“Sound intensity”的意思、由来-开放百科全书

词条

Sound intensity

释义

Mathematical definition
Inverse-square law
Sound intensity level
Measurement
References
External links

{{Sound measurements}}Sound intensity level also known as acoustic intensity is defined as the power carried by sound waves per unit area in a direction perpendicular to that area. The SI unit of intensity, which includes sound intensity, is the watt per square meter (W/m²). One application is the noise measurement of sound intensity in the air at a listener's location as a sound energy quantity.^[1]

Sound intensity is not the same physical quantity as sound pressure. Hearing is directly sensitive to sound pressure which is related to sound intensity. In consumer audio electronics, the level differences are called "intensity" differences, but sound intensity is a specifically defined quantity and cannot be sensed by a simple microphone.

The rate at which sound energy passes through a unit area held perpendicular to the direction of propagation of sound waves is called intensity of sound.

Mathematical definition

Sound intensity, denoted I, is defined by

where

p is the sound pressure;
v is the particle velocity.

Both I and v are vectors, which means that both have a direction as well as a magnitude. The direction of sound intensity is the average direction in which energy is flowing.

The average sound intensity during time T is given by

Also,

Where,

is frequency of sound,

is the amplitude of the sound wave particle displacement,

is speed of sound, and

is density of medium in which sound is traveling

Inverse-square law

For a spherical sound wave, the intensity in the radial direction as a function of distance r from the centre of the sphere is given by

where

P is the sound power;
A(r) is the surface area of a sphere of radius r.

Thus sound intensity decreases as 1/r² from the centre of the sphere:

This relationship is an inverse-square law.

Sound intensity level

{{Other uses|Sound level (disambiguation){{!}}Sound level}}Sound intensity level (SIL) or acoustic intensity level is the level (a logarithmic quantity) of the intensity of a sound relative to a reference value.

It is denoted L_I, expressed in dB, and defined by^[2]

where

I is the sound intensity;
I₀ is the reference sound intensity;
{{no break|1=1 Np = 1}} is the neper;
{{no break|1=1 B = (1/2) ln(10)}} is the bel;
{{no break|1=1 dB = (1/20) ln(10)}} is the decibel.

The commonly used reference sound intensity in air is^[3]

being approximately the lowest sound intensity hearable by an undamaged human ear under room conditions.

The proper notations for sound intensity level using this reference are {{nobreak|L_{I /(1 pW/m²)}}} or {{nobreak|L_I (re 1 pW/m²)}}, but the notations {{nobreak|dB SIL}}, {{nobreak|dB(SIL)}}, dBSIL, or dB_SIL are very common, even if they are not accepted by the SI.^[4]

The reference sound intensity I₀ is defined such that a progressive plane wave has the same value of sound intensity level (SIL) and sound pressure level (SPL), since

The equality of SIL and SPL requires that

where {{nobreak|1=p₀ = 20 μPa}} is the reference sound pressure.

For a progressive spherical wave,

where z₀ is the characteristic specific acoustic impedance. Thus,

In air at ambient temperature, {{nobreak|1=z₀ = 410 Pa·s/m}}, hence the reference value {{nobreak|1=I₀ = 1 pW/m²}}.^[5]

In an anechoic chamber which approximates a free field (no reflection) with a single source, measurements in the far field in SPL can be considered to be equal to measurements in SIL. This fact is exploited to measure sound power in anechoic conditions.

Measurement

One method of sound intensity measurement involves the use of two microphones located close to each other, normal to the direction of sound energy flow. A signal analyser is used to compute the crosspower between the measured pressures and the sound intensity is derived from (proportional to) the imaginary part of the crosspower.

References

1. ^{{cite web|title=Sound Intensity|url=http://hyperphysics.phy-astr.gsu.edu/hbase/sound/intens.html|accessdate=22 April 2015}}
2. ^"Letter symbols to be used in electrical technology – Part 3: Logarithmic and related quantities, and their units", IEC 60027-3 Ed. 3.0, International Electrotechnical Commission, 19 July 2002.
3. ^Ross Roeser, Michael Valente, Audiology: Diagnosis (Thieme 2007), p. 240.
4. ^Thompson, A. and Taylor, B. N. sec 8.7, "Logarithmic quantities and units: level, neper, bel", Guide for the Use of the International System of Units (SI) 2008 Edition, NIST Special Publication 811, 2nd printing (November 2008), SP811 PDF
5. ^Sound Power Measurements, Hewlett Packard Application Note 1230, 1992.

External links

How Many Decibels Is Twice as Loud? Sound Level Change and the Respective Factor of Sound Pressure or Sound Intensity
Acoustic Intensity
Conversion: Sound Intensity Level to Sound Intensity and Vice Versa
Ohm's Law as Acoustic Equivalent. Calculations
Relationships of Acoustic Quantities Associated with a Plane Progressive Acoustic Sound Wave
Table of Sound Levels. Corresponding Sound Intensity and Sound Pressure
What Is Sound Intensity Measurement and Analysis?

4 : Acoustics|Sound|Sound measurements|Physical quantities

随便看

开放百科全书收录14589846条英语、德语、日语等多语种百科知识，基本涵盖了大多数领域的百科知识，是一部内容自由、开放的电子版国际百科全书。