Volume 107, Issue 4, April 2000
Index of content:
- ACOUSTICAL MEASUREMENTS AND INSTRUMENTATION 
107(2000); http://dx.doi.org/10.1121/1.428490View Description Hide Description
The standardized method for measurement of complex impedances according to ISO 10534 Part 2 is applied to the acoustic impedance of the ear with an “open-pinna” condition and with different types of headphones. The method is based on measurement of the transfer function of two microphone locations in an impedance tube and subsequent signal processing of the complex signal spectra. The termination of the tube is interpreted as ear canal entrance, while the measurement direction is, apparently, from “inside” the head towards outside. A tube which was specifically designed for this purpose works well, even though extremely small impedances must be measured. The impedances of the free pinna are similar to the “soft” end condition in the open tube, approximately following the radiation impedance of a piston into free space. The headphone impedances can be separated according to the type of headphone. In addition, the absolute impedances as the differences to the open ear compared with a number of headphones are interesting and may be starting point for further investigations. One possibility is, of course, quality control of headphones. The results are also expected to be useful for psychoacoustic research, for better understanding of sound perception, and for use in development of audio equipment.
107(2000); http://dx.doi.org/10.1121/1.428491View Description Hide Description
This paper outlines a method that has been implemented to predict and measure the acoustic radiation generated by ultrasonic transducers operating into air in continuous wave mode. Commencing with both arbitrary surface displacement data and radiating aperture, the transmitted pressure beam profile is obtained and includes simulation of propagation channel attenuation and where necessary, the directional response of any ultrasonic receiver. The surface displacement data may be derived directly, from laser measurement of the vibrating surface, or indirectly, from finite element modeling of the transducer configuration. To validate the approach and to provide experimental measurement of transducer beam profiles, a vibration-free, draft-proof scanning system that has been installed within an environmentally controlled laboratory is described. A comparison of experimental and simulated results for piezoelectric composite, piezoelectric polymer, and electrostatic transducers is then presented to demonstrate some quite different airborne ultrasonic beam-profile characteristics. Good agreement between theory and experiment is obtained. The results are compared with those expected from a classical aperture diffraction approach and the reasons for any significant differences are explained.