Sound intensity

	Email		Print
	Bookmark		Link

Sound intensity

The sound intensity, I, (acoustic intensity) is defined as the sound power

Sound power

Sound power or acoustic power Pac is a measure of sound energy E per time t unit.It is measured in watts, or sound intensity I times area A:...
P_ac per unit area A. The usual context is the noise measurement of sound intensity in the air at a listener's location. For instantaneous acoustic pressure p_inst(t) and particle velocity

Particle velocity

Particle velocity is the velocity v of a particle in a Transmission medium as it transmits a wave. In many cases this is a longitudinal wave of pressure as with sound, but it can also be a transverse wave as with the vibration of a taut string....
v(t) the average acoustic intensity during time T is given by

Notice that both v(t) and I are vectors, which means that both have a direction as well as a magnitude.

Discussion

Ask a question about 'Sound intensity'

Start a new discussion about 'Sound intensity'

Answer questions from other users

Full Discussion Forum

Encyclopedia

The sound intensity, I, (acoustic intensity) is defined as the sound power

Sound power

Particle velocity

Si, si, or SI may refer to :...
units of intensity are W/m² (watt

WATT

WATT is a radio station broadcasting a News radio-Talk radio-Sports radio format. Licensed to Cadillac, Michigan, it first began broadcasting in 1945....
s per square metre

Square metre

The square metre is the SI derived unit of area, with symbol m?. It is defined as the area of a square whose sides measure exactly one metre....
).

For a spherical sound source, the intensity in the radial direction as a function of distance r from the centre of the source is:

Here P_ac (upper case) is the sound power and A the surface area of a sphere of radius r. Thus the sound intensity decreases with 1/r² the distance from an acoustic point source, while the sound pressure decreases only with 1/r from the distance from an acoustic point source after the 1/r-distance law.

where p (lower case) is the RMS

RMS

RMS may refer to:...
sound pressure (acoustic pressure).

Hence

The sound intensity I in W/m² of a plane progressive wave is:

where:

Symbol	Units	Meaning
p	pascal Pascal (unit) The pascal is the SI derived unit of pressure, stress , Young's modulus and tensile strength. It is a measure of force per unit area i.e. equivalent to one newton per square meter or one joule per cubic meter.... s	RMS sound pressure Sound pressure Sound pressure is the local pressure deviation from the ambient pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water....
f	hertz Hertz The hertz is a measure of frequency per unit of time, or the number of list of cycles per second. It is the SI base unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts....	frequency Frequency Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency....
?	m, metre Metre The metre or meter is a Unit of measurement of length. It is the SI base unit of length in the metric system and in the International System of Units , used around the world for general and scientific purposes.... s	particle displacement Particle displacement Particle displacement or particle amplitude is a measurement of distance of the movement of a particle in a medium as it transmits a wave....
c	m Metre The metre or meter is a Unit of measurement of length. It is the SI base unit of length in the metric system and in the International System of Units , used around the world for general and scientific purposes.... /s Second The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time....	speed of sound Speed of sound Sound is a vibration that travels through an elasticity medium as a wave. The speed of sound describes how much distance such a wave travels in a certain amount of time....
v	m/s	particle velocity Particle velocity Particle velocity is the velocity v of a particle in a Transmission medium as it transmits a wave. In many cases this is a longitudinal wave of pressure as with sound, but it can also be a transverse wave as with the vibration of a taut string....
? = 2pf	radians/s Second The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time....	angular frequency Angular frequency In physics , angular frequency ? is a scalar measure of rotation rate. Angular frequency is the magnitude of the vector quantity angular velocity....
?	kg Kilogram The kilogram or kilogrammeThe spelling kilogram is used by the International Committee for Weights and Measures and the U.S.... /m Metre The metre or meter is a Unit of measurement of length. It is the SI base unit of length in the metric system and in the International System of Units , used around the world for general and scientific purposes.... ³	density of air Density of air The density of air, ? , is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics and other sciences. Air density decreases with increasing altitude, as does air pressure....
Z = c · ?	N Newton The newton is the International System of Units SI derived unit of force, named after Isaac Newton in recognition of his work on classical mechanics.... ·s Second The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time.... /m Metre The metre or meter is a Unit of measurement of length. It is the SI base unit of length in the metric system and in the International System of Units , used around the world for general and scientific purposes.... ³	characteristic acoustic impedance Acoustic impedance The acoustic impedance Z is a frequency f dependent parameter and is very useful, for example, for describing the behaviour of musical wind instruments....
a	m Metre The metre or meter is a Unit of measurement of length. It is the SI base unit of length in the metric system and in the International System of Units , used around the world for general and scientific purposes.... /s Second The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time.... ²	particle acceleration Particle acceleration In a compressible sound transmission medium - mainly air - air particles get an accelerated motion: the particle acceleration or sound acceleration with the symbol a in metre per second squared....
I	W WATT WATT is a radio station broadcasting a News radio-Talk radio-Sports radio format. Licensed to Cadillac, Michigan, it first began broadcasting in 1945.... /m²	sound intensity
E	W WATT WATT is a radio station broadcasting a News radio-Talk radio-Sports radio format. Licensed to Cadillac, Michigan, it first began broadcasting in 1945.... ·s Second The second , sometimes abbreviated sec., is the name of a units of measurement of time, and is the International System of Units SI base unit of time.... /m Metre The metre or meter is a Unit of measurement of length. It is the SI base unit of length in the metric system and in the International System of Units , used around the world for general and scientific purposes.... ³	sound energy density Sound energy density The sound energy density or sound density is an adequate measure to describe the sound field at a given point as a sound energy value. The letter "lower case w" sign is easily mixed with the sign , therefore we choose the letter E....
P_ac	W, watt WATT WATT is a radio station broadcasting a News radio-Talk radio-Sports radio format. Licensed to Cadillac, Michigan, it first began broadcasting in 1945.... s	sound power Sound power Sound power or acoustic power Pac is a measure of sound energy E per time t unit.It is measured in watts, or sound intensity I times area A:... or acoustic power
A	m Metre The metre or meter is a Unit of measurement of length. It is the SI base unit of length in the metric system and in the International System of Units , used around the world for general and scientific purposes.... ²	area Area Area is a quantity expressing the two-dimensional size of a defined part of a surface, typically a region bounded by a closed curve. The term surface area refers to the total area of the exposed surface of a 3-dimensional solid, such as the sum of the areas of the exposed sides of a polyhedron....

Sound intensity level

Sound intensity level or acoustic intensity level is a logarithmic measure of the sound intensity , in comparison to the reference level of 10-12 W/m2....
, L_I, is the magnitude of sound intensity, expressed in logarithmic units (decibel

Decibel

The decibel is a logarithmic units of measurement that expresses the magnitude of a physical quantity relative to a specified or implied reference level....
s). (dB-SIL), where I_o is the reference intensity, 10^-12 W/m²

Note 1: The term "intensity" is used exclusively for the measurement of sound in watts per unit area.
To describe the strength of sound in terms other than strict intensity, one can use "magnitude

Magnitude (mathematics)

The magnitude of a mathematical object is its size: a property by which it can be larger or smaller than other objects of the same kind; in technical terms, an ordering of the class of objects to which it belongs....
" "strength", "amplitude

Amplitude

Amplitude is the magnitude of change in the oscillating variable, with each oscillation, within an oscillating system. For instance, sound waves are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation....
", or "level

Level

Level may refer to:*A floor of a building*A level of a mine, see shaft mining. A level with access to the surface is an adit*Level , a stage of the game...
" instead.

Sound intensity is not the same physical quantity as sound pressure

Sound pressure

Sound pressure is the local pressure deviation from the ambient pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water....
. Hearing is directly sensitive to sound pressure which is . In stereo the level differences have been called "intensity" differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone, nor would it be valuable in music recording if it could.

Sound intensity

See also

External links