Sound Power -, Intensity - and Pressure Level

An introduction to decibel, sound power, sound intensity and sound 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) 

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 - Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation
  • Physiology - Human physiology, air quality and comfort temperatures, activity and metabolic rates, health effects of carbon monoxide and more
  • Noise and Attenuation - Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems

Related Documents

  • Decibel - Decibel is a logarithmic unit used to describe the ratio of a signal level - like power, sound pressure, voltage, intensity etc. to a reference level
  • Sound Intensity - Sound intensity is the acoustic power of sound per unit area
  • Sound Power - Sound Power Level from common sources like fans, jet engines, cars, humans and more ..
  • Blade Dampers - Noise Generated - Sound power level generated in blade dampers
  • Maximum Sound Pressure Levels in Rooms - Maximum recommended sound pressure levels in rooms with activities like kindergartens, auditoriums, libraries, cinemas and more ...

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