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{{Language_sel|LANG=ger|ARTIKEL=Schalldruck}}
{{PSM_Infobox}}
Sound pressure
__FORCETOC__

==Definition==

Sound pressure ''p'' is a physical [[Measured Variable|measured variable]] (see also: [[Material Parameter|material parameter]]) that is widely used in [[Non-destructive Polymer Testing|non-destructive polymer testing]], particularly in [[Resonance Analysis|acoustic resonance analysis]] and [[Ultrasound Testing|ultrasound testing]], and is expressed in pascals (Pa). Because specifying sound pressure in pascals is rather impractical (it covers a very large range from 10-5 to 102 Pa), the logarithmic sound pressure level (abbreviated to sound level) is used. This is the logarithmic effective value of the sound pressure ''p'', which is related to the sound pressure ''p''0 of the human hearing threshold at a frequency of 1 kHz. The specification of the level in dB (decibels) has been introduced and has proven itself in practice. The level is calculated from:

{|
|-
|width="20px"|
|width="500px" | <math>L_p\,=\,20\, log\frac{p}{p_0}</math>
|}

Here,

{|
|-
|''p''
|width="15px" |
|is the sound pressure
|-
|''p''0
|
| is the reference value for airborne sound (2 • 10-5 Pa).
|}

Normalisation to a reference value is necessary because the logarithm can only be calculated from a dimensionless number.

'''Reference'''

* Hertlin, Ingolf: Informationsschriften zur zerstörungsfreien Prüfung – ZfP kompakt und verständlich. Volume 5: Akustische Resonanzanalyse. Castell-Verlag GmbH, Wuppertal (2003)

==Acoustic sound field variables==

The sound pressure (more precisely: sound alternating pressure) ''p''s is a scalar quantity and describes the change in pressure in a medium as a function of place and time. Together with the normal pressure ''p''0 (= air pressure), it forms the total pressure:

{|
|-
|width="20px"|
|width="500px" | <math>P\left(\vec r,t \right)\,=\,p_0\,+\,p_s\, \left(\vec r,t \right).</math>
|}

Sound pressure is of fundamental importance for calculating the [[Reflection Sound Waves|reflection]] or [[Transmission Sound Waves|transmission of sound waves]]. For example, the reflection coefficient is derived from the sound pressure ratio of the wave entering the [[Material & Werkstoff|material]] and the reflected wave.

Due to HUYGENS' principle, the elementary waves in the environment of the [[Piezoelectric Ceramic|oscillator]] overlap until only one interference maximum is formed, which represents the beginning of the far field, in which the sound pressure decreases to ~1/z (see '''Figure''').

[[File:Schalldruck.JPG|600px]]
{|
|- valign="top"
|width="50px"|'''Figure''':
|width="600px" |Dependence of sound pressure on location in units of near-field length ''N'' on the acoustic axis (according to [1])
|}

The sound pressure in [[Ultrasound Testing|ultrasound testing]] is small compared to the total pressure, so that the [[Density|density]] in the medium through which the sound travels can be regarded as constant, thus simplifying the equations for calculating acoustic variables such as [[Sound Velocity|sound velocity]] and specific attenuation [2, 3].

==See also==

* [[Sound Power|Sound power]]
* [[Ultrasonic Birefringence|Ultrasonic birefringence]]
* [[Ultrasonic Standard Sensors|Ultrasonic standard sensors]]
* [[Absorption Sound Waves|Absorption sound waves]]
* [[Reflection Sound Waves|Reflection sound waves]]
* [[Transmission Sound Waves|Transmission sound waves]]

'''References'''

{|
|-valign="top"
|[1]
|Deutsch, V.: Platte, M.; Vogt, M.: Ultraschallprüfung – Grundlagen und industrielle Anwendungen. Springer Verlag, Berlin Heidelberg (1997), (ISBN 3-540-62072-9; see [[AMK-Büchersammlung|AMK-Library]] under M 45)
|-valign="top"
|[2]
|Matthies, K. u. a.: Dickenmessung mit Ultraschall. DGZfP, Deutsche Gesellschaft für zerstörungsfreie Prüfung, (Hrsg.), DVS-Verlag, Berlin (1998) 2nd expanded Edition (ISBN 3-87155-940-7; see [[AMK-Büchersammlung|AMK-Library]] under M 44)
|-valign="top"
|[3]
|&Scaron;utilov, V. A.: Physik des Ultraschalls. Springer, Vienna New York (2012), ISBN 978-3-7091-8751-7; First Edition: Akademie Verlag, Berlin (1984)
|}

[[Category:Acoustic Test Methods_Ultrasonics]]

Sound Pressure - Revision history