The standards DIN EN ISO 3744 (accuracy class 2) and DIN EN ISO 3746 (accuracy class 3) define the procedure for determining the sound power or sound energy level of a noise source. For this purpose, sound pressures are measured on an enveloping surface surrounding the noise source. The noise source is located in an environment that is approximately equivalent to an acoustic free field and has one or more reflecting planes. The following chart lists the symbols valid according to this standard.
| Symbol | Dimension | Unit |
| sound pressure level | ||
| sound power level | ||
| sound power | ||
|
reference sound power ( |
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| sound pressure |
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reference sound pressure |