Jean-Pierre Jallet Car Active Noise Cancellation for improved car efficiency, From/In/To car voice communication and music listening experience NXP Semiconductors
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.
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: mostenergyliesunder500Hz.
Adaptive Feedback controller: using only an error microphone. Adaptive Feedforward controller: using a noise reference signal and an error microphone (for adaptive ANC). ANC ANC 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 (analogue+digital) or digital ANC filters. • Fixed or adaptive ANC filters.
V6 Variable Displacement V8 Car Active Noise Cancellation: Motivation (1) • In current cars, noise is reduced by passive countermeasures like car structure/stiffness/dynamics 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.
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.
d y r e filter w c ANC 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.
m AEC d - y + e c w - + rd ANC 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.
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.
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
Car Active Noise Cancellation: Benefit (3)Hands-free voice communication noise reduction • Noiseremainsaproblemforcarhands-freevoicecommunication: • 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. • Additionallowfrequency, diffuse noise reduction methods are needed. 4 microphone endfire array vs. 1 cardioid microphone
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.
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.
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.
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 Jeanemail@example.com Interleuvenlaan, 80 B-3001 Leuven Belgium