Appeal No. 1998-2675
Application 08/779,024
Chaplin discloses that in a prior art "virtual earth"
active attenuation system shown in figure 1, a
pressure-sensing microphone 1 is located a distance d from a
loudspeaker (or driver) 2 in a closed feedback path to create
a constant or null pressure. The smaller the distance d, the
higher will be the critical frequency fr, and the greater is
the bandwidth over which the system can be used to achieve
active attenuation (col. 1, lines 34-39). In order to
minimize the delay around the loop and hence reduce
instability, the microphone must be placed as close as
possible to the loudspeaker (col. 1, lines 50-52). Thus, the
microphone is located in the near field of the speaker where
the sound pressure changes much more rapidly with distance as
compared to some position more remote from the speaker
(col. 1, lines 52-56). This means that the pressure waveform
at the microphone 1 by the driver 2 matches the primary
vibration field only over a very localized region of space,
thus limiting the region of null pressure.
Chaplin discloses that the working range of a virtual
earth system can be enhanced by feeding the vibrations of the
driver into an acoustically partially closed volume whose
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