Page 359 - Mechatronics for Safety, Security and Dependability in a New Era

Ch70-I044963.fm Page 343 Friday, July 28, 2006 1:50 PM
Page 343
1:50 PM
Ch70-I044963.fm
Friday, July 28,2006
343
343
ACTIVE NOISE CANCELLATION HEARING
PROTECTOR WITH IMPROVED USABILITY

Mika Oinonen, Harri Raittinen, Markku Kivikoski

Tampere University of Technology, Institute of Electronics, P.O. Box
692, FIN-33101, Tampere, Finland

ABSTRACT

Low frequency performance of a passive hearing protector can be improved significantly by active
noise control methods. Tn a laboratory environment, the low frequency attenuation of a passive hearing
protector can be improved by as much as 25 dB. However, when the device is used in real-life
situations, several usability aspects must also be considered. A prototype of an active noise
cancellation hearing protector was constructed. The main goal was to achieve comfortable and
unnoticeable operation. Stability issues were also taken into account. The developed prototype is
comfortable to use because of its improved stability and reduced nonlinear distortion.

KEYWORDS

Hearing protector, stability, active control of sound, safety, wearable, human friendly

INTRODUCTION
With a passive hearing protector, noise attenuation decreases at frequencies lower than 500 Hz.
However, it is relatively easy to attenuate noise having a frequency of 500 Hz or less by implementing
active noise control (ANC) methods. Thus, it is possible to construct a very efficient hearing protector
by combining an active noise cancellation system with a high-quality passive hearing protector.

Active control of sound is based on destructive interference. When two sound waves of equal
magnitude but opposite in phase interfere, they cancel each other out. A personal hearing protector is
well suited to the application of active noise control, because the space is confined and the
characteristics of the plant are known, or at least can be measured. A relatively simple system can be
used. For example, a feedback system implemented by means of analog electronics can be used for
generating a quiet zone near the ear. When designing an active noise cancellation hearing protector, it
is relatively easy to achieve good performance in a laboratory environment. For practical purposes,
there are several design aspects that must be taken into account. First, there is a trade-off between
active attenuation, usability, and the operating frequency range. A large amount of active attenuation
requires a large amount of gain, which may lead to stability problems and increased distortion.