Measurement Techniques

Sound Intensity

In general there are two possible configurations to measure sound intensity. A combination of a pressure microphone and a Microflown, the pu probe and a configuration where two pressure microphones are closely spaced, the pp probe.

The pp probe has become a well-known and established method, described in several standards, the pu probe is relatively new and no standards have been written for this probe yet.

The quality of the methods depends on the sound field. The pp method has difficulties with large values of the pressure-intensity index (caused by reflections, diffuse sound fields or extraneous sound sources), but not to high values of the reactivity.By contrast the pu method is sensitive to high values of the reactivity (caused by sound fields with a high phase shifts, e.g. in the near field of a sound source), but not to large values of the pressure-intensity index. Both limitations can be serious in practical measurements, and one cannot conclude from these considerations that one method is superior to the other.

The size of the probes is different. The pp probe is very much larger than a pu probe. The size of a pp probe is in the order of 10cm and a pu probe can be as small as several millimeters. Small size is essential if the measurement space is small (measurement in small cavities) or the sound source is small (e.g. a hearing aid).

The bandwidth of a pu probe is larger (20Hz-20kHz) than the bandwidth of a pp probe. The pp probe can normally be used in a 100Hz-10kHz bandwidth.

At lower frequencies (f<3kHz) the selfnoise of a pu probe is better than a pp probe. This is only an issue with very low level noise sources.

Unlike the pp method, it is easy to construct a three dimensional broad banded pu sound intensity probe. Such 3D probe only requires 4 measurement channels (the pp method requires 6 channels).