Gunnar Rasmussen
Bruel & Kjaer, 2850 Naerum, Denmark
Intensity measurements are especially valuable when used for the detection of phenomena of a fast-varying nature. Sound pressure levels are greatly influenced by the character of the environment, the reverberation time, the location of acoustic centers, and other reactive conditions. The intensity data may be largely independent of these conditions especially when obtained in very short periods of time. Some standards require a wide frequency range, which often requires at least two spacings between the microphone acoustic centers and thus two measurements carried out with the same acoustical center for the probe. Many sources are not stationary and will therefore not offer the repeatability required for two measurements to be carried out with the required accuracy. Examples are vehicle passby, explosions, nonlinear structural vibration, reciprocating engines, and measurements during breakdown as well as run up and down. Techniques involving FFT technique may have serious shortcomings under such conditions. They will work extremely well during calibration and laboratory conditions. When there is a need for actual tests, possible errors may be difficult to assess. Spatial transformation will also require steady continuous signals. The response of transducers to very short duration signals, the influence of the transducer size and configuration, as well as ways of assuring a wide frequency range as required for standardized sound power measurement will be reported. The importance of the probe configuration for practical field measurements will be discussed.