PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Design of optimal car interior sound improving driving feeling
Su-Ho CHA1; Sung-Hwan SHIN1; Jae-Ho YANG1
1

Kookmin University, South Korea

ABSTRACTThe virtual driving sound has been used as a tool improving the driving feeling of passenger car. The purposeof this study is to suggest the strategy generating virtual driving sound that is closely connected with thedriving condition. To this end, previous studies ascertained that powerful feeling of car interior sound wasrelated to the roughness during the acceleration and that dynamic feeling and pleasant feeling of car interiorsound were negative correlation. In this study, the acoustic factor affecting dynamic feeling was extracted
based on the subjective listening evaluation. And by using the acoustic factor, it was identified to improvethe driving feeling when a virtual sound should be generated and what order components should besupplemented. It is expected that the method can improve driving feeling related to not only dynamic feelingbut also pleasant feeling.
Keywords: Sound Quality, Driving Feeling, Driving Sound, Dynamic Feeling

1. INTRODUCTION
The studies to improve the driving feeling of the car have been on-going as the expectation on theperceived quality have been surged. For this issue, there is a way to control the actual engine soundby using the intake and exhaust system, but recently, the virtual driving sound is produced in use of
the audio system by the reasons of physical restrictions such as the structure of engine and car bodyand regulating on the external sound. In order to improve the driving feeling of the car with virtualdriving sound, the strategy of designing the driving sound tuned to the driving conditions is required.
When the precedent studies for designing the driving sound were looked into, Frank et al. (1)described that the roughness was related to a powerful sound for the sporty car, and the roughness
was increased by amplitude modulation when the neighboring half-order of the main order on theengine was amplified. Wang et al. (2) described that the roughness improve the powerful feeling of
the sporty car, but it induce the annoying in the usual car. Cha (3) and Kim (4) classified the dynamicfeeling into a powerful feeling and a speedy feeling based on the subjective listening evaluation anddescribed the reduced pleasant feeling at the improved the dynamic feeling. This study is to suggest
the virtual driving sound design strategy to improve the driving feeling that improves dynamic feeling
during the acceleration and keeps the pleasant feeling during the constant or deceleration.
To this end, Section 2 measured the driving sound for the cars with different engine performancein various driving conditions, Section 3 performed the subjective listening evaluation on the measured
driving sounds for the driving feeling evaluation. Section 4 derived the acoustic factors to analyze thesubjective listening evaluation as objective, and Section 5 proposed the strategy generating virtual
driving sound to improve the driving feeling.

2. Measurement of driving soundAs shown in Table 1, the driving sound was measured for Car A and Car B differed in engine
performance (Power & Torque) through the microphone installed at the driver ’s ear position.
The cars were driven in acceleration after initial cruising speed, and the specific driving conditions
were constructed in 1-4 steps as shown in Table 2. In this paper, the driving conditions were describedin sequence of initial cruising speed and acceleration conditions. For example, as shown in Figure 1,15204

soulshin@kookmin.ac.kr

it was expressed in 80km/h MTI in case of 80km/h initial cruising (constant) speed and Middle Tip In(MTI) acceleration.
Table 1 – Comparison of the engine performances
Engine performance

Maximum power

Maximum torqueCar A

332 kW @ 5,500 rpm

651 N∙m @ 1,800 ~ 4,500 rpm

Car B

272 kW @ 6,000 rpm

510 N∙m @ 1,300 ~ 4,500 rpm

Cars sample

Table 2 – Driving conditions for recording
Driving condition

Initial cruising speed

Accelerating condition

Step1

80km/hMTI

Step2100km/h

MTI

Step380km/h

WOT

Step4100km/h

WOT

StepFigure 1 – Method of the driving according to speed vs. time

3. Subjective listening evaluation for driving feeling
3.1

Construction of subjective listening evaluation
As shown in Figure 2, the test sound was made for the measured driving sound in Chapter 2 byediting in acceleration interval after keeping the initial cruising speed for 1 second. Because thesesounds were more than 10 second and are non-stationary sound, the listening evaluation program wasmade as shown in Figure 3(a) and used to evaluate on each section. Magnitude Estimation Method(MEM) was applied to the listening test, and the levels of dynamic feeling were evaluated with thecriteria of Figure 3(b). The jury was composed of 30 people in 20-30s having normal hearing ability.

Figure 2 – Change of test sound in time domain5205

(a)

(b)

Figure 3 – (a) Listening evaluation program for acceleration sound,(b) Rating guide for dynamic feeling
3.2

Results of dynamic feeling evaluation
As shown in Figure 4, Car A with more good in engine performance was evaluated higher than Car
B in the overall acceleration section. The result can be analyzed for the reflection of engine
performance to the dynamic feeling.
Next, the relation between the driving characteristics and dynamic feeling was analyzed as follows.
Firstly, the dynamic feeling had more than 3.5 points for Car A and less than 2.8 points for Car B at
the initial acceleration section, and later, tended to increase for both Car A and B at the acceleration.
In the other words, the improvement on the dynamic feeling at the initial acceleration section will beeffective to improve the overall dynamic feeling during accelerated driving.
Secondly, the driving conditions of the Car A with high performance engine were compared in theacceleration section as follows. The dynamic feeling was increased comparatively in linear form in
Wide Open Throttle (WOT) acceleration, but increased in log form in MTI acceleration. In the otherwords, it is considered that the difference in acceleration amount (or supplied power) made influenceon the dynamic feeling at the acceleration section.
(a)

(b)

Figure 4 – Result of Subjective listening evaluation(a) WOT acceleration conditions, (b) MTI acceleration conditions

4. Method to analyze dynamic feeling objectively
For objective analysis on dynamic feeling, Section 4.1 analyzed driving sound by means of Short
Time Fourier Transform (STFT) and Section 4.2 suggested the TNR order for acoustic factor to reflect
the features in engine sound. And Section 4.3 analyzed the influential factors to the dynamic feelingobjectively with TNR order .
4.15206

STFT Analysis
Figure 5 showed the calculated STFT of each car (Car A & Car B) for 80km/h WOT. In case of Car

A, the 1st firing order of the engine was mainly occurred in the initial acceleration section and the 2 nd
firing order was mainly occurred at the section after 8s. For Car B, however, the firing ordercomponents were not dominant or clear through the acceleration section.
(a)(b)

Figure 5 – Results of STFT analysis for 80km/h WOT (a) Car A, (b) Car B
4.2TNRorder
Tone to Noise Ratio (TNR) (6) presented the log scale for the calculated ratio of tone power tonoise power as shown in Eq. 1 to reflect the features of tonal sound. The critical-bandwidth, 1Barkfor ISO 532-1 (7) was adopted for the frequency range of the noise power.
But, differently with TNR, TNR order presented the log scale for the calculated ratio of tone power
to noise power of target engine order. The Equivalent Rectangular Bandwidth (ERB), 1Cam for ISO
532-2 (8) was adopted for the frequency range to obtain the noise power. The reason was to minimize
the interference of neighboring engine order for the target engine order on the frequency range
corresponding to noise power because 1Cam is narrower than 1Bark on the frequency below 500Hz
where engine sound occurred mainly. Figure 6 showed the frequency range of 1Cam for obtain thenoise power of the 1 st and 2nd firing order. TNRorder was calculated through Eq. 1 after obtaining tone
power and noise power.

TNR = 10𝑙𝑜𝑔10

𝑇𝑜𝑛𝑒 𝑝𝑜𝑤𝑒𝑟
𝑁𝑜𝑖𝑠𝑒 𝑝𝑜𝑤𝑒𝑟(1)

Figure 6 – Order components and frequency ranges of noise component for TNRorder
4.3

The relation between TNRorder and dynamic feeling
The influential factors on the dynamic feeling was analyzed objectively with TNR order . First, Figure7(a) shows the calculated TNR order of the 1 st firing order when the acceleration conditions of Car A5207

and Car B were WOT. It shows TNR order was more than twice higher in Car A than Car B in the initialacceleration section. This result can be regarded as TNR order difference of the 1 st firing order by thedifferences of engine and the performance of NVH control of Car A and B. In other words, the soundpressure level (SPL) of the engine sound is reinforced when the 1st firing order was emphasized at theinitial acceleration section, and such phenomenon was concluded as the differences of dynamicfeelings for each car (Car A & Car B) in the initial acceleration section.
Next, Figure 7(b) shows the calculated TNR order of the 2 nd firing order on all driving conditions ofCar A. It shows TNR order of the 2 nd firing order was decreased after the certain timing during
acceleration in MTI condition, but TNR order of the 2 nd firing order was high after the certain timing
during acceleration (WOT 80km/h: near to 11.5s, WOT 100km/h: near to 9.5s). This result can be
regarded as TNR order difference of the 2 nd firing order by difference of acceleration amount for WOT
acceleration and MTI acceleration in high speed driving and the structural resonance of the Car A in
the frequency range corresponding to the 2 nd firing order at certain timing during WOT acceleration.
In other words, the tone color was changed when the high order was emphasized at the certain timing
during acceleration, and such phenomenon was also concluded as the increased dynamic feeling on
WOT acceleration duration.
(a)(b)

Figure 7 – Results of TNRorder vs. Time(a) TNRorder for 1st firing order, (b) TNRorder for 2nd firing order of Car A

5. DISCUSSION
In section 4.3, the reinforcement of SPL for the engine sound by using the 1 st firing order at theinitial acceleration section and the change of tone color for the engine sound by using the high orderat certain section during acceleration were known to be effective for the dynamic feeling enhancement.
In other words, the reinforcement of the appropriate order component of engine sound and itsgenerating timing were more effective to improve the dynamic feeling than the overall level wassimply increased.
In addition, Kim (4) classified the dynamic feeling into the speedy feeling and the powerful feeling,and Cha et al. (3) described that the reinforcement of the high engine order was effective to improvefor the speedy feeling. And, Frank et al. (1) described that the reinforcement of the half engine order
was effective to improve for the powerful feeling. It is considered that the dynamic feeling can beimproved by strengthening the speedy feeling as supplementing higher order and/or the powerfulfeeling as doing the roughness while the SPL of the car interior is kept when the virtual driving soundis generated
Based on the descriptions in the above, the strategy for creating the virtual driving sound to
improve the driving feeling was proposed in Figure 8. In the constant speed section corresponding to‘T 0 ~ T1 ’, the virtual driving sound is not generated, thereby keeping the pleasant feeling . In theacceleration section corresponding to ‘T1 ~ T2 ’, because the acceleration amount is kept, the dynamic
feeling is improved by generating a virtual driving sound to increase TNRorder of the 1st firing order.
In the acceleration section corresponding to ‘T2 ~ T3 ’, because the SPL on the car interior is high, the5208

dynamic feeling is improved through the speedy feeling is improved by generating a virtual driving
sound to increase TNR order of the high order. (Otherwise, the dynamic feeling is improved through thepowerful feeling is improved by generating a virtual driving sound to increase TNR order of the halforder.)

(a)

(b)

(c)

Figure 8 – Strategy generating virtual driving sound(a) Speed vs. Time, (b) TNRorder vs. Time, (c) Perceptual feeling vs. Time

6. CONCLUSIONSIn the study, TNR order was suggested as the acoustic factor reflecting the dynamic feeling of engine5209

sound and, in use of the suggested TNR order , it was known the reinforcement of SPL for the engine
sound at the initial acceleration section and the change of tone color for the engine sound at c ertain
section during acceleration were effective for the dynamic feeling enhancement.
Based on these results, the strategy of generating the virtual driving sound was suggested that thedynamic feeling can be improved by generating the virtual driving sound at the specific acceleration
condition such as passing drive. In addition, the pleasant feeling can be kept when the virtual driving
sound was not generated.

ACKNOWLEDGEMENTS
This research was financially supported by Korea Evaluation institute of industrial technology(KEIT, Korea) and the Ministry of Trade, Industry & Energy (MOTIE, Korea) through the coretechnology development program of Automotive Industry (10082561, Development of Sound Field
Control System for Realizing Personal Sound Zone in Vehicle Interior).

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3. Su-Ho Cha, Sung-Hwan Shin, and Chanho Kim, Improved dynamic feeling of engine sound usingmodifying its harmonic order. Proc ASK 2018; 9-11 May 2018, South Korea. 2018:106.
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