A cheap earphone for small animals with good frequency response in the ultrasonic frequency range
Introduction
Investigation in binaural hearing requires dichotic stimulation of individual ears which is generally achieved by earphones positioned in close opposition to the individual ears or by direct coupling the earphones to the external ear canal. Sound sources are either commercially available loudspeakers of different types in the lower frequency range or capacitive microphones driven as loudspeakers that also cover the ultrasonic frequency range. Especially when complex acoustic stimuli are used, an optimally flat frequency response of the system is desired. In the ultrasonic frequency range, high quality condenser microphones have relatively flat frequency response curves when used as loudspeakers and cover the necessary frequency range to 100 kHz and beyond, e.g. for investigations in bats. Schlegel (1977)has presented a coupler for commercially available quarter-inch condenser microphones suitable for bats with a smooth frequency response curve declining by approximately 35 dB in the range between 100 kHz and 10 kHz.
In an attempt to improve the frequency response, a new type of ultrasonic earphones was developed that needs no expensive high quality condenser microphones as sound source. Accidental damage to the speaker unit provokes, therefore, only minimal costs. The earphones were developed for auditory stimulation of bat's ears in neurophysiological experiments covering the frequency range between 10 and 120 kHz.
Section snippets
Material, methods and discussion
In conventional earphones, the adaptation of a condenser microphone as transducer to the ear coupler necessarily creates a small air volume between membrane and coupling tube as closer apposition would eventually damage the microphone membrane by mechanical impact. This geometry creates cross-sectional changes between the sound emitting surface and the coupling tube. The discontinuity creates frequency-dependent disturbances in sound transmission (through e.g. reflection and diffraction)
Conclusion
The new type of earphones is suitable for dichotic auditory stimulation of small mammals from 10 kHz up to the ultrasonic frequency range. The frequency range is flat from 10 to 120 kHz within 6 dB and up to 200 kHz within 10 dB. Distortions are moderate at medium (70 dB SPL) to low sound pressure levels of the fundamental and prominent mainly for low frequencies.
The earphones can be manufactured easily and no expensive parts are used (estimated total material costs below US$20). The low costs
Acknowledgements
We are indebted to Mr. Zaschka and the mechanical workshop of the department for excellent machining of the mechanical equipment. Supported by the Deutsche Forschungsgemeinschaft (DFG), Sonderforschungsbereich 204 `Gehör', TP 10.
References (2)
- Bruel and Kjaer (1972) Manual for 1/4″-Condenser...
- Schlegel, P. (1977) Calibrated earphones for the echolocating bat, Rhinolophus ferrumequinum, J. Comp. Physiol., 118:...
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