Sensor Design for Physiological Low-Frequency Vibration Measurement: A Study of
Sensor Parameters, Signal Amplification, and Fixation Methods in Measuring
Mechanomyography (MMG) in Clinical Environments
Alex Posatskiy1,2,3, BEng (Hons); Tom Chau2,3,4, PhD., P.Eng.
1
Department of Mechanical and Industrial Engineering, University of Toronto
Institute of Biomaterials and Biomedical Engineering, University of Toronto
3
Pediatric Rehabilitation Intelligent Systems Mobile (PRISM) laboratory, Holland Bloorview Kids Rehab,
4
Holland Bloorview Kids Rehab
2
Abstract
Objective: Measuring low-frequency physiological signals, such as mechanomyography (MMG
or “muscle sounds”), is difficult because of ambient noise, improper fixation to the subject, and
the small scale of the measurement. This study aimed at investigating (A) how sensor
parameters can be optimized to amplify the measured signal and, to some degree, isolate it
from outside noise sources, and (B) the variability in measurement with different fixation
methods. Target Population –This study focuses at the healthy adult population, and the results
will later be used for studying MMG in children in need of access solutions.
Background: Typically MMG is measured with accelerometers or condenser microphones
placed inside simple cylindrical housings. However, accelerometers are inherently sensitive to
limb movement, which makes extracting the actual signal of interest cumbersome in clinical
environments (i.e. where limb movement is prevalent). Condenser microphones are less
sensitive to limb movement, but are typically not designed for low-frequency measurement.
Furthermore, the microphone housings are chosen without particular attention to dimensions
and materials, and their corresponding effects on the quality of measurement.
Methods: To investigate the effect of sensor parameters on signal quality, conical and
cylindrical chamber housings were manufactured. A tensioned membrane (four microns in
thickness) transmits surface vibration to an intra-chamber pressure field. Sensors were clamped
and held by an articulating arm over a nylon stinger. The stinger was attached to an
electrodynamic transducer which sweeped through a range of frequencies at a constant
amplitude of five microns. Three to five trials were performed for each sensor. Each sample was
analyzed by calculating the power spectral density (PSD) and fitting the trials with an
approximate frequency response function. The secondary part of this research involved
investigating the variance of an acquired vibrational signal with different fixation methods. Of
particular interest was considering how loading pressure with a strap affects the sensor and, in
turn, the acquired signal.
Results/Conclusions: Preliminary results indicate that some cylindrical chamber geometries
may attenuate surface vibrations. Furthermore, results demonstrate that conical chambers can
be used to amplify surface vibrations. Also, fixation pressure has been shown to drastically
affect measurement. This fact is critical in clinical measurement, and can be of assistance in the
design of a new strap and/or the standardization of fixation pressure. Further work is currently
being performed to complete the understanding of chamber parameters and fixation pressure.