WS PRESENTATION TEMPLATE

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Jean-Pierre Jallet Car Active Noise Cancellation
for improved car efficiency, From/In/To car
voice communication and music listening
experience NXP Semiconductors
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Content

• Noise in the Car.
• Active Noise Cancellation.
• Car Active Noise Cancellation.
• Motivation.
• Technology.
• Benefit.
• In-car music listening experience improvement.
• Hands-free voice communication noise reduction.
• Integration of the car ANC, car audio and car
  hands-free voice communication systems.
• Conclusion.

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Noise in the car

• Car is a noisy environment noise level between
  65 and 75 dBA.
• Three main sources of noises
• Road noise
• Caused by the interaction between the tires and
  the road surface.
• Drumming/Booming structure borne, occurs when
  one of the first resonance frequencies of the car
  is excited.
• Low-frequency, stochastic noise.
• Engine noise
• Caused by the firing of the engine.
• Booming when resonance frequencies are excited.
• Mainly low-frequency noise, periodic with
  peaks correlated with
  the engine rotational speed.
• Wind noise
• Caused by the interaction between the wind and
  the car structure.
• Higher frequency, stretches beyond a few hundred
  Hz, stochastic noise.
• Road noise is dominant, engine noise is second.
  In some circumstances,
  uphill, variable displacement
  engine, engine noise can become dominant.
• The resulting noise is
• Mainly diffuse comes from all directions.
• Low frequency most energy lies under 500Hz.

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Active Noise Cancellation

• The principle of Active Noise Cancellation is to
  reduce the noise perceived by the car passengers
  by means of superposition of the same noise
  signal but in anti-phase, i.e. by playing
  anti-noise.
• For diffuse noise, ANC can provide 10dB
  cancellation in a sphere with a diameter of
  1/10th of the wave length (ex. 30cm for 100Hz)
  ANC is perfectly suited to cancel low frequency
  noise.
• ANC structures
• Analogue, hybrid (analoguedigital) or digital
  ANC filters.
• Fixed or adaptive ANC filters.

• Adaptive Feedback controller using only an error
  microphone.

• Adaptive Feedforward controller using a noise
  reference signal and an error microphone (for
  adaptive ANC).

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Car Active Noise Cancellation Motivation (1)

• In current cars, noise is reduced by passive
  countermeasures like car structure/stiffness/dynam
  ics modifications, sealing, damping/absorption
  material
• Passive countermeasure effectiveness is good for
  higher frequencies but decreases with decreasing
  noise frequency.
• Most of these passive countermeasures increase
  the car weight.
• Passive countermeasures cannot reduce the noise
  generated by energy saving variable displacement
  engines (engines that deactivate part of their
  cylinders in light load operation)
• When cylinders are deactivated, the
  engine sound level is
  higher in magnitude
  and lower in frequency.
• Variable displacement engines are like
  two/three different engines in
  one car.
• Current demand for energy consumption and
  emission reduction lead to search for car weight
  reduction.

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Car Active Noise Cancellation Motivation (2)

• Active Noise Cancellation fits well for car noise
  reduction
• Car noise is mainly diffuse, low frequency noise
  ANC cancels efficiently diffuse low frequency
  noise.
• Car ANC is able to reduce car engine noise and
  car road noise
• Engine noise can be reduced by adaptive
  feedforward ANC using the engine rotational speed
  as noise reference.
• Road noise can be reduced by adaptive feedforward
  or feedback ANC.
• ANC is able to reduce the noise of variable
  displacement engines
• High magnitude low frequency noise in variable
  displacement operation ANC cancels efficiently
  low frequency noise.
• VD engines have different noise characteristics
  in normal operation
  and VD operation
  ANC can be adaptive.
• ANC adds very little weight.

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Car Active Noise Cancellation Technologies (1)

• Active car engine Noise Cancellation
• Adaptive digital feedforward controller.
• Uses a noise reference the engine rotational
  speed.
• Uses an error microphone placed close to the
  passenger ears within 1/10th of the wave length
  of the highest cancelled frequency.
• Feedforward filter w is computed such that the
  power in the error microphone signal is
  minimized.
• Adaptive algorithm like filtered-x Normalized
  Least Mean Square or filtered-x Single-frequency
  Adaptive Notch.

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Car Active Noise Cancellation Technologies (2)

• Active road Noise Cancellation two approaches
• Adaptive multiple references digital Feedforward
  controller
• Uses several (typically 8) accelerometers as
  noise references to provide enough correlation
  between the noise references signals and the
  ambient noise component.
• Same algorithm as for engine noise but much more
  complex and computing intensive due to the
  multiple noise references.
• Algorithmically complex and expensive solution.
• Adaptive Feedback
• No noise reference needed.
• Uses only an error microphone.
• Fits very well for drumming noise because the
  first resonance frequency of the car structure is
  typically around 50Hz.
• Issue ANC should not cancel the low frequencies
  of the music.
• Requires the cancellation of the acoustic echo of
  the car music in the error microphone signal.

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Car Active Noise Cancellation Benefit (1)

• Weight saving
• less damping/absorption material.
• Car structure weight reduction.
• Leads to
• Energy consumption reduction.
• Emission reduction.
• Cost saving
• Car manufacturing less damping material and
  cheaper car structure.
• Car use energy consumption reduction.
• Better noise reduction
• Limited effectiveness of passive countermeasures
  for low frequency noise.
• Variable displacement engines noise cannot be
  reduced by passive countermeasures.
• Improved car dynamics.

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Car Active Noise Cancellation Benefit (2)
In-car music listening experience improvement

• The music bass are masked by the high level low
  frequency car noise. Car noise produces a high
  pass filtering perceptual effect for the car
  music listener.
• State of the art (adaptive) bass-boost solutions
  de-masks the bass but at the expense of a
  modified frequency balance.
• ANC de-masks the music bass without frequency
  balance modification and restores a listening
  experience closer to the one obtained in
  quiet/low noise environment.
• Original Bass
  boost Active Noise Cancellation

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Car Active Noise Cancellation Benefit (3)
Hands-free voice communication noise reduction

• Noise remains a problem for car hands-free voice
  communication
• At medium and high speed, the speech to noise
  ratio for low frequencies can drop below 0 dB.
• The noise is mainly diffuse and located in the
  low frequency range.
• Sending path noise reduction technologies provide
  only limited low frequency diffuse noise
  reduction
• Single microphone technologies only reduce
  stationary noises and can provide a maximum
  reduction of about 13dB.
• Superdirectional microphone array technologies
  attenuation for diffuse low frequency noise is
  limited by
• Polar selectivity in the low frequency range.
• Uncorrelated low frequency noise amplification.
• Additional low frequency,
  diffuse noise
  reduction
  methods are needed.

4 microphone endfire array vs. 1 cardioid
microphone
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Car Active Noise Cancellation Benefit (4)
Hands-free voice communication noise reduction

• Active Noise Cancellation at the voice pick-up
  microphone location provides additional low
  frequency noise reduction, on top of sending
  path noise reduction
• The same microphone is used as ANC error
  microphone and for voice pick-up.
• The quiet zone created by the ANC around the
  voice pick-up microphone
• Improves the speech to noise ratio of the
  microphone captured signal.
• Is centered around the voice pick-up microphone
  ANC is then able to provide noise cancellation
  for the voice pick-up up to higher frequencies
  than for the car passengers.
• The ANC microphone is ideally placed close to
  the car passengers, providing the best possible
  speech to noise ratio.
• For microphone array hands-free voice
  communication, ANC with multiple error
  microphones can create multiple quiet zone.

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Car ANC, car audio and car hands-free voice
communication systems integration

• The same microphone is used as car ANC error
  microphone and to capture the speech for car
  hands-free voice communication.
• One AEC is used for ANC and hands-free voice
  communication.
• The car audio head unit can run the AEC and ANC
  processing.
• The same car audio playback system is used to
  playback music, speech and
  anti-noise.
• Very little additional weight.

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Conclusion/Summary

• Energy saving and emission reduction create a
  need for car weight reduction.
• Passive countermeasures for car noise reduction
  increase the car weight.
• Car Active Noise Cancellation is able to reduce
  the low frequency car engine and road noise
  without car weight increase.
• Car Active Noise Cancellation is the only
  solution to cancel the noise of variable
  displacement engines.
• Car Active Noise Cancellation brings additional
  benefits
• Car audio listening experience de-masking of the
  music bass to restores a listening experience
  close to the one obtained in quiet/low noise
  environment.
• Car hands-free voice communication additional
  low frequency noise reduction on top of sending
  path noise reduction.
• Car Active Noise Cancellation can ideally be
  integrated with the car audio and car hands-free
  voice communication systems.

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Car Active Noise Cancellation Benefit (2) Link
with car audio and car hands-free communication

• Additional information
• Corporate Innovation Technology / Research
• Sound System Group
• Jean-Pierre Jallet
• Jean-pierre.jallet_at_nxp.com
• Interleuvenlaan, 80
• B-3001 Leuven
• Belgium

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