressed as:

L = 10 log (S / Sref)                                        (1)

where

L = signal level (dB)

S = signal (signal unit)

Sref = signal reference (signal unit)

The Decibel Scale

### Sound Power Level

Sound power is the energy rate - the energy of sound per unit of time (J/s, W in SI-units) from a sound source.

Sound power can more practically be expressed in a logarithmic scale named Sound Power Level as the ratio of sound power to the sound power at the threshold of hearing - 10-12 W:

LN = 10 log (N / Nref)                                   (2)

where

LN = Sound Power Level (decibel, dB)

N = sound power (W)

Nref = reference sound power (10-12 W)

• The lowest sound power persons with excellent hearing can discern is about 10-12 W -  defined as 0 dB in the decibel scale
• The loudest sound power generally possible to encounter is that of a jet aircraft with a sound power of 105 W - 170 dB

### Sound Intensity Level

Sound intensity is the ratio acoustic or sound power to area. The SI-unit for Sound Intensity is W/m2

The Sound Intensity Level can be expressed as:

LI = 10 log (I / Iref)                                (3)

where

LI = sound intensity level (dB)

I = sound intensity (W/m2)

Iref = 10-12 - reference sound intensity (W/m2

### Sound Pressure Level

The sound pressure is the force (N) of a sound on a surface area (m2) perpendicular to the direction of the sound. The SI-unit for the Sound Pressure is N/m2 or Pa.

The Sound Pressure Level in decibel can be expressed as

Lp = 10 log (p2 / pref2)

= 10 log (p / pref)2

= 20 log (p / pref)                                  (4)

where

Lp = sound pressure level (dB)

p = sound pressure (Pa)

pref = 2 10-5 - reference sound pressure (Pa)

• If the pressure is doubled the sound pressure level is increased with 6 dB (20 log (2))

## Related Topics

• ### Acoustics

Room acoustics and acoustic properties. decibel A, B and C calculations. Noise Rating (NR) curves. Sound transmission through walls. Calculate sound pressure, sound intensity and sound attenuation.
• ### Noise and Attenuation

Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems.
• ### Physiology

Human physiology vs. air quality, comfort temperatures, activity and metabolic rates. Health effects of gases adn polutions like carbon monoxide and more.

## Related Documents

• ### Decibel

Logarithmic unit used to describe ratios of signal levels - like power or intensity - to a reference level.
• ### Decibel A, B and C

Sound pressure filters that compensates for the hearing sensed by the human ear.
• ### Maximum Sound Pressure Levels in Rooms

Maximum recommended sound pressure levels in rooms like kindergartens, auditoriums, libraries, cinemas and more.
• ### NC - the Noise Criterion

Noise Criterion - NC - level is a standard uesd to describe the relative loudness of a space within a range of frequencies.
• ### Noise - Acceptable dBA Level

Acceptable noise - dBA - level at typical common locations.
• ### Noise Exposure - Permissible Levels and Duration

Exposure to noise should be controlled so that it is less than a maximum combination of exposure level and duration.
• ### Noise generated in Blade Dampers

Sound power noise generated by blade dampers in ventilation systems.
• ### NR - the Noise Rating Curve

The Noise Rating (NR) Curve used to determine acceptable indoor environments for hearing preservation, speech communication and annoyance.
• ### Phonetic Alphabet

The phonetic alphabet used in international aircraft communications.
• ### Signals - Adding Decibels

The logarithmic decibel scale is convenient when adding signal values like sound power, pressure and others from two or more sources.
• ### Sound - Hearing Threshold vs. Age

Shift in hearing threshold for men and women vs. age.
• ### Sound Intensity

Acoustic power of sound per unit area.
• ### Sound Power

Sound power from sources like fans, jet engines, cars, humans and more.
• ### Sound Pressure

Sound Pressure is the force of sound on a surface perpendicular to the propagation of sound.
• ### Subjective Effect of Change in Sound Pressure Level

Change in sound pressure level and the subjective human perception.

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## Citation

• The Engineering ToolBox (2003). Sound Intensity, Power and Pressure Levels. [online] Available at: https://www.engineeringtoolbox.com/sound-power-intensity-pressure-d_57.html [Accessed Day Month Year].

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8.21.8

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