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Experiments were conducted on a reciprocating
tester (Fig.1), in which frictional sound was generated by
the contact of the flat side of mm disk on a flat bar,
attached to a moving stage. To reduce background noise
levels, the motor was placed in a separate sound insulated
compartment and magnetic screws were used to move the
stage. The inside of the test chamber was covered with
sound-absorbing material to eliminate sound reflections.
Normal load was applied by the elastic deformation of a
leaf spring when the XZ-stage is lowered down. The sound
was measured by a microphone, placed at about 2.5 cm
from the center of the upper specimen holder.
Specimens used in the experiment were made of
steel JIS - SUS 303. Experiments were conducted for all
the roughness combinations of the two specimens (Ta-
ble 1) under load settings of 0.5 and 1 N and sliding speed
40 mm/s.
An example of raw data acquired by the microphone
measurements is shown in Fig. 2. Long time averaged root
mean square (RMS) values are estimated from the raw
sound pressure signals and compared. Direct RMS values
comparison is preferred in this study to the more com-
mon comparison of decibel levels (logarithmically related
to RMS values), because the former represent the physi-
cal process, while the latter are used for convenience.
By comparison of the frequency spectra of frictional
sound against the background noise, it was found that in
the frequency band from 0.5 to 5 kHz the signal-to-noise
ratio was highest. All further analysis was carried in this
frequency band only.