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Active noise compensation is accomplished in the following manner: Each ear cup
includes a microphone, a feedback control circuit, and a transducer to reproduce both the
communication and the noise cancelling signal. The feedback control microphones sense
the total sound pressure within each ear cup resulting from both the desired radio signal
from the receiver and the undesired noise that has come through the ear cup. The
microphone signal is amplified and the radio signal is subtracted from it. The remaining
signal (noise) is then filtered and inverted and the radio signal is added back in.
Eventually, the entire signal is amplified and fed back to the transducer in each ear cup.
Since the noise component of the signal is inverted, it cancels the noise signal coming
through the ear cup. The radio signal remains unaffected, as it was not processed through
the cancellation circuits.
Noise compensation with NoiseGard
Attenuation
dB
0
10
20
active
30
40
50
25
50
100
The figure illustrates the noise compensation provided by NoiseGard: A conventional
noise attenuating headset provides a substantial noise reduction at frequencies above 500
Hz but not below. Unfortunately, some strong components of aircraft motor and wind
noise occur at frequencies below 500 Hz. By means of active noise compensation, noise
within the frequency range from 25 - 500 Hz is attenuated by about 25 dB. The combined
active/passive noise compensation response of the system amounts to approximately 30
dB over the entire audio range.
A noise reduction of about 10 dB is subjectively perceived as a halving of the noise level.
If the noise is reduced by additional 10 dB, this is again perceived as a halving of the noise
level, etc.
THE
passive
200
500
1000
2000
PRINCIPLE
5000
10000
15000 Hz
Frequency
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