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)
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. -
Noise and Attenuation
Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems. -
Physiology
Human physiology, air quality and comfort temperatures, activity and metabolic rates, health effects of 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.
