Mackie SP-DSP1, Reference Manual

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Mackie Industrial White Paper Noise Sensing
September 2000
which signicantly reduced the adaptation amount when the
error was small, extending the adaptation time to well over 30
minutes. Our
beta function (patent pending) gives stability to
the algorithm when the error is large, but effectively shortens
the adaptation time dramatically when the error becomes
small. This allows the algorithm to adapt to a room in only
a few minutes.
For small rooms a signicant portion of the music energy due
to the paths from speaker to microphone (including reec-
tions) are removed by the nLMS algorithm, as there would
have to be many reections before the sound could have trav-
eled this far. Each reection reduces the energy of the music
signal, and the amount of reduction of the energy is related
to the shape and material of the reecting surface. The more
‘live’ the room, the longer and more dense the RTF is, the
less the energy of the music signal that can be removed by the
nLMS lter [6]. Even without a lot of reections a very large
room may have poor cancellation due to the small number of
reections before the music signal has traveled the maximum
‘distance’ of the FIR.
Diagram 1: Matlab simulation Block Diagram and Results