Sound power measurements at low frequencies

The sound power level of a sound source is usually determined by sound intensity or sound-pressure measurements either in a free-field environment or in enclosures. Although these measurements provide good comparable results for the middle- and high-frequency range, there are large discrepancies at low frequencies. Using the sound intensity method as reference, it can be shown that these discrepancies are systematic. In the case of measuring sound pressure in a free field, they are due to the near-field error depending on the distance of the measurement surface to the source. Measuring sound pressure in enclosures—especially in rooms with hard walls—interference effects cause a change in the sound power output of the source, depending on the distance of the source to the room boundaries. The effects described above have been investigated theoretically and the results have been confirmed by accurate measurements.