The Fully Networked Car Geneva, 4-5 March 2009 1 Jean-Pierre Jallet Car Active Noise Cancellation for improved car efficiency, From/In/To car voice communication.

The Fully Networked Car Geneva, 4-5 March 2009

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Jean-Pierre JalletCar Active Noise Cancellation for improved car

efficiency, From/In/To car voicecommunication and music listening experience

NXP Semiconductors

The Fully Networked Car Geneva, 4-5 March 2009

2Content

o Noise in the Car.o Active Noise Cancellation.o 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.

o Conclusion.

The Fully Networked Car Geneva, 4-5 March 2009

3Noise in the car

o Car is a noisy environment: noise level between 65 and 75 dBA.o 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.

o Road noise is dominant, engine noise is second. In some circumstances, uphill, variable displacement engine…, engine noise can become dominant.

o The resulting noise is:• Mainly diffuse: comes from all directions.• Low frequency: most energy lies under 500Hz.

The Fully Networked Car Geneva, 4-5 March 2009

4Active Noise Cancellation

o 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.

o 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.

o ANC structures:

• Analogue, hybrid (analogue+digital) or digital ANC filters. • Fixed or adaptive ANC filters.

ANCANC

• Adaptive Feedback controller: using only an error microphone.

• Adaptive Feedforward controller: using a noise reference signal and an error microphone (for

adaptive ANC).

The Fully Networked Car Geneva, 4-5 March 2009

5Car Active Noise Cancellation: Motivation (1)

o In current cars, noise is reduced by passive countermeasures like car structure/stiffness/dynamics modifications, sealing, damping/absorption material…

o Passive countermeasure effectiveness is good for higher frequencies but decreases with decreasing noise frequency.

o Most of these passive countermeasures increase the car weight.o 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.

o Current demand for energy consumption and emission reduction lead to search for car weight reduction.

VariableDisplacement

V6

V8

The Fully Networked Car Geneva, 4-5 March 2009

6Car Active Noise Cancellation: Motivation (2)

o 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.

o ANC adds very little weight.

The Fully Networked Car Geneva, 4-5 March 2009

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o 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.

Car Active Noise Cancellation: Technologies (1)

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The Fully Networked Car Geneva, 4-5 March 2009

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o 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.

Car Active Noise Cancellation: Technologies (2)

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The Fully Networked Car Geneva, 4-5 March 2009

9Car Active Noise Cancellation: Benefit (1)

o Weight saving: • less damping/absorption material.• Car structure weight reduction.Leads to:• Energy consumption reduction.• Emission reduction.

o Cost saving:• Car manufacturing: less damping material and cheaper car

structure.• Car use: energy consumption reduction.

o Better noise reduction:• Limited effectiveness of passive countermeasures for low frequency

noise.• Variable displacement engines noise cannot be reduced by passive

countermeasures.o Improved car dynamics.

The Fully Networked Car Geneva, 4-5 March 2009

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o 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.

o State of the art (adaptive) bass-boost solutions de-masks the bass but at the expense of a modified frequency balance.

o 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

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

The Fully Networked Car Geneva, 4-5 March 2009

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o 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:o Polar selectivity in the low frequency range.o Uncorrelated low frequency noise amplification.

Additional low frequency, diffuse noise reduction methods are needed.

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

4 microphone endfire array vs. 1 cardioid microphone

The Fully Networked Car Geneva, 4-5 March 2009

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o 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.

Car Active Noise Cancellation: Benefit (4) Hands-free voice communication noise reduction

The Fully Networked Car Geneva, 4-5 March 2009

13Car ANC, car audio and car hands-free voice communication systems integration

o The same microphone is used as car ANC error microphone and to capture the speech for car hands-free voice communication.

o One AEC is used for ANC and hands-free voice communication.o The car audio head unit can run the AEC and ANC processing.o The same car audio playback system is used to playback

music, speech and anti-noise. Very little additional weight.

Audio playback

Head Unit

Quiet zone

Car audio

MAEC

Left channel

Right channel

Adaptive Filter and adaptation

control

Adaptive Filter and adaptation

control

Post-processing:residual echo & fast

stationary noise cancelation

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

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The Fully Networked Car Geneva, 4-5 March 2009

14Conclusion/Summary

o Energy saving and emission reduction create a need for car weight reduction.

o Passive countermeasures for car noise reduction increase the car weight.

o Car Active Noise Cancellation is able to reduce the low frequency car engine and road noise without car weight increase.

o Car Active Noise Cancellation is the only solution to cancel the noise of variable displacement engines.

o 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.

o Car Active Noise Cancellation can ideally be integrated with the car audio and car hands-free voice communication systems.

The Fully Networked Car Geneva, 4-5 March 2009

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Additional information:

Corporate Innovation & Technology / ResearchSound & System Group

Jean-Pierre [email protected]

Interleuvenlaan, 80B-3001 Leuven

Belgium

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