Mechanical behaviour of condenser microphone in mechanomyography

Condenser microphones (MIC) have been widely used in mechanomyography, toge ther with accelerometers and piezoelectric contact sensors. The aim of the present investigation was to clarify the mechanical variable (acceleration, velocity or displacement) indicated by the signal from a MIC transducer us ing a mechanical sinusoidal vibration system. In addition, the mechanomyogr am (MMG) was recorded simultaneously with a MIC transducer and acceleromete r (ACC) during voluntary contractions to confirm the mechanical variable re flected by the actual MMG and to examine the influence of motion artifact o n the MMG. To measure the displacement-frequency response, mechanical sinus oidal vibrations of 3 to 300 Hz were applied to the MIC transducer with dif ferent sizes of air chambers (5, 10, 15 and 20 mm in diameter and 15 20 or 25 mm long). The MIC transducer showed a linear relationship between the ou tput amplitude and the vibration displacement, however, its frequency respo nse declined with decreasing diameter and decreasing length of the air cham ber. In fact, the cut-off frequency (-3 dB) of the MIC transducer with the 5 mm-diameter chamber was 10, 8 and 4 Hz for the length 15 20 and 25 mm, re spectively. The air chamber with at least a diameter of 10 mm and a length of 15 mm is recommended for the MIC transducer. The sensitivity of this MIC transducer arrangement was 92 mV mum(-1) when excited at 100 Hz. During vo luntary contraction, the amplitude spectral density function of the MMG fro m the MIC transducer resembled that of the double integral of the ACC trans ducer signal. The angle of the MIC transducer was delayed by 180 degrees in relation to the ACC transducer signal. The sensitivity of the MIC transduc er was reduced to one-third because of the peculiar volume change of air ch amber when the MMG was detected on the surface of the skin. In addition, th e MIC transducer was contaminated by a smaller motion artifact than that fr om the ACC transducer. The maximal peak amplitude of the MIC and ACC transd ucer signal with the motion artifact was 7.7 and 12.3 times as much as the RMS amplitude of each signal without the motion artifact, respectively Thes e findings suggest that the MIC transducer acts as a displacement meter in the MMG. The MIC transducer seems to be a possible candidate for recording the MMG during dynamic muscle contractions as well as during sustained cont ractions.