3D Traffic Noise Simulator for VR Environment
Koji Makanae

Koji Takahashi

School of Project Design, Miyagi University
Miyagi, Japankoji@makalab.org

Abstract
A simulator system to predict and hear road traffic noise was developed. The system places sound sources onvehicles in traffic flow simulation and reproduces sound data by using a three-dimensional sound capability. By
using this system, the effect of road construction work and that of sound barriers as a countermeasure can bepresented as easily-understood information; this capability should be helpful in building consensus with localcommunities.
Keywords
Traffic noise, Audio simulator, Virtual reality, Traffic flow simulation

1. INTRODUCTION
In a road construction project, consensus must be formed
between the project conductor and the local communities
along the road. To this end, the conductor needs to explain the effect of the project to the local people in aneasily-understood way. The effect of such a project can
now be presented visually with high levels of reality bylandscape simulation using computer graphics and virtual
reality technologies. On the other hand, noise and vibration, which may affect living environments greatly, aretypically shown as numeric estimates, which are compared with environmental criteria to evaluate the severityof the effect. Such numeric data, however, is very difficult to perceive intuitively.
The purpose of this study is to construct a simulator system to predict and hear road traffic noise. The systemshould be able to present predicted noise, which used to
be given as numeric data, as sound information audible tohumans.
2. PRECEDING STUDIESRoad traffic noise is a very complicated phenomenon
affected by various factors such as road structure, traffic
conditions, vehicles, and propagation. Various methods
to construct noise prediction models have been proposedin Japan. The Acoustical Society of Japan [ASJ99] integrated past findings and presented an expression fornoise prediction based on a traffic noise model withequally spaced sources in a row (equally spaced model),which is adopted in Road Environment Improving Manual [JRA88], which is used for environmental impact
assessment. Model experiment is a way to investigate thedistribution of sound levels and the propagation ofsound--when road structure and surrounding topography
are too complex-- by experimentally studying models
that incorporate such complex factors. This approach,which allows flexible setting of conditions, is useful instudying the effects of different road structures [Shimizu87].
On the other hand, road noise prediction systems basedon computers allow vehicles--assumed to be point soundsources--to run on a road of arbitrary structure andalignment. These systems, which require many conditions to be set beforehand, are still not in the stage ofpractical use. The road noise prediction system by Shojiet.al, [Shoji68] is an example of this approach. They
simulated the flow of traffic on an expressway by theMonte Carlo method to predict the propagation of noisefrom running vehicles. Other systems [Nagano99] tried
to present road and vehicle noise as audible information.
They constructed and evaluated a system that allowed theuser to hear noise by manipulating the regeneration levelsof recorded video and sound. However, there has been
few attempts to develop a simulator that allows the user
to hear noise in synchronization with traffic flow simulation; this is the attempt made in the present study. For
driving simulator, some traffic noise simulators havebeen developed in recent year [Mourant03].
3. CONCEPT OF SIMULATOR FOR NOISE
PREDICTION AND HEARING
3.1 Conceptual model
The simulator for road traffic noise prediction and hearing in this study is a virtual reality system that allows theuser to hear noise, which have been presented only as
numeric values or visual information. The concept of thesystem is shown in Figure 1.

Figure 1: Concept of the system
The system defines a three-dimensional virtual space andallows the observer to hear sound at a certain point of thespace by using a three-dimensional sound capability.
3.2 Conditions for noise prediction
The process of the generation, propagation, and receptionof road traffic noise is affected by various factors including road and traffic conditions, vehicles, environment,
weather, buildings, and people. Because it is difficult totake all the factors into account, only the major factors
are usually used in the prediction of road traffic noise.
This study aims to construct a prototype simulator fornoise prediction and hearing by only using the conditions
such as the sound of engine, traffic volume, Vehicle
types and their ratio, velocity, and noise barrier.
Although vehicle tires are a significant source of noise,
they are excluded from consideration here because ofdifficulty in sampling their sound.
4. CONSTRUCTION OF SIMULATOR FOR NOISE
PREDICTION AND HEARING
4.1 Modular configuration of the system
The simulator system consists of a traffic simulation
module to compute vehicle behavior, a noise prediction
module to predict sound as numeric values, a threedimensional sound module to convert the calculation
results to audible information, and a sound module toconvert the calculation results to visual information(Figure 2).
The computer configuration for developing the system
are as follows.
Operating system: Microsoft Windows 2000
Language: Microsoft Visual Basic 6.0
Graphic library: OpenGL
Three-dimensional sound environment: MicrosoftDirectX 7.0
4.2 Outline of the DirectSound threedimensional sound library
DirectSound of Microsoft DirectX is used in this study asa three-dimensional sound library.
DirectSound is a general sound API that provides
applications with many functions, such as regeneration ofvarious sounds including three-dimensional sound,capturing of sounds, synthesis of different sounds, andchanges in localization and distance [Microsoft02].

Figure 2: Modular configuration
The three-dimensional sound capability means simulated
sound can be heard not only from right and left but fromall directions. DirectSound calculates the levels of soundheard by the listener based on three-dimensional soundlocations. Because DirectSound does not take environmental geometries into account, discrepancies occur between the simulated and real sound environments. These
discrepancies, however, can be compensated, for example, by adjusting roll-off factors, which control soundattenuation. In addition, DirectSound can cause Doppler
effect based on three-dimensional sound velocity and cancontrol the effect over a wide range by specifying Doppler factors.
4.3 Traffic flow simulation
To predict noise on a microscopic basis, the behavior ofeach vehicle within a certain observation area isdescribed in traffic flow simulation.
1. Vehicle description
The behavior of each vehicle within the observation
area is described based on microscopic traffic flow
simulation. The time interval of simulation (scanningcycle) is set to 0.1 s.
2. Road structureTwo-lane and one-lane, straight, flat roads with a lane
width of 3.5 m are postulated.
3. Generation of vehicles
Vehicles are fed into the observation area from traffic
inlets. The vehicles generated at each inlet conform toPoisson distribution based on the average headway
determined from hourly traffic volume.
4. Vehicle types and their ratio
Two types of vehicles are assumed: large vehicle (12.0m long and 2.5 m wide) and small vehicles (4.7 m longand 1.7 m wide). The percentage of large vehicles canbe set arbitrarily.
5.Velocity and behavior of vehicles
The velocity of vehicles is either 30, 40, or 50 km/h,
and all the vehicles run at the same constant speed. Novehicles stop or make turns.

Table 1:Sound samplingvehicle typevelocity

noise levelsmall vehicle

30 km/h

67 dB

40 km/h

71 dB

50 km/h

74 dB

30 km/h

85 dB

40 km/h

93 dB

50 km/h96 dBlarge vehicle

Figure 4: Noise distribution map
4.4 Regeneration of noise from running vehicles
4.4.1 Sound reception area
An area within 50 m from the centerline of a 100-m longroad on both sides is set as the area of observation, andthe listener can move freely within the area. The point ofsound reception is set at a height of 1.2 m.

4.5.2 Sound attenuation by noise barrier
Whether sound is attenuated by a noise barrier depends
on the spatial relationship among the listener, the vehicle(sound source), and the noise barrier. Sound attenuationby the noise barrier is assumed when the barrierintersects the straight line connecting the listener and thevehicle.

4.4.2 Location of sound sourceEach vehicle generated in the traffic flow simulation isprovided with such information as location and type.
DirectSound calculates the level of sound received by thelistener based on the location of each sound source,which is determined from the vehicle location
information. The source is set at the planar center of thevehicle, and the height of the source is 0.3 m, which is atypical value adopted in previous studies.

If sound attenuation is assumed, the noise level
prediction method by JSA is applied. That means soundlevels at the reception point with and without the barrier
are determined, and then an attenuation factor iscalculated from the ratio of the sound levels. The effectof attenuation is taken into account by assuming thatsound level decreases by 6 dB as distance is doubled inquasi-free space.

4.4.3 Sampling
The sound generated by a vehicle should be emitted fromthe sound source. However, because there is noestablished mathematical function that precisely
regenerates the sound of the vehicle according to its
speed and other conditions, the sound of the engine was
sampled for use as sources. Using jacked-up vehicles, theactual sound of its engine was sampled and recorded aswave files for each velocity and vehicle type. Noiselevels were measured at the same time with a noise meter.
The different cases of noise sampling and the measured
noise levels are shown in Table 1. The noise levelchanges according to load on the engine in actual driving,
but this effect is not taken into account in the sampling
and measurements in this study.
4.4.4 Doppler effect
The velocity of sound sources must be specified toregenerate Doppler effect. The velocities set in the traffic
flow simulation were used.
4.5 Suppression of noise by noise barrier
4.5.1 Installation of noise barrier
A noise barrier of any height within the range of 1.5 to
5.0 m and any length can be installed.

4.6 Noise distribution mapping
In addition to the audible information, noise distribution
maps are created to provide visual information on theprediction made by the system. A noise distribution map
shows the distribution of noise at a certain time.
In noise distribution mapping, an observation area of 100
m x 100 m is divided into 1 m x 1 m lattices, and thenoise level at each lattice point is determined according
to the prediction model of Road Environment Improving
Manual.
The calculated noise levels are converted to color data
and displayed as numeric information on noisedistribution. The numeric information is converted tocolor values and displayed as color information. Figure 4
shows an contour map obtained by extracting contours,
by using space filters, from a noise distribution map
displayed by the system.
4.7 Simulator interface
The listener wears a pair of headphones to hear the traffic
noise simulated by the system. The behavior of vehicles
generated by the traffic flow simulator is shown in realtime on the computer screen (Figure 5). The listener canchange freely the point of reception by mouse operation,and the sound information changes accordingly in realtime. It is also possible, if necessary, to set or change
simulation conditions and display noise distribution.
5. SIGNIFICANCE OF THE SYSTEM AND
FUTURE CHALLENGES
5.1 SignificancePrediction of road noise has been presented in the formof numeric data or maps created from the data; such information, however, is difficult to perceive intuitively.
The present system allows the results of sound prediction
to be perceived directly as audible information, which
can be easily-understood by ordinary people without expert knowledge. This system, when applied to an actualroad construction project, should be useful not only forpredicting the effect of the project on living environments but also for explaining the effect of countermeasures such as barrier installation.
5.2 Challenges
The present system has been developed as a prototype.
There are some challenges to be addressed for practicaluse.
1. Improvement in noise regeneration
This study assumed that the only source of noise is thesound of the engine, whose contribution in the entire
noise is significant. In actual traffic noise, however,tires are another significant source of noise. To improvenoise regeneration, therefore, sampling of tire sound as
an additional source of sound needs to be considered.
We also need to have a mechanism that reflects roadsurface conditions, such as wet surfaces in rainy
weather.
The sound sources sampled in this study were mostly
dependent on vehicle velocity because they were usedwithout modification. Mathematical functions to express vehicle noise thus require further study.
2. Consideration of complicated road structure and traffic
conditions
Although only straight, flat roads were used for simulation in this study, we need in the future to regeneratesound in consideration of planer features (intersections,alignment, etc.), vertical features (viaducts, fills, cuts,etc.), and roadside conditions. We also need to develop
a system that can handle more complex traffic flows,
although a simple model was adopted for traffic flow
simulation in this study.
3. Evolution to multimedia environment simulator
The present system, which aimed only at regenerating
sound information, may evolve into an integrated multimedia landscape simulator by adding visual information and using virtual reality technologies. The integrated simulator is expected to be useful in building
consensus with local communities.
6. SUMMARY
A simulator system with a three-dimensional sound capability was developed. The system allows prediction ofFigure 5 : Simulator Interface

road traffic noise and sound barrier effect and allows theuser to perceive noise as audible information. In building
consensus with local communities regarding a road construction project, the system allows the project conductor
to show in an easily-understood way the effect of roadnoise and noise barrier installation.
Future challenges include adaptation to more complex
road and traffic conditions, improved sound regeneration,and evolution into a multimedia simulator.
7. REFERENCES[ASJ99]Research commitee of road traffic noise inAcoust.Soc.Jpn. ASJ prediction model 1998 for roadtraffic noise report, J.Acoust.Soc.Jpn., 55, 297-310,
1999.
[Nagano99]T.Nagano, Y. Yasuda, K.Furihata andT.Yanagisawa. The examination of virtual reality system for noise experience, The Technical Report of
The Proceeding of The IEIC(Japan), EA99-56, 1999.
[JRA88]Japan Road Association. Road environment
management manual, 1998.
[Shimizu87]H. Shimizu. Road environemt, Kogakusha,
1987 (in Japanese).
[Microsoft02]Microsoft. DirectX SDK. 2002.
<http://www.microsoft.com/windows/directx/>[Mourant03].Mourant, R. and D.Refsland. "Developing a
3D Sound Environment For a Driving Simulator",
Proceedings of the Ninth International Conference on
Virtual Systems and Multimedia, Montreal, Canada,
October, 2003, pp. 711-719.