Page 361 - Mechatronics for Safety, Security and Dependability in a New Era

Ch70-I044963.fm Page 345 Friday, July 28, 2006 1:50 PM
Friday, July 28,2006
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acoustic plant was to avoid sharp resonance peaks in the transfer function. Any cavities, which could
store acoustic energy were avoided.
The developed prototype was installed inside a high-quality passive hearing protector. The secondary
source was installed in a small enclosure, and the error sensor was placed near the secondary source.
The complete secondary source and error sensor -assembly was installed so that it is located near the
ear. The assembly described above was installed in both earcups of the hearing protector. A 3.6 V
lithium-ion -battery was installed inside one earcup and the ANC controller was installed inside the
other earcup.

IN-EAR MEASUREMENTS
The performance of the implemented prototype was measured using in-ear method. The sound
pressure level at the entrance of the ear canal was measured without hearing protection, with passive
hearing protection and with active hearing protection.
A Sennheiser KE 4-211-1 electret microphone capsule was placed at the entrance of the ear canal. The
microphone was connected to a self-made batteiy-powered microphone pre-amplifier. The amplified
signal was recorded by a Sony DCD-D8 portable DAT recorder and analysed using a Briiel & Kjaer
2260 Investigator sound level meter. Fast time constant and A-weighting were used. The
measurements were made in a room equipped with sound-absorptive material covering the walls and
ceiling. Two different kinds of noise stimuli were used. The first stimulus was a 30-second sample of
pink noise generated by a self-made digital pink noise generator. The other stimulus was a 30-second
sound recorded inside a moving tank, and it was played by a Sony ZA5ES DAT player. The signal
sources were connected to an active speaker system via an AMC Stereo Preamplifier 1100. The active
speaker system consists of two Genelec 1030A two-way monitors and one Genelec 1092A subwoofer
unit.

The first stimulus was pink noise. The sound pressure levels without hearing protection (solid line),
with passive hearing protection (dotted line) and with active hearing protection (dash-dot -line) are
presented in Figure 2a. A sound sample of a moving tank served as the second stimulus. The sound
pressure levels without hearing protection (solid line), with passive hearing protection (dotted line) and
with active hearing protection (dash-dot -line) are shown in Figure 2b.

The dynamic range of the device was also tested. Pink noise was used as a stimulus and the sound
pressure level was increased gradually from 80 dB SPL to 100 dB SPL. It was clear that the effect of

a) 90

10 F(Hz) F(Hz) 10" 10'

Fig. 2. Third octave band levels of the noise with pink noise (a) and tank noise (b).