Low-frequency sound intensity measurement and free-field sound-pressure propagation

Classical acoustical equations relate the sound power of a source to sound-pressure level at a given distance from that source. Sound intensity of a source can be measured, sound power determined by integration over the radiation area, and the sound-pressure levels at a distance from the source can be calculated. An assumption is made when the sound power to sound-pressure calculation is made, that at low frequencies (20–100 Hz) the waves have propagated and the resultant sound-pressure level is from the source. This work tests the reliability of that assumption. A speaker was suspended inside three different size boxes, and powered by a noise generator bandpassed at discrete frequencies. Sound intensity measurements were recorded near field at the same discrete frequency. Free-field sound pressure measurements were then recorded at distances from the boxes. This paper reports the results of the measurements.