Sound Pressure

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Sound pressure

Definition

Sound pressure p is a physical measured variable (see also: material parameter) that is widely used in non-destructive polymer testing, particularly in acoustic resonance analysis and 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 p₀ 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:

$L_{p}\,=\,20\,log{\frac {p}{p_{0}}}$

Here,

p		is the sound pressure
p0		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 p₀ (= air pressure), it forms the total pressure:

$P\left({\vec {r}},t\right)\,=\,p_{0}\,+\,p_{s}\,\left({\vec {r}},t\right).$

Sound pressure is of fundamental importance for calculating the reflection or transmission of sound waves. For example, the reflection coefficient is derived from the sound pressure ratio of the wave entering the material and the reflected wave.

Due to HUYGENS' principle, the elementary waves in the environment of the 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).

Figure:

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 is small compared to the total pressure, so that the 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 and specific attenuation [2, 3].

See also

• Sound power
• Ultrasonic birefringence
• Ultrasonic standard sensors
• Absorption sound waves
• Reflection sound waves
• Transmission sound waves

References

[1]	Deutsch, V.: Platte, M.; Vogt, M.: Ultraschallprüfung – Grundlagen und industrielle Anwendungen. Springer Verlag, Berlin Heidelberg (1997), (ISBN 3-540-62072-9; see AMK-Library under M 45)
[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-Library under M 44)
[3]	Šutilov, V. A.: Physik des Ultraschalls. Springer, Vienna New York (2012), ISBN 978-3-7091-8751-7; First Edition: Akademie Verlag, Berlin (1984)