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Sound Intensity, Power and Pressure Levels

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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)


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)


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)


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)


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))
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Related Topics


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.


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

Related Documents


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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