RESEARCH ON VIRTUAL STEERING SOUND IN PARKING MODE

Hyeon-Cheol Jo, Jae-Yong Seo, Kyung-Hwan Park, Bo-Seok Kang, Tom Van

Wayenberge, Markus Bodden, Torsten Belschner

► To cite this version:

Hyeon-Cheol Jo, Jae-Yong Seo, Kyung-Hwan Park, Bo-Seok Kang, Tom Van Wayenberge, et al. RESEARCH ON VIRTUAL STEERING SOUND IN PARKING MODE. ICSV30, IIAV, Jul 2024, Amsterdam, Netherlands. [hal-04739965](https://hal.science/hal-04739965v1)

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Submitted on 16 Oct 2024

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# RESEARCH ON VIRTUAL STEERING SOUND IN PARKING MODE

Hyeon-Cheol Jo, Jae-Yong Seo, Kyung-Hwan Park, Bo-Seok Kang and In-Je Cho

Applied NVH Technology Cell, Hyundai Mobis, republic of Korea

email: clfclfdlsp@mobis.com

Tom Van Wayenberge

Simulation & Test Solutions, Siemens Digital Industries Software DI SW, Belgium

email: tom.vanwayenberge@siemens.com

Markus Bodden, Torsten Belschner

Neosonic, Germany

email: markus.bodden@neosonic.eu

In a vehicle, the Electric Power Steering (EPS) is a steering system that utilizes an electric motor to assist the driver's steering efforts. EPS systems are categorized on the location of the motor and the method of power transmission. The type where the motor is mounted on the rack and power is transmitted via a belt is referred to as Rack-Assisted EPS(R-EPS). This R-EPS occasionally leads to noise issues, which are especially noticeable in electric vehicles due to their lower background noise. This requires stringent management and attention to the emotional quality to ensure the sounds do not detract from the overall driving experience. Due to the global trend of increasing electric vehicle market share as a response to climate change, a fundamental solution for managing noise in R-EPS is necessary. However, realistically, resolving these issues through design improvements is quite limited. In this paper, research on the development of a system that generates virtual steering sounds when driver is steering is introduced. Based on the conclusions drawn from analyzed the operational noise of R-EPS, we designed sounds to mask the operational noise. Additionally, we focused on designing the sound by establishing a concept based on our symbol to reflect our brand identity and considered the noise characteristics of R-EPS. In actual vehicles, the virtual steering sound system was developed to function based on steering angle, steering speed, steering torque, and vehicle speed, with the sound fine-tuned to suit the actual driving environment.

**Keywords:** Electric Power Steering, Virtual Sound, Masking, Granular Synthesis, Sound Quality

## 1. Introduction

In vehicles, noise is a crucial aspect linked to the quality of automobiles. Notably, the ambient noise within the cabin is a direct sensory experience for drivers, significantly influencing the vehicle's perceived emotional quality and the driver's level of fatigue. [1] Consequently, vehicle component manufacturers and automobile companies have exerted considerable effort to minimize the noise from parts as well as external sounds, striving to enhance the overall acoustic experience within the vehicle.
The Electric Power Steering (EPS) system plays a role in assisting the driver's steering efforts, providing aid for easier and more precise manoeuvring. EPS is categorized based on the location of the motor and the method by which power is transmitted. An EPS where the motor is situated on the rack on the rack bar and power is delivered through a belt is referred to as Rack-assisted EPS (R-EPS). R-EPS is often installed in large or luxury vehicles due to its reputation for providing superior steering feel and driving stability. [2] As shown Fig.1, R-EPS consists of a structure comprising the motor, belt and RBNA (Rack Ball & Nut Assembly). Based on the torque level transmitted from the driver, as well as the vehicle's speed and steering angle, the Electronic Control Unit (ECU) determines the output of the motor. The power generated from the motor is then transmitted through the belt and RBNA. During the transmission process, noise from the belt and RBNA becomes an issue and is managed to be kept below a certain level. This noise is especially sensitive in Electric Vehicles (EVs) because the ambient noise of EVs is about 10 dBA lower compared to that of internal combustion engine vehicles (ICEs). Therefore, noise standards for R-EPS in EVs are managed more strictly compared to those in ICEs, and the sensory quality of noise has also become more critical. As the rise in EV market share is a global trend in response to climate change, there's a fundamental need for addressing noise management in R-EPS. However, realistically, resolving issues through design improvements is quite limited. Therefore, this paper proposes a different method for resolving noise issues by designing virtual steering sound and ensuring this virtual sound is generated through the vehicle's speakers, instead of reducing the noise.

<figure><img src="image_4.png" /><figcaption>Figure 1: Composition of R-EPS</figcaption></figure>

Most current research and development on virtual sound using the audio system inside vehicles are primarily focused on engine sounds, with Active Sound Design (ASD) playing virtual engine sounds to enhance the driving experience and emotional quality for drivers. [3] The development of virtual steering sound will expand the scope of ASD applications and offer a new alternative for resolving noise issues of automotive parts. It also holds potential for extension to other vehicle components. Moreover, through the virtual steering system, designing sounds with consumer preferences in mind and providing various options can lead to a natural enhancement of emotional quality.

## 2. Analysis of R-EPS operating noise

Before developing the virtual steering sound, an analysis of the noise characteristics of R-EPS in actual vehicle conditions was conducted. The noise evaluation was conducted at constant speeds ranging from 0.4 to 1.5rps, with intervals of 0.2 to 0.3rps. In Fig.2, the left graph shows the results of the FFT (Fast Fourier Transform) for each steering speed and the right graph shows the noise levels of the overall (O/A), RBNA noise, main motor orders, and belt noise. Two microphone sensors were installed at the right side
of driver's head and in the middle of the rear seat. Measurements were taken four times for each steering speed in both LH (Left-Hand) and RH (Right-Hand) directions, and the average was calculated.

Summarizing the evaluation, firstly, the noise at speeds below 0.7rps was lower than the vehicle's ambient noise, making it very difficult for the driver to detect. It was determined that a very low volume setting is necessary for the sound design in the section below 0.7rps. Secondly, the noise from the R-EPS was not pure tone noise but rather band noise, and regardless of the steering speed, most of the energy was concentrated in the [150, 900] Hz range. Taking this into account, it was considered necessary to adjust the gain for the [150, 900] Hz range during sound design. Additionally, the noise level during LH steering was about 2dBA higher compared to RH steering. Therefore, it was reasonable to focus on the LH steering when tuning the volume. Also, the evaluation revealed that the O/A level increased with higher steering speeds, with the most significant contributions to the O/A level being in the order of RBNA noise, motor noise, and then belt noise. For belt noise, the level was low across all speed ranges. As for motor noise, it decreased beyond 1.0rps from the driver's seat perspective and beyond 1.3rps from the rear seat perspective. Of course, the frequency band of RBNA noise is larger compared to other noises, which can be disadvantageous when comparing energy levels. However, due to structural issues with RBNA, it has a characteristic that sounds grinding, which is also emotionally unpleasant.

<figure><img src="image_5.png" /><figcaption>Figure 2: FFT and Levels of R-EPS Noise by Each Steering Speed</figcaption></figure>

Additionally, a modal testing was conducted to understand the dynamic characteristics of the R-EPS, and a separate evaluation was performed to analyze the Operational Deflection Shapes (ODS). As shown in Fig.3, there were four significant modes between [200, 500] Hz, with the motion of the motor, belt housing, and bush mounting being the primary factors. ODS analysis conducted at the frequencies where modes occurred, and it was confirmed that, even during the actual operation of the R-EPS, there is significant relative displacement in the frequency ranges where large modes occur. Considering all these evaluations, we concluded that the problematic noise of the R-EPS is primarily due to RBNA, and the major resonance components of the R-EPS contribute to the problematic frequency range.
<figure><img src="image_6.png" /><figcaption>Figure 3: Mode Shape and ODS of R-EPS</figcaption></figure>

## 3. Sound Design for Virtual Steering Sound

The methods of sound design, as shown in Fig.4, include Masking and Embedding. Masking means covering the target noise with a sound that is louder and has a broader frequency range. Related research includes a study on how the sound of fountains in urban parks in Stockholm masks road traffic noise. [4] Embedding is a method that involves generating harmonious pure tones through harmonic analysis of unpleasant pure tone noises, transforming them into more pleasant sounds. This approach is being researched in the field of electronic music based on the analysis of harmony. [5][6] An advantage of embedding is that the generated sound does not necessarily have to be louder than the existing noise. In the case of R-EPS noise, as previously analyzed, it was determined that masking is the suitable method due to the broad band of R-EPS noise in the [150, 900] Hz range.

<figure><img src="image_7.png" /><figcaption>Figure 4: Masking and Embedding Methods for Sound Design</figcaption></figure>

After determining the method for sound design, a concept was established for creating virtual steering sounds. We found a concept suitable for sound design that is also related to the company, and ultimately decided on sounds related to archery and basketball.

For the archery-related sounds, we used the sound of an arrow in flight as the base. As shown in Fig.5, we created a total of four sound signatures by adjusting pitch variations considering [150, 900] Hz and time stretching. Also, we created four more sound signatures based on whether brown noise generator and pink noise generator were used with granular synthesis generator. Granular synthesis generator is a sound generator that divides sound into very small grains and rearranges them in various ways to create new sounds. It is also generally applied and used in generating virtual engine sounds. [7] Pink noise is a softer version of white noise. The higher frequencies are not as powerful, so there's less
of a high-pitched hissing sound. There is research suggesting that pink noise can reduce the complexity of brain activity and improve the quality of sleep. It is believed that if this sound is heard by drivers during vehicle steering, it may contribute to stability. [8] Next, the characteristic of brown noise is that it includes all frequencies of sound, but low frequencies are more pronounced, while high frequencies sound softer and create a deep, rumbling noise. To summarize, brown noise is a gentler sound compared to pink noise and provides even greater stability for drivers due to its smoothness. Furthermore, for the basketball-related sounds. We extracted the sounds of dribbling and the sound of the ball hitting the net to create sound signatures. In the case of the sound of the ball hitting, since the length of the sound source was very short, we used granular synthesis generator. From this basic sound source, we created a total of four sound signatures by varying the application of gain filters and mold.

<figure><img src="image_8.png" /><figcaption>Figure 5: STFT of Sound Signatures</figcaption></figure>

We conducted a survey among experts and non-experts on the popularity, stability, and brand identity of the created sound signatures, as shown in Fig.6, with each category rated out of five points.

In the case of archery, the basic sound signature with longer period and pitch adjustment received higher scores. For the sound sources where granular synthesis generator was applied, the sound with brown noise generator and mold tuning received higher scores.

In the case of basketball, the sound of ball hitting the net, particularly the sound with mold and weighted filters applied, received high scores in terms of stability and popularity. In terms of brand identity, sounds without pitch adjustment received slightly higher scores. However, the final sound sources selected were those with higher stability and popularity ratings.

<table><caption>Figure 6: Results of Survey Regarding the Sound Signatures</caption><thead><tr><th rowspan="3">Signature</th><th colspan="10">Archeory</th><th colspan="4">Basketball</th></tr><tr><th colspan="4">Arrow - Play Back</th><th colspan="4">Arrow - Granular Synthesis</th><th rowspan="2">Dribble</th><th rowspan="2">Net Swoosh</th></tr><tr><th>Time ↑</th><th>Time ↓</th><th>Brown</th><th>Pink</th><th>X</th><th>O</th><th>X</th><th>O</th><th>X</th><th>O</th><th>X</th></tr></thead><tbody><tr><td>Mold</td><td>X</td><td>X</td><td>X</td><td>X</td><td>O</td><td>X</td><td>O</td><td>X</td><td>X</td><td>X</td><td>O</td><td>O</td></tr><tr><td>Pitch or Filter</td><td>X</td><td>O</td><td>X</td><td>O</td><td>X</td><td>X</td><td>X</td><td>X</td><td>X</td><td>O</td><td>O</td><td>X</td></tr><tr><td>Popularity</td><td>2.2</td><td>2.3</td><td>2.3</td><td>2.0</td><td>3.6</td><td>3.4</td><td>3.0</td><td>3.3</td><td>1.6</td><td>1.8</td><td>3.0</td><td>2.9</td></tr><tr><td>Comfort</td><td>2.0</td><td>2.3</td><td>2.0</td><td>1.7</td><td>3.8</td><td>3.6</td><td>2.8</td><td>3.6</td><td>1.4</td><td>1.7</td><td>3.0</td><td>2.9</td></tr><tr><td>Brand</td><td>3.7</td><td>3.5</td><td>3.4</td><td>3.5</td><td>3.3</td><td>2.5</td><td>2.9</td><td>2.8</td><td>2.9</td><td>2.5</td><td>2.5</td><td>2.6</td></tr></tbody></table>
4. System Implementation and Sound Tuning

In the end, three sound signatures were selected and fine-tuned in the actual vehicle. The virtual steering sound system was designed and tuned to operate based on steering angle, steering velocity, steering torque, and vehicle speed, as depicted in Fig. 7. [9]

During the survey, which was conducted blindly, many participants expressed that the sound of arrows flying with granular synthesis generator and the sound of a basketball hitting the net sounded like waves and wind, respectively. When the sound signatures were applied directly in the actual vehicle, there were emotionally rough segments. To enhance the naturalness of the sound, we blended the original sources with wave and wind sounds through tuning tool. Moreover, since steering speed had the most significant impact on operational noise, we assigned the highest weight to the volume based on steering speed. Besides, we incorporated algorithm to adjust the left and right speaker volumes based on the steering angle during LH and RH steering to create a three-dimensional spatial audio experience."

<figure><img src="image_9.png" /><figcaption>Figure 7: The Diagram of the Virtual Steering Sound System (KR Patent Application 10-2021-0188623)</figcaption></figure>

5. Demonstration and Survey

After the completion of development, we conducted demonstrations for approximately 100 participants, including both experts and non-experts. As shown in Fig.8, the first archery sound received the highest evaluation, with a 77% response rate. When surveyed based on the audio signature at the studio, the first signature received high scores for brand appeal, but relatively lower scores for comfort and popularity. However, when participants experienced it directly in the actual vehicle while steering and listening through the vehicle's speakers, it received a lot of feedback for being the most popular. Many participants noted that it had a space-like ambiance that complemented electric cars well. Regarding the second and third audio sources, due to the application of granular synthesis generator, participants had no prior knowledge of the sounds. However, overall, they expressed that these signatures were comfortable to listen to, despite not knowing exactly what they represented.

We also conducted a survey regarding the manufacturability of the virtual steering sound system. Among the respondents, 85.6% were positive and 14.4% were negative. There was a common consensus that further tuning was necessary to create a more sophisticated sound for commercialization. Many participants also expressed a desire for various sound options. Initially, when establishing the initial sound concept, we aimed to align it with the brand's identity. However, after conducting demonstrations and gathering feedback, the most significant realization was that people's preferences are truly diverse and varied.
Based on the survey results, our future development towards mass production involves providing
high-quality sound options that cater to diverse customer preferences. To achieve this, it is essential to
diversify virtual sounds and perform fine-tuning, considering the vehicle's speaker system and interior
space. In addition, while our current research focused on parking modes at speeds below 20 kph, to en-
hance overall completeness, it is imperative to develop and tune virtual sounds for regular driving and
autonomous driving modes as well.

The concept of future vehicles is believed to revolve around Purpose Built Vehicles (PBV) designed for specific purposes. These vehicles are centered around the user's experience, providing personalized services and convenience. [10] Virtual sounds of automobile parts, such as virtual steering sounds, can be utilized in various ways as part of the applications within PBVs.

<figure><img src="image_10.png" /><figcaption>Figure 8: Final Survey Results of Virtual Steering Sound</figcaption></figure>

**6. Conclusions**

In this study, we conducted research on the creation of virtual sounds as a method to mask the noise, rather than reducing noise through design improvements, in order to address the operational noise of the electric vehicle's R-EPS system.

1) The main frequency range of the operational noise in R-EPS is [150, 900] Hz and four major modes in the [200, 500] Hz and RBNA contribute to the noise.

2) Approaching sound design with consideration for the characteristics of broad-spectrum noise for masking, sounds were extracted, and audio samples were created, drawing inspiration from archery and basketball, and applying granular synthesis and a brown noise filter. Subsequently, through surveys, the final three audio samples were selected.

3) A virtual steering sound system was created based on vehicle speed, steering angle, steering ve-
locity, and steering torque, and tuning was conducted in real vehicle conditions.

4) After completing the development, we conducted demonstrations and surveys in real vehicle con-
ditions, and 85.6% of respondents showed a positive response. To prepare for future commercial-
ization, fine-tuning is necessary, considering the entire vehicle's speaker system and interior envi-
ronment. Finally, algorithm for virtual steering sound should be added to the ASD system.

5) This research was targeted for parking mode at speeds below 20 kph, but there is a need for further development and tuning of virtual steering sound to consider regular driving and autonomous driving modes.
6) By designing virtual steering sounds considering consumer preferences, it's possible to offer various options, providing drivers with a new experience.

7) Using ASD, it's possible to apply virtual steering sound to mass-produced vehicles, and there's also ample potential for expansion to virtual sounds of other parts. Additionally, mitigation of noise complaints and enhancement of natural emotional quality are expected.

# 7. ACKNOWLEDGEMENTS

This research was supported by Hyundai Mobis.

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