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 Ω, and its resonance frequency f0 was 2.7 kHz. At f0, the range of the input voltage level over which the output sound pressure of the driver unit increased linearly was −10 dBV (reference 1 V) or less. Below f0, the frequency response of the driver unit decreased by 15dB/oct, while above f0 it did not drop significantly. When the input signal level to the driver was −12 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.