Research ArticleArtificial Engine Sound Synthesis Method for Modification ofthe Acoustic Characteristics of Electric Vehicles

Dongki Min, Buhm Park, and Junhong Park

Department of Mechanical Engineering, Hanyang University, Seoul 133-791, Republic of Korea

Correspondence should be addressed to Junhong Park; parkj@hanyang.ac.kr

Received 15 January 2018; Accepted 6 March 2018; Published 10 April 2018

Academic Editor: Marcello Vanali

Copyright © 2018 Dongki Min et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Sound radiation from electric motor-driven vehicles is negligibly small compared to sound radiation from internal combustionengine automobiles. When running on a local road, an artificial sound is required as a warning signal for the safety of pedestrians.In this study, an engine sound was synthesized by combining artificial mechanical and combustion sounds.Themechanical soundswere made by summing harmonic components representing sounds from rotating engine cranks. The harmonic components,including not onlymagnitude but also phase due to frequency, were obtained by the numerical integrationmethod.The combustionnoise was simulated by random sounds with similar spectral characteristics to the measured value and its amplitude wassynchronized by the rotating speed. Important parameters essential for the synthesized sound to be evaluated as radiation fromactual engines were proposed. This approach enabled playing of sounds for arbitrary engines. The synthesized engine sounds wereevaluated for recognizability of vehicle approach and sound impression through auditory experiments.

1. Introduction

With the increasing use of electric motor-driven vehiclesdue to their advantages such as environmental friendlinessand fuel-efficient performance [1], it is necessary to generateartificial sounds to inform pedestrians and cyclists of thevehicle’s approach. In addition, drivers prefer sounds similarto internal combustion engines for auditory satisfaction, andthe sounds should represent the current status (accelerationor deceleration) of the vehicle, for example, start-up or rapidacceleration and deceleration. A simplemethod of generatingartificial engine sounds is to play recorded combustion enginesounds according to engine rotating speeds. However, thismethod exhibits limited performance for reproducing thecomplex conditions of a vehicle and requires an impracticalnumber of recordings for generating artificial sounds forvarious vehicle statuses from a single machine. An artificialengine sound generator is considered the most effectivesolution. This sound generator should be designed to makea positive impression on people to minimize the possibilityof noise pollution.

Subjective evaluations of various engine sounds wereperformed in previous studies [2–5]. Amman and Das[6] proposed deterministic and stochastic components and

used synchronous discrete Fourier transform and subopti-mal multipulse excitation approaches to generate realisticengine sounds. Airplane engine sounds were generated bycombining broadband and individual tonal contributions[7, 8] to determine the annoyance level and how to reduceit efficiently. Hastings [9] used deterministic and randomvariation in sound amplitude to regenerate diesel enginesounds. With amplitude variation following a rotating crankshaft, the random signals generated with impulse trains wereused to generate artificial sounds for annoyance evaluation.Artificial engine sound generators mounted on low noisevehicles have used simple sounds for warning pedestriansoutside the vehicle.The playing of sampled engine sounds forvarious rotating speeds was widely used in previous studieson artificial engine sound synthesis [10–18]. A system hasbeen developed to warn pedestrians by using sine wavesweep signals [10–13], motor sounds [14], and future-orientednew sounds using instrument sounds [15]. The synthesizingmethod utilizing the premeasured actual engine sounds hasthe advantage of using lessmemory, but there is a limitation increating a variety of sounds in relation to the vehicle speeds.

Active noise control and active sound design techniqueshave been used to reduce low-frequency engine noise and

HindawiShock and VibrationVolume 2018, Article ID 5209207, 8 pageshttps://doi.org/10.1155/2018/5209207

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synthesize harmonics to enhance existing engine soundquality [16]. The artificial complex sounds were required tominimize noise according to the driving conditions of thevehicle [17, 18]. Equalizers have been developed to allowsetup of parameters adjusting the amplitude and range of thespectrum of basic sound building blocks.

In this study, an artificial engine sound generator wasdeveloped using a sample-based algorithm. With the sys-tematic data processing, engine sounds that are similar tothe actual engine sound are generated. Artificial enginesound synthesis method was implemented by analyzing theactual engine sound generation mechanism. The numericalintegration method allows construction of an optimal dataset generating the engine sound. Synthesized artificial enginesounds according to the vehicle engine speed were generatedby applying a sample-based algorithm.

Many researches recorded and used the actual vehicleengine sound for sound synthesis.The artificial engine soundgenerator requires a CAN (controller area network) buscommunication module and a sound module. This methodis simple to use and implements actual engine sounds (orpresynthesized beeps), but the types and scalability of soundsare limited and require a lot ofmemory.The auditory satisfac-tion of the user is also limited because it is difficult for the userto change the sound. To generate engine sounds, parametersrelated to engine characteristics such as number of cylinders,rotating speed, engine type, combustion pressure profile,timing, and amplitude modulation should be considered.For generation of engine sounds to be unique for eachvehicle, important parameters that influence engine soundperception should be fully understood.

In this study, a mathematical formulation to generateartificial engine sounds to meet consumer preferences isproposed. With this approach, flexible generation of enginesounds without playing sampled sounds is possible. To ana-lyze the characteristics of engine sounds, gasoline and dieselengine sounds were recorded. After spectrum analysis, themechanical and combustion engine sounds were separated.The mechanical sounds were synthesized using a summationof tonal sounds. The combustion sounds were generatedusing spectrum amplitude-modulated random sounds toexhibit a specific frequency envelope. The artificial enginesound was synthesized by combining these mechanical andcombustion engine sounds. For illustration, several differentengine sounds were generated and used in the evaluation ofperception. Recognizability of vehicle approaches and per-ceived quality of the artificial engine sounds were evaluatedby comparison to actual recorded engine sounds. This alsoprovided information on the sound generation mechanismfor actual engines and its perceived characteristics. Enginesound characteristics must be understood for both design ofquiet vehicle engines and artificial engine sound-producingdevices.

2. Measurements of Acoustic Characteristics ofEngine Sounds

Gasoline and diesel engine sounds were measured usingthe test engines in the laboratory [19, 20] for spectrum

Control room

Engine room Window Window

Engine controller Recorder

MicrophoneGasolineengine engine

Diesel

ExhaustExhaust

Figure 1: Overall schematic view of the laboratory chamber.

analysis. The measured engine noise was used to definethe mechanical and combustion noise. Figure 1 shows theexperimental setup to measure the engine sound. Figure 1shows an overall schematic view of the laboratory chamberthat has the controller adjusting operating conditions. Themicrophone used to measure sounds from each engine waslocated 1 meter from the engine. The engines used in theexperiment were equippedwith 4 cylinders.The engine roomwas surrounded bywallsmade of glass andporous panels.Thesounds were measured for 10 seconds when the engine speedwas 1000, 1500, 1800, and 2100 rpm, with idle conditions of850 and 820 rpm for gasoline and diesel engines, respectively.

Figure 2 compares the soundof the gasoline engine to thatof the diesel engine at various rotation speeds. The measuredresults were focused on the spectrum characteristics of themechanical sound and the combustion sound. The high-frequency component of the diesel engine combustion noisewas higher than that of the gasoline engine.

The combustion noise propagates into the air throughthe vibration of the various structures that make up theengine. The factors affecting the pressure profile includeinjection strategy, combustion chamber geometry, air-fuelmixture ratio, compression ratio, and amplitude changesdue to explosion [20–24]. The resonance frequency of thecrankcase and the cylinder block in the range of 200 to1000Hz, the timing gear, the rocker cover in the range of 1000to 5000Hz, and the cast aluminum cover and the thin castiron in 5000Hz were analyzed. The engine structures havedifferent emission efficiencies. The frequency componentswere attenuated by more than 10 dB after 1 kHz for the dieselengines [25]. The contribution of the noise generated inthe wide band over 1 kHz is relatively low compared to themechanical sound of the tonal components. The sound levelis reflected in frequencies and bands magnitude multipliedby artificial mechanical and combustion sounds. The low-frequency region of the measured sound was masked bythe mechanical sound. The mechanical and the combustionsounds were separated with respect to the frequency of 1 kHz.This distinction provided a simple but accurate algorithm forgenerating artificial engine sounds.

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Figure 2: Spectral characteristics of (a) gasoline and (b) diesel engine sounds measured in the laboratory.The spectrum showed a decreasinglevel with increasing frequency, and the combustion noise at high frequencies over 1 kHz was larger for the diesel engine.

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)In

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Figure 3: Comparison between (a) in-cylinder static pressure and (b) acoustic pressure for a single-cylinder diesel engine. For the acousticpressure, the gray and black lines showmeasured and filtered (from 1.1 to 2.9 kHz and 1.9 to 2.1 kHz) acoustic pressures to minimize influencefrom the dynamometer used in the experiments.

2.1. Spectral Characteristics of Engine Sounds. The frequencycomponents of the measured engine sounds are shown inFigure 2. The measured sound was divided into mechanicaland combustion sounds to produce artificial engine sound.The mechanical sound clearly shows the frequency shift inaccordancewith the engine speed in the low-frequency range.The combustion noise was defined for spectral componentshigher than 1 kHz, where the increase in the noise level ofthe broadband noise is noticeable depending on the enginespeed. Although the combustion sound also affects the low-frequency range according to the cylinder internal pressureconditions [23, 24], the variation of the combustion soundlevel by the mechanical sounds was negligibly small. Atfrequencies below 1000Hz, the engine sound level showedminimal dependence on the engine speed but showed asignificant influence from the first engine order defined as

𝑓0 = 𝑁 ⋅ 𝑛2 ⋅ 60, (1)

where 𝑁 is the engine speed which is represented in revo-lutions per minute (rpm) and 𝑛 is the number of cylinders.The harmonic components at multiples of the first engineorder had a significant influence at low-frequency bands,below 600Hz, and are known as mechanical sounds [3,21, 22, 25, 26]. The combustion sounds measured for thediesel engine at frequencies between 1 and 10 kHz were muchhigher compared to those of the gasoline engine. The high

in-cylinder pressure of the diesel engine for autoignitioncaused this increased level of the combustion sound [26,27]. The basic spectrum envelope of the frequency responsebetween combustion and acoustic pressure remains the samefor most diesel engines [4, 25]. The combustion sound levelis proportional to the engine speed in the fixed injectionstrategy and idle state.

2.2. Transient Variation in Engine Sounds. One importantcharacteristic of diesel engine sounds is the temporal varia-tion [3, 15]. Direct injection diesel engines are often short-circuited to the cylinder prior to the top dead center (TDC),often expressed as zero degrees of crank. Ignition occurs at10 degrees prior to the combustion [28]. The first peak ofthe combustion pressure is due to the mechanical action ofcompressing the air in the second stroke. The second peak iscaused by the sudden change in the sound pressure during thecombustion process. For further investigation of combustionsound, a single-cylinder diesel engine [29, 30] was usedfor recording. To prevent reflection of engine sounds, anabsorbing chamber was installed around the engine system.The engine sounds were recorded using a microphone whichwas located in front of the engine at a height of 1m. The in-cylinder pressure was measured at the same time.

Since the measured engine sound shown in Figure 3included noise from various components of the test engine,the measured engine sound was bandpass-filtered from 1.1

4 Shock and Vibration

to 2.9 kHz and from 1.9 to 2.1 kHz. The temporal variationin filtered sound due to the combustion cycle is clearlydemonstrated in Figure 3. Comparing the temporal varia-tion to the time history of the in-cylinder pressure showsthat combustion sound is boosted periodically with the in-cylinder pressure change. The first maximum in the in-cylinder pressure was from the piston’s mechanical move-ment during the compression stroke; the secondmaximum isfrom the combustion during the explosion stroke. Therefore,it is apparent that a combustion sound has a certain frequencyenvelope above 1 kHz and it is amplitude-modulated in accor-dance with the mechanical cycling motion of the cylinder.

3. Development of a Synthesis Method forArtificial Engine Sound

Based on the measured spectral characteristics of enginesound, mechanical sound below 1 kHz was especially dom-inant for the gasoline engine. The combustion sound above1 kHz had significant influence on the diesel engine sound.According to this measured variation, an artificial enginesound was synthesized by summing synthesized mechanicaland combustion sounds.

3.1. Mechanical Sound Synthesis. Mechanical engine soundbelow 1 kHz was synthesized by summing the harmoniccomponents according to first engine order as

𝑝𝑚 (𝑡) =𝐾

∑𝑘=1

(𝑎𝑘 cos(2𝜋𝑘𝑓0𝑡𝑁𝑚 ) + 𝑏𝑘 sin(2𝜋𝑘𝑓0𝑡𝑁𝑚 )) , (2)

where 𝑝𝑚 is the mechanical sound and 𝑁𝑚 is the resolutionfor obtaining spectral components lower than the first engineorder (𝑁𝑚 = 𝑛2). The magnitude of each frequency compo-nent, 𝑎𝑘, and phase, 𝜙𝑘 (the Fourier coefficients of the cosineand sine terms, 𝑎𝑘 and 𝑏𝑘), was obtained from spectrum anal-ysis of the actual engine sounds measured for 10 s. Becausea typical spectrum analysis such as fast Fourier transformprovides information only about the magnitude withoutphase, an integration method was applied to obtain both ofthem in this study as

𝑎𝑘 = 1𝑀𝑀

∑𝑚=1

( 1𝑇 ∫𝑇

0𝑝 (𝑡 + (𝑚 − 1) 𝑇) cos 2𝜋𝑘𝑡𝑇 𝑑𝑡)

𝑏𝑘 = 1𝑀𝑀

∑𝑚=1

( 1𝑇 ∫𝑇

0𝑝 (𝑡 + (𝑚 − 1) 𝑇) sin 2𝜋𝑘𝑡𝑇 𝑑𝑡) ,

(3)

where 𝑑𝑡 is the discrete-time interval of the recorded enginesound𝑝(𝑡),𝑇 is the integration period considering resolutionfor obtaining spectral components, and𝑀 is the number ofsamples of time averaging as recording time over𝑇. Note thatthe summation of (3) means a time averaging. To solve theabove integration numerically, Simpson’s 1/3 rule was applied.Using the integration and time averaging, randomnoise com-ponents including the combustion noise become negligible.Therefore, only the mechanical sound components can beobtained. The magnitude of each frequency component 𝐴𝑘

0.1 0.2 0.3 0.4 0.50.0Time (s)

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Figure 4: Comparison between (a) measured engine sound and(b) synthesized engine sound. The mechanical sound was varied byadjusting the amplitude (the amplitudes at frequencies 2𝑓0 and 10𝑓0were amplified 2 and 10 times and are shown in (c)).

and phase 𝜙𝑘 was expressed for convenience of calculation ofthe mechanical sound synthesis.

𝐴𝑘 = √𝑎2𝑘 + 𝑏2𝑘𝜙𝑘 = tan−1 𝑏𝑘𝑎𝑘 .

(4)

Figure 4(a) shows the measured mechanical sounds andFigure 4(b) shows the synthesized engine sound under idleconditions. Note the low-frequency envelope of 8Hz (repeat-ing every 0.15 s) in the signal. This low-frequency envelopeoriginated from the cyclic motion of the engine pistonand differences in the firing process between the cylindersand significantly influenced the development of realisticengine sounds. This low-frequency component is crucial formaking specific engine sounds. In addition, by changing thefrequency components and their magnitude and phase in(2), the synthesized engine sound changed as shown inFigure 4(c).

3.2. Combustion Sound Synthesis. Combustion sound has aspecific frequency envelope above 1 kHz and is amplitude-modulated with piston movements. Considering the fixedaspects of the changes in the amplitudes and the time

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Table 1: Stimuli used in the auditory experiments.

Type Number Description

Synthesized engine sound(a) Mechanical sound of a gasoline engine(b) Mechanical sound of a diesel engine(c) Mechanical and combustion sound of a diesel engine

Recorded real engine sound

(d) Gasoline engine, 2000 cc displacement (reference stimulus)(e) Diesel engine, 2000 cc displacement(f) Sports car engine, 6000 cc displacement(g) Sports car engine, 4000 cc displacement

Measured combustion noiseArtificial combustion noise

103 104102

Frequency (Hz)

10−15

10−10

10−5

Acou

stic p

ress

ure (

Pa)

Figure 5: Typical combustion sound spectrum, which is composedof magnitude-adjusted noise bands from sampled white noise with1/3-octave band.

characteristics of the mechanical sounds and the randomsignal characteristics of the broadband frequencies of thecombustion noise, it is shown that they can produce soundssimilar to those of real diesel engines [9]. The coefficients toadjust the total noise level and to change the fixed time char-acteristic of the mechanical sound as shown in (5) were used.It was possible to produce noise that is similar to the actualcombustion noise. According to the spectrum characteristicsof the combustion sound, the artificial spectrum, 𝑝𝑒, wasgenerated using white noise sampled with 1/3-octave bandsand the magnitude of each band was adjusted for generatingan arbitrary combustion engine as shown in Figure 5. Afterinverse Fourier transform, the combustion sound was madeas

𝑝𝑐 (𝑡) = 𝐶 ⋅ 𝑀𝑓 ⋅ 𝑝𝑒 (𝑡) ⋅ [𝑝𝑚 (𝑡) + 𝐶𝑟 ⋅ 𝑀𝑓] , (5)

where 𝑝𝑐 and 𝑀𝑓 are the combustion sound and the mag-nitude of the first engine order component, respectively. Themagnitude coefficient,𝐶, is a ratio of𝑀𝑓 andwas determinedto be between 0.005 and 0.05, based on measured typicaldiesel engine-driven vehicle noises at low and high rotationspeeds. The broadband noise component considering rever-beration effects by resonance in the combustion chamber

×10−4

Measured combustion noiseArtificial combustion noise

0 0.02 0.03 0.04 0.050.01

Time (s)

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Figure 6: Acoustic pressure fluctuations of the measured andsynthesized combustion sounds.

and repetitive explosive excitations was considered to be aconstant𝐶𝑟.The reverberation coefficient𝐶𝑟 was determinedto be between 2 and 4 at low and high rotation speeds.Consequently, the combustion sound was synchronized inmagnitude with the mechanical sound. Figure 6 shows anexample of synchronized engine sound in Figure 4(a) underconstant rotation of the engine. Total engine sound wasgenerated by summing the mechanical and combustionengine sounds.

4. Validation of Synthesized Engine Soundthrough an Auditory Experiment

To compare the performance of actual and artificial enginesounds, auditory experiments were performed using themagnitude estimation method [31]. Seven sound sampleswere used in the experiments. Figure 7 shows the acousticpressure profile of the stimuli during idle condition forvarious recorded and synthesized engine sounds in Table 1.The engine rotating speed was increased gradually from800 rpm and reached a maximum rpm of 3000 rpm anddecreased to the idle condition again. The maximum sound

6 Shock and Vibration

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Figure 7: Acoustic pressure profiles of stimuli used in the auditoryexperiment. The sounds from (a) to (c) are the synthesized soundsand the sounds from (d) to (g) are the measured sounds. The 𝑥-axisof each layer indicates time in seconds of 0–0.5, respectively.

level of each stimulus was adjusted to be 70 dBA. Figure 8shows an example of acoustic pressure variation for the soundsample shown in Figure 7(a) during playback.

The stimuli were presented to 23 participants with normalhearing using loudspeakers in a soundproof room in whichthe sound level was lower than 38 dBA. Participants were

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Figure 8: Acoustic pressure variation due to rpm change of stimuliused in the auditory experiment.

a b c d e f g1234567

Reco

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b c d e f gaStimulus

123456

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Figure 9: Results of the auditory experiments to evaluate therecognizability of vehicle approach and preference for engine sound.

asked to evaluate the recognizability of the vehicle approachand a preference for engine sounds when a stimulus wasgiven. A typical gasoline engine sound (Figure 7(d)) wasplayed three times as a reference stimulus that was set to fourpoints for participants who were not used to evaluating usinga seven-point scale method. Note that points 1 and 7 mean“extremely poor” and “extremely good,” respectively.

The results of auditory experiments are summarized inFigure 9. Participants’ consistency analysis was performedfor subjective evaluation. Because stimulus (d), which wasgiven as a reference, has an average of 4.48 and the standarddeviation was small at 0.8, the reliability of the result wasverified. A synthesized diesel engine (stimulus c) shows thehighest performance for achieving a recognizable vehicleapproach. Compared to synthesized mechanical sounds only(stimuli a and b), the inclusion of the combustion soundssignificantly increased the recognizability of engine sounds.The recorded sounds of sports cars (stimuli f and g) showedhigh performance, but not higher than that of stimulus c.

Shock and Vibration 7

Synthesized gasoline engine sounds (stimuli a and b)exhibited better performance than that of the diesel engine(stimulus c). This suggested that a more annoying soundmade pedestrians clearly recognize a vehicle approach. Thesports car engine sound (stimulus g) has better engine soundpreference than the other sounds (stimuli c, d, e, and f). Thestrongly fluctuating sounds of the sports vehicles were onereason for this large variation.

5. Conclusions

In this study, the transient and spectrumvariation of recordedsounds from the test engines in a lab was analyzed to inves-tigate the effects of piston cyclic motion on the radiation ofmechanical and combustion engine sounds and their corre-lation to frequency components. Artificial enginemechanicalsound was synthesized using summation of sinusoidal wavesrepresenting mechanical sounds. Due to small differences inthe frequency of sounds radiating from each cylinder, a mod-ulated mechanical sound was generated. For representingmechanical sounds with harmonic sums without influence ofthe combustion noise, the Fourier series was used with thefundamental frequency smaller than the first engine order.After considering these frequency components less than therotating speeds, the synthesizedmechanical sounds exhibitedsimilar characteristics to actual gasoline engine sounds.The combustion sound was synthesized using a spectrum-enveloped random signal. The engine sound was generatedby combining these mechanical and combustion sounds.This approach enables generation of arbitrary engine soundsto customer preference for their own vehicles. From theresults of the auditory experiments, recognizability of vehicleapproach and preference for engine sound were comparedto actual engine sounds. The synthesized sounds, especiallyfor the diesel engine, showed excellent performance forinforming participants of vehicle operation. The proposedalgorithm utilizes variations in frequency components withrotating speeds and allows the use of various sound sourcesfrom musical instruments. Consequently, pleasant soundsmay inform pedestrians of a vehicle approach.

Data Availability

The data used to support the findings of this study areavailable from the corresponding author upon request.

Conflicts of Interest

The authors declare no conflicts of interest.

Acknowledgments

This work was supported by the Research Fund of HanyangUniversity (HY-2017) and LG Electronics Company.

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