A Comparison of Three Methods of Measuring the Volume Velocity of an Acoustic Source
Measurement of the volume velocity of an acoustic source allows the acoustic transfer impedance seen by the source and its acoustic power output to be determined. An investigation of three sources is described whose volume velocity can be determined in different ways: using laser velocimetry, using measurement of the internal source pressure, and using a moving-coil loudspeaker as an output transducer (Salava's method). Practical implementation of each method is discussed. Using laser velocimetry as a reference measurement, the accuracy of the other two sources is determined. The total harmonic distortion at the acoustic output is also measured. Salava's method is shown to be superior in both respects. Example measurements of acoustic transfer impedance within a duct and in a well-damped room demonstrate the use of such sources as measurement tools. The former is shown to adhere well to theoretical predictions. Preliminary experiments are also reported concerning the practical measurement of acoustic power output, and the use of this measurement to maximize the acoustic power absorption of the source when exposed to an external sound field.
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