Mechanical behaviour of condenser microphone in mechanomyography

Abstract

Condenser microphones (MIC) have been widely used in mechanomyography, together 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 using a mechanical sinusoidal vibration system. In addition, the mechanomyogram (MMG) was recorded simultaneously with a MIC transducer and accelerometer (ACC) during voluntary contractions to confirm the mechanical variable reflected by the actual MMG and to examine the influence of motion artifact on the MMG. To measure the displacement-frequency response, mechanical sinusoidal vibrations of 3 to 300 Hz were applied to the MIC transducer with different 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 output amplitude and the vibration displacement, however, its frequency response declined with decreasing diameter and decreasing length of the air chamber. In fact, the cut-off frequency (-3dB) of the MIC transducer with the 5-mm-diameter chamber was 10, 8 and 4 Hz for the length 15, 20 and 25 mm, respectively. 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 microm(-1) when excited at 100 Hz. During voluntary contraction, the amplitude spectral density function of the MMG from the MIC transducer resembled that of the double integral of the ACC transducer signal. The angle of the MIC transducer was delayed by 180 degrees in relation to the ACC transducer signal. The sensitivity of the MIC transducer was reduced to one-third because of the peculiar volume change of air chamber when the MMG was detected on the surface of the skin. In addition, the MIC transducer was contaminated by a smaller motion artifact than that from the ACC transducer. The maximal peak amplitude of the MIC and ACC transducer 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. These 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 contractions.

Influence of Force Tremor on Mechanomyographic Signals Recorded with an Accelerometer and a Condenser Microphone during Measurement of Agonist and Antagonist Muscles in Voluntary Submaximal Isometric Contractions

“…Mathematically, each of the physical values of acceleration and displacement can be converted to the other's using integral or differential computation. Watakabe et al (2001) proved that the amplitude spectral density function (ASD) of the MIC is similar to the double integral of the ACC transducer signal, and suggested that the MIC acts as a displacement meter in the MMG. The MMG signals recorded by a MIC and an ACC showed differences in frequency response (Orizio, 1993;.…”

“…However, the sensitivity of the MIC may be influenced by the size of its air chamber because the MIC detects the mechanical activity of the muscle via the air chamber. Watakabe et al (2001) proved in a mechanical vibration test that the sensitivity of frequency response did not change if the air chamber is at least 10 mm in diameter and 15 mm in length, though there existed a cut-off frequency of 2-4 Hz. In the present study we used an air chamber of 10 mm in diameter and 15 mm in length.…”

“…2c). It has been suggested that the MMG signal might include FT and motion artifacts, which may affect the interpretation of MMG signals (Barry, 1987;Oster and Jaffe, 1980;Itoh et al, 2000;Goldenberg et al, 1991;Orizio, 1993;Tarata, 2003;Watakabe et al, 2001). Because different MMG transducers have different inherent characteristics, the influence of FT and motion artifacts may differ.…”

“…However, FT (Barry, 1987;Oster and Jaffe, 1980;Itoh et al, 2000;Goldenberg et al, 1991;Orizio, 1993) and motion artifacts (Tarata, 2003;Watakabe et al, 2001) have been…”

Biomechanical Assessments in Sports and Ergonomics

“…Piezoelectric contact sensors are mostly obsolete due their weight, and the applied pressure to obtain a mechanical coupling dampens the recorded MMG signal. Condenser microphones acting as a displacement meter are still used from time to time but they also require a coupling, e.g., air or gel between the muscle and the microphone (Watakabe et al, 2001). Further, the volume of the air-chamber influences the amplitude and the frequency content of the recorded MMG signal.…”