Introduction to Active Sound Management Technology

DENSO TEN Technical Review Vol.1

Sanjay Singh Takehiro WAKAMATSU Keizo ISHIMURA

Abstract Active Sound Management is a technology that enables to control sound effectively leveraging the existing audio system. More recently, it has been expanding to be applied as a technology to provide in-cabin acoustic environment with comfort, such as eliminating uncomfortable low frequency“boom” sound resulting from powertrains and enhancing powerful engine sound. In this article, the system developed by DENSO TEN, which includes the DSP with Active Sound Management algorithm by Müller BBM GmbH coded, is discussed. Based on the existing audio system, configuration parts added microphone, speaker, and others, system design requirement such as signal processing of CAN-BUS, and analysis of tuning results on actual vehicles (reduction of boom sound and enhancement of engine sound) are introduced.

1. Introduction

1.1 Active Sound Management Technology 1.2 Active Noise Cancellation (ANC)

Active Sound Management enables OEMs to Loud engine/powertrain sounds can result from control sound effectively, leveraging the existing mass reduction of components that result in increased audio system. It allows providing a wide range of vibration. Uncomfortable sounds also result from acoustic options. For the past decade, Noise Vibration early torque converter lock, cylinder deactivation, (NV) teams have used Active Sound Management to transition from electric to Internal Combustion reduce driver fatigue by eliminating uncomfortable Engine (ICE), as well as Continuously Variable low frequency “boom” sound resulting from Transmission (CVT) drivetrain. Technologies powertrains. More recently it is also used to create designed to improve fuel efficiency typically have in-cabin acoustic brand images such as refined a side effect of causing uncomfortable sounds and luxury, familiar comfort, and sporty excitement. render the vehicle non-salable. ANC tool is to reduce Active Sound Management software continuously these sounds and associated harmonics. creates appropriate constructive and destructive acoustic fields to enhance or create desired 1.3 Engine Sound Enhancement (ESE) powertrain sounds and to reduce or eliminate unwanted powertrain sounds. This significantly One of the desired acoustic signatures to be reduces the reliance on expensive mechanical enhanced is notes of powerful engines. ESE can solutions such as tuned exhaust manifolds, change the acoustic sound signature of small fuel waveguides, mufflers, and sound dampeners. Two efficient, high RPM, high power engines to sound aspects of Active Sound Management will be signature of older traditional large I5/V6/V8 engine discussed below. sound with lots of modulated low rumbling bass notes. Uncomfortable sounds of 3 cylinder engine operation can be masked by creating familiar 4 or 6

23 Introduction to Active Sound Management Technology

cylinder engine sound signatures. For electric cars, the vehicle could provide large cancellation below this technology also allows us to create a pleasant 60Hz and taper off to minimal effect at 90Hz in all sound character. seats. Similarly a single e-mic placed directly over the driver position could provide large cancellation 2. Outline of Active Sound Management up to 240Hz and taper off at about 300Hz. MY18 and beyond it is common for OEMs to use 3 to 4 2.1 Structure of the System e-mics to create good cancellation effect for every seat, see Fig. 2. Fig. 1 shows a simplified least mean square (LMS) system diagram. [Ref.1] The advantage for Active Sound Management in automotive is that the rotational speed (RPM) of various vehicle Tweeters components are typically known to within 12 to

25ms period. This advantage allows targeting the Woofers engine and powertrain rotational sounds with low computational effort. The resulting software is ANC Mic ANC Mic therefore very lightweight and can be added to an infotainment or audio system CPU or DSP relatively easily. Typically, multiple ANC and ESE datasets ANC Mic Mids operate concurrently and can seamlessly adapt and transition from cylinder deactivation, to various sports modes. Sub

Engine rpm Engine noise

S Fig. 2 Typical 3 error mic. system configuration

2.3 Components of the System: Speakers

Error microphone signal The desired sound image determines Active Sound eitω Ŝ ＊ Management applications. Ideas such as filling in a Fig. 1 Block Diagram of ANC and ESE 2dB notch to restore linearity to the engine sound, to cancelling over 20dB from 30-90Hz, to creating 2.2 Components of the System: Microphones a roaring modulated sound of a powerful naturally aspirated engine. Typical Active Sound Management application While all vehicles can benefit from stock speakers, uses the existing vehicle audio system. Error typically 1 to 6 speakers are actively driven in Active feedback microphone (e-mic) is applied for ANC Sound Management applications; 4 speaker, plus 2 function and is not necessarily needed for ESE tweeter stock configurations being the most common, function. The e-mics used for ANC function have see Fig. 2. Usually, dedicated subwoofers are added to a very limited frequency response, typically only larger SUV applications where there is a need to cancel to 1 KHz. Therefore, ANC e-mics are inexpensive high levels of low frequency sound resulting as a side- compared to telephony microphones. effect of fuel efficient powertrains. High end sports cars The ANC frequency cancellation range also tend to have upgraded audio systems that allow is defined by distance of the e-mic from the for good low frequency extension for ESE application. occupants head location (e.g. ¼ wave 94cm = ~90Hz). Therefore, a single e-mic in the center of

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2.4 Components of the System: Signals those systems, ANC cancellation is generally less than 300Hz with good performance starting to taper off Active Sound Management is required to match above 240Hz. This is due to the limitation of the e-mic the continuously varying RPM of the engine and distance to the occupant head location, see Fig. 2. The powertrain. Any delay would cause undesired typical ESE performance designed for high volume audio artifacts and ghost sounds. Therefore, the mainstream production vehicles do well to 800Hz and Active Sound Management system requires real- taper off at approximately a 1000Hz as localization (the time response to powertrain signals. ability of a listener to recognize the distance from and ANC function requires RPM signal as input location of the sound source) issues become a factor. signal which can be either analog input or via While it is possible to achieve some cancellation or CAN bus. ESE functions, in addition to RPM, also enhancements with traditional low quality 15 cm would need the torque, vehicle speed, and percent speakers, some OEMs choose to use higher quality accelerator information. The RPM signal is used for woofers or a dedicated subwoofer to achieve >25dB frequency matching, whereas the other information cancellation at 30Hz. Below 30Hz, which is 2nd order is used to adjust the gain. Additional signals, frequency at 900rpm on a 4cylinder car or a 3rd order such as door and window open or closed status, frequency of a 6 cylinder car at 600rpm, the RPM cabin temperature, gear information and various signal itself is not considered accurate. Therefore, diagnostics, help to improve the performance. the lower bound cut off frequency is decided by the accuracy of the RPM signal and not necessarily the 3. System Design size of the required subwoofer. There are other advanced ANC and ESE options 3.1 System Design Aspects including near field technology that can achieve ANC < 800Hz and ESE < 3000Hz. The software for these There are many technical issues to contend systems is similarly light-weight; however dedicated with when designing an Active Sound Management speakers and e-mics in non-standard locations make system. To list a few are LMS algorithms, number these systems cost prohibitive and a realm of luxury of datasets, SNR and group delay for e-mics, vehicles. e-mic placement, measuring transfer functions, convergence, tracking, impulse noise rejection, 3.3 Audible distortion of Multimedia Content wind noise mitigation over e-mics, stability versus divergence detection and mitigation, various In Active Sound Management system, distortion diagnostics for speaker and e-mic polarity and of music content is considered a non-issue and will not performance, retry mechanisms, and more. impact the audio systems tuning from the multimedia Tuning effort of the Active Sound Management group. system depends on the complexity of the ANC cancels specific frequencies by using notch requirement. Tuning is made easier by the use of a filters. Properly designed notch filters with well dynamometer, as it allows simulating transitions for managed energy distribution will not have a perceptible every gear and vehicle load condition. Final check detrimental impact on the music content. ANC generally is made on a track with professional driver and NV enhances the clarity of the music content since it uses acoustic engineer. RPM signal to target sounds of the multiple engine orders (harmonics) already present as background noise 3.2 Acoustic Performance Range to the music content. ESE on the other hand either emphasizes desired engine orders (make louder) and The typical mainstream Active Sound adds many new orders that would have traditionally Management systems use the traditional vehicle been created by modulating sounds using complex audio system, meaning no dedicated speakers in the lengths of mechanical exhaust manifolds and baffles. headrest. This is classified as far-field systems. Under ESE sounds are carefully and subtly introduced with

25 Introduction to Active Sound Management Technology

33 40 50 60 70 80 90 100 110 120 130 140 150 very precise times during heavy acceleration, or high 80 □ F Order 2.00 Seat: Driver （Ｈｚ） torque demand. Psychoacoustic perception is that of a 75 □ F Order 2.00 Seat: Driver Mid Size CUV, 4 Cylinder, MY 2018 Production Tuning powerful engine instead of music distortion. 70 Front Door Speakers: 150x225mm, Rear Door Speakers 150mm In some fuel efficient small car applications, end 65 users can download“fun sound-fonts” to mimic sounds 60 （Ａ）Ｂ of Formula-1 racing cars, etc. This model allows end ｄ 55 user fun and residual income generation as telematics 50 ANC Target Line service. 45 Effect of ESE can be seen in Fig. 7 which shows 40 Typical Cruising RPM • 35 （ＲＰＭ） the high performance vehicle (350HP @440N m). The 1000 1500 1750 2000 2250 2500 2750 3000 3250 3500 3750 4000 4250 4500 right side chart shows the added orders to enhance 67 80 90 100 110 120 130 140 150 160 170 the low frequencies of the sports car sound. The 70 （Ｈｚ） 66 uneducated driver would enjoy the near 100dB muscle- □ F Order 4.00 Seat: Driver Mid Size CUV, 4 Cyl. ! 64 □ F Order 4.00 Seat: Driver MY 2018 Production Tuning car engine notes during street racing in drift mode, and 62 60 58 would likely not notice music distortion. The educated 56 54 sports car enthusiast on the other hand might be very 52 Baseline 50 4th Order （Ａ）

Ｂ 50．00 surprised on how a 4 cylinder car sounds so amazing ｄ 48 Engine 46 when there are no mechanical baffles, manifolds, and 44 42 wrong number of cylinders to create such a sound 40 ANC Applied 38 signature. An OEM in the US has found Active Sound 36 34 Management to be appealing and profitable. 30 （ＲＰＭ） 1000 1200 1400 1600 1800 2000 2200 2400 2600 4. Performance and Effect of the System Fig. 3 ANC effect with Mid-Size CUV, 4 cyl. Engine

4.1 Analysis of ANC Measurements 75.00 3rd Order Drivers Seat Mid Size SUV, 6 Cylinder, MY 2019 Production Tuning Fig. 3 shows ANC production tuning made by 70.00 Front Door Speakers: 150mm, Rear Door Speakers 150mm DENSO TEN of a mid-size MY2018 North America 65.00 cross-over vehicle with 4-cynder engine. The stereo system is a basic DENSO TEN system driving 60.00 4 channels, two front 6 × 9” with tweeters, and 55.00 Baseline V6 two rear 6” speakers. There are 4 e-mics in this nd 50.00

application, one per seat. 10dB to 20dB of 2 order dB(A) and up to 10dB of 4th order noise reduction can be 45.00 seen in the cruising speed RPM range in the driver 40.00 seat. The performance in the remaining passenger With ANC seats is similarly good, but not shown in this article. 35.00 Fig. 4 shows ANC production tuning of a very 30.00 Typical Cruising RPM popular mid-size SUV using the same system as Fig. 3. This vehicle is larger than the one in Fig. 3. 25.00 47 70 90 1000 120140 0150 （Ｈｚ） 170 This SUV has four main 6” speakers and 2 tweeters 20.00 and also has 4 e-mics installed, one per seat. Since 950.00 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 this is a 6 cylinder vehicle, the 3rd order engine rpm noise is the most dominant. Although this vehicle is Fig. 4 ANC effect with Mid-Size SUV, V6 Engine equipped with smaller speakers, a stable 3rd order cancellation up to 20dB can be seen in the cruising RPM range.

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Fig. 5 uses Campbell Charts to show results of an 4.2 Analysis of ESE Measurements ANC demonstration using the same basic radio as in Fig. 3 and Fig. 4. The vehicle selected was a The DENSO TEN demonstration vehicle, basic 4 cylinder minivan. This vehicle was very Fig.6., was used to show different kinds of loud with higher RPMs and had poor speakers, European sports car sounds with and without thus the front speakers were upgraded to 6.5” the aid of a subwoofer with up to 10 orders of aftermarket speakers. ANC was applied only enhancement to 600Hz. The lower-left chart using two front speakers and 4 e-mics for the in Fig. 6 shows the status before tuning; the 4 seats in the first 2 rows. The 3rd row of the upper-right is the result of enhancement on low minivan was not utilized for this experiment. frequency by sub woofers; and the lower-right is The results shown above are for the driver seat the result of enhancement on higher frequency. only. Up to 10dB on 1st, up to 20dB on 2nd, and up The zones surrounded by red-colored dotted line 10dB on 4th orders cancellations were measured are enhanced, which represents the effect of ESE. respectively. Fig. 7 is an actual 4 cylinder turbo sports car with 350HP@440nm torque. This was measured Front right seat, ANC Oﬀ 1/min 123 dB for benchmark by DENSO TEN. As the effect of 6000 90 emphasis can be seen in the Campbell Chart, this

5000 vehicle has ~21 orders of enhancement with loud 4 80 100dB sound levels that might finally sound like

4000 a 5 cylinder engine. 5 70 6 3000 7 8 9 2000 10 60 11 12 13 14 1000 50 050 100 150 200 250 300 Hz Front right seat, ANC On 1/min 123 dB 6000 90

5000 4 80

4000 5 70 6 3000 7 8 9 2000 10 60 11 12 13 14 1000 50 050 100 150 200 250 300 Hz Fig. 5 4 cylinder Minivan – Campbell Chart

27 Introduction to Active Sound Management Technology

12345 dB SPL DENSO TEN ESE demo vehicle 6 80 -Soccer Mom Minivan -Quiet 4 Cylinder Engine (below) 7 70 -8 Channel Premium Audio System 8 60 -20cm Dual voice coil Subwoofer 9 10 50 ESE Eﬀect 1: Right Top -Low Frequency Emphasized (Subwoofer) 40

ESE Eﬀect 2: Right Bottom 30

-Higher Frequency Emphasized (No Subwoofer) 20

dB SPL Orders 1 to 10 1 2 3 4 5 6 RPM 12345 dB SPL 6 6000 80 80 7 7 70 5000 70 8 8 9 9 60 60 4000 10 10 50 50 3000 40 40 2000 30 30 1000 20 20 0 100 200 300 400 500 600 Hz

Fig. 6 ESE Effect using Campbell charts (Normal, w/ Sub woofers, w/high-frequency enhancement)

1/min 1 23456789101112 dB 1/min 1 2345678910 11 12 dB 5000 100 5000 100

13 13 90 90 4500 4500 14 14

80 80 4000 4000

70 70

3500 3500

60 60

3000 3000 50 50

2500 2500 40 40

2000 30 2000 30

1500 20 1500 20 0100 200 300400 500600 700800 9001000Hz 0100 200 300400 500600 700800 9001000Hz

Fig. 7 ESE Effect in 4 cyl. Turbo 350HP North America Production Sports Car (3rd Gear shown)

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5. Conclusion

Active Sound Management technology has been in automotive luxury segment for a decade and those drivers have enjoyed the benefits of luxury and excitement. Recently it has been applied to Mid to Low grade vehicles. Active Sound Management application is expected to be enhanced since it also has a potential to be telematics service contents that improve acoustics environment by Over The Air (OTA) or USB download. Active Sound Management is also considered as a solution for wind and road noise cancellation. We would like to continuously develop this technology for future.

References: Fig. 1 – Rolf Schirmacher of Müller-BBM Active Sound Technology GmbH,”Active Noise Control for the 4.0 TFSI with Cylinder”in SAE International, 2012-01-1533,2012,

Profiles of Writers

Sanjay Singh Takehiro Keizo WAKAMATSU ISHIMURA DENSO TEN AMERICA Limited DENSO TEN DENSO TEN AMERICA Limited AMERICA Limited