Acoustic Testing

• Sound velocity
  It tells us how fast the sound propagates in a medium. The speed of sound depends on temperature – at room temperature (20 degrees Celsius), for example, it is 343 meters per second.
• Sound power
  It describes the total sound energy emitted by a sound source per unit of time.
• Sound intensity
  The technical term stands for the sound power that passes through a sound absorbing surface per unit area.
• Sound pressure level
  The sound pressure level is a ratio, formed from the reference sound pressure (hearing threshold) and the instantaneous sound pressure.
• Sound power level
  It describes the noise generated by a product under operating conditions, for example.
• Averaging level
  Because the sound pressure level is not constant for many sound processes, but varies over time, they are often combined into a single value: the temporal average or average level.
• Rating level
  It can be used to consider subjective assessments of different types of noise pollution. For this purpose, the average level for the assessment period is often used. In addition, there may be surcharges for impulsiveness, tone and rest periods. The rating level is used, for example, for comparison with the immission guide values.

The following three methods are used in audio equipment testing: the enveloping surface method, the reverberation chamber method and the comparative sound source method.

• Enveloping surface method
  This is the most frequently used procedure, because I usually apply it directly at the location of the machines and devices to be tested. It involves measuring the sound power emitted by a noise source by measuring at numerous points on an imaginary enveloping or measuring surface that encloses the source.
• Reverberation chamber method
  The basis for this method is the fact that the sound power radiated into the room by a noise source must be as great as that absorbed by the room’s boundary surfaces. Thus the sound power can be determined from the absorbed sound energy, calculated from the average sound pressure level of the room and its sound absorption capacity.
• Comparison sound source method
  In it the sound power is determined by comparing the noise source to be investigated with a reference sound source. For this purpose, the sound pressure levels of both sound sources are recorded at several measuring points and averaged. In practice, this method is rarely used – the basic prerequisite is that a reference sound source and its exact sound power are known.