AN-000056
MEMS Microphones for Active Noise Cancellation Applications
InvenSense reserves the right to change the detail
specifications as may be required to permit improvements
in the design of its products.
InvenSense Inc.
1745 Technology Drive, San Jose, CA 95110 U.S.A
+1(408) 988–7339
www.invensense.com
Document Number: AN-000056
Revision: 1.0
Release Date: 5/12/2015
MEMS microphones are an attractive choice to use in active noise cancellation (ANC) applications. They provide high-performance
audio-acoustic specifications in a small size, and have very stable part-to-part performance and across a part’s lifetime. A description of
specific performance characteristics and their relevance to ANC follows.
FREQUENCY RESPONSE – MAGNITUDE
The microphone’s magnitude response at low frequencies is important for several reasons, mostly related to its phase response. A
microphone optimized for ANC should have a low-frequency corner, as low as possible. A microphone with a good low frequency
extension has well-matched part-to-part phase characteristics, minimal phase lead, and maximum low frequency SNR. The phase
effects are discussed in the next section. In general, the lower the microphone’s low frequency corner, the lower the limit frequency
at which the ANC algorithm is stable and performs well.
The microphone’s high-frequency response is not as important for ANC since active noise cancellation is not practical at frequencies
above a few kHz.
FREQUENCY RESPONSE – PHASE
Understanding a microphone’s phase response, and its effects on the performance of an ANC algorithm, is critical to selecting an
appropriate microphone and designing a high-performance system. Both the absolute phase response of a single microphone, and
the phase response variations from one microphone to the next, are important.
Absolute Phase
A microphone is a minimum-phase device, and its phase response is directly related to its magnitude response. For this discussion,
we will just consider the low frequency phase response. The microphone’s low frequency corner is defined as the point at which the
amplitude response is at −3 dB from the nominal response at 1 kHz, and also the point at which the phase response is at +45°. At
frequencies below this point, the amplitude response continues to roll off at −6 dB/octave and the phase lead continues to grow. In
Figure 1, you can see that the low frequency corner of this microphone is at 22 Hz, both from the −3 dB point of the magnitude
response and the +45° phase shift.
Figure 1. Example Microphone Phase and Magnitude Response
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Document Number: AN-000056
Revision: 1.0
Relative Phase
The relative phase from part-to-part describes the difference in response between all microphones that may be used in production.
This is usually specified as a deviation (±X°) from the nominal phase response; see an example in Figure 2. As the microphone’s
phase response is directly related to the magnitude response, a tightly matched magnitude response results in a similarly tightly
matched phase response. At frequencies a couple of octaves above the low frequency corner, the phase variation specification is
excellent; it may be only ±2-3°. Even at the corner frequency, the phase variation from the typical response may be only ±5°. This
tightly matched response allows the ANC processing to be done with greater confidence in a consistent output from any
microphone.
Figure 2. Example of Phase Variation from Typical Response of a MEMS Microphone
ECMs have a wide manufacturing tolerance for low-frequency corner, resulting in greater low frequency phase response variations
between microphones. To avoid the undesired phase variations within the ANC algorithms functional frequency range, an ECM’s
low-frequency corner has to be much lower than is strictly necessary for the microphone’s operation in the system. This results in
less part-to-part phase variation at frequencies higher than the low-frequency corner. In turn, this often results in undesired
overload of the ANC system by low frequency signals with very high amplitudes, such as wind noise. This limits the ANC
performance, and causes unpleasant audible artifacts.
Codec Contribution to Phase Response
The codec connected to the microphone’s output also contributes to the low frequency phase response. The connection between
the microphone and the codec is typically AC-coupled, so the system includes another high pass filter that adds to the low frequency
phase lead. The corner frequency of this high pass filter should be very low to minimize its effect. When the codec input’s first order
high pass filter corner is designed to be one decade below that of the microphone’s, the effect will be an additional 5° of phase lead
at the microphone’s corner frequency. Figure 3 shows the phase response of the microphone itself (same as in Figure 1), as well as
the phase of the microphone and the codec input, where the codec’s high-pass filter corner is one decade below that of the
microphones.
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