ANR Module
The
electronic ANR Module uses a closedloop feedback circuit. A
small 'sense' microphone is positioned in the earshell cavity to
detect the
noise appearing at the ear. The signal is filtered and amplified
and is then used to drive the earphone, also inside the earshell,
to generate an identical acoustic signal which is in anti-phase
(180 degrees phase shifted to the original sound).
The introduction of the real time anti-phase signal within the
earshell results in a high level of cancellation, which can reduce
the low frequency energy by
up to 97%. This, when combined with the passive attenuation of
the headset, provides a broadband noise reduction performance of
30
dB(A). Understanding Decibels
The heart of the module is a Racal designed electronic circuit,
which incorporates the speech pre-amplifier, feedback filter and
drive amplifier, and additionally provides
the overall system stability control essential for correct
operation in a dynamic environment. The circuit is mounted on a
small Surface Mount Technology PCB, which also accommodates the
passive components needed to optimise
and interface the ANR system for specific applications.
By using this approach, the overall size and weight of the ANR
system is minimised, allowing its incorporation in the earshell of
the headset without the typical degradation of passive attenuation
associated with other methods of integration.
The electronic PCB is mounted on a small plastic moulding, which
also houses the earphone transducers and sense microphone. The
board is sealed to the moulding to
maintain the integrity of the acoustic transfer function and the
component side of the board is internal to the module to provide a
higher level of handling and environmental protection.
Fail Safe
In addition to the ANR earphone, a second conventional service
type earphone is provided for communication such that, should the
ANR circuit or ANR earphone fail, communications will still be
received. This reversionary mode enables the headset to be used as
a conventional headset when required. If required, a switch can be
provided on the headset to disable the ANR circuit when used in the
reversionary mode.