We measured the input impedance characteristics, input voltage versus output sound pressure characteristics, harmonic distortion characteristics, frequency characteristics, and impulse response of a currently available miniature electrodynamic driver unit (Foster Electric, MT006B) when used as a loudspeaker with an open space load. The nominal input impedance of the driver unit was 16 , and its resonance frequency f 0 was 2.7 kHz. At f 0 , the range of the input voltage level over which the output sound pressure of the driver unit increased linearly was À10 dBV (reference 1 V) or less. Below f 0 , the frequency response of the driver unit decreased by 15dB/oct, while above f 0 it did not drop significantly. When the input signal level to the driver was À12 dBV, the signal-to-noise ratio between the sound pressure level produced by the driver and the background noise level of the soundproof room was 0 dB at a frequency of 130 Hz for a distance 0.2 m, and at 230 Hz for a distance 1.0 m. These results indicate that the MT006B can be used as an earplug speaker for a fast head-related transfer functions measurement system via reciprocity.

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