Acoustic load on the ear caused by headphones

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.