Measured specifications from protype
Frequency response: 20 Hz to 20 kHz +-1dB
Noise level (A-weighted): -85 dBm
Amplification: 35 dB
Because of the simplicity of the design the distortion performance is not very good. At signal levels typically used by electret microphones the distortion is about 2-3%. With dynamic microphones the distortion level is lower (not measured). Here is the frequency response as measured by LoudSpeaker LAB software DEMO version with Sound Blaster 16 PNP card:
The bass frequency attenuation is caused by the microphone preamplifier circuit. The high end attenuation is caused by Sound Blaster 16 card. As seen in the measured performance, the microphone preamplifier is suitable for speaker measurements made using suitable measurement software and sound card. Using this preamplifier connected to line level input the problems caused by poor microphone preamplifier in many sound cards can be avoided.