PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Auralisations for Outdoor Noise Sources
Environmental Auralisations
Erik Thysell1; Per Finne2
1&2

FORCE Technology, Copenhagen, Denmark

ABSTRACT
Auralisations have for years been used in addition to mandatory noise maps and calculated number of noise
exposed dwellings in the EAA-processes carried out by e.g. the Danish Road Directorate. Auralisations makeit possible to get a realistic impression of the future noise during the planning of new roads or other importantnoise sources. Until now, only major road projects have been published using auralisations. Other noise
sources like local roads, wind turbines, railways, tramways and light rails can obviously benefit fromauralisations in communicating noise consequences to people influenced such as neighbours, decision makers
etc.
Auralisations make it possible to aurally explain the mix of noise combined by two or even more noise
sources. Normally, noise from various noise sources are treated and explained individually and have evendifferent noise limits to meet. Auralisations should represent the total sound from all sources. This is muchmore realistic as the total sound will be perceived and experienced by the future neighbours.
This presentation deals primarily with auralisations made for road noise, but challenges of auralising othernoise sources like wind turbines and the combined noise generated by various noise sources will be covered
as well.
Keywords: Auralisation, Auralization, Environmental Noise

1. INTRODUCTION
Auralisations make it possible to hear how planned outdoor sound sources such as roads, windturbines and railways will sound in different landscapes and situations. The most well-known analogyto this is a visualisation. Who will allow the construction of a new road or bridge without having seenin advance how the road will affect the landscape? But when it comes to sound and noise, you normallyuse visual measures like noise maps to explain the consequences. Why not use the ears?
Auralisations can also be used to clarify, how noise can vary in the surroundings due to different
conditions. How does the road sound for example, in upwind or downwind situations, with anincreased traffic volume or with a completely different traffic mix? Auralisations can answer all that.
From experience it is well-known that commonly used noise indicators like L den are difficult toexplain to layman; Hence noise is one of the issues that is most often discussed intensively, whenplanning new noisy sources. Naturally, the neighbours are concerned about, how the future noise willaffect their lives – and by nature they will try to affect the project, so that noise is minimised.
From a planner’s point of view, the authorities must provide as good and adequate information as
possible, when the consequences of e.g. a road project are presented for the neighbours. This applies
of course to the future noise level with respect to the noise limit, but certainly also to the explanationof possibilities and limitations to reduce the noise. Information on the effect of possible noise
protection measures, such as noise barriers or noise reducing pavement, as well as changes in thespeed and traffic volume, is important information. The neighbours and other stakeholders must beprovided with most realistic and obtainable picture of the consequences at the planning stage. In12

erth@force.dk
pfi@force.dk

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addition to describing the consequences in words, pictures, graphics and tables, the benefit ofauralisations can be used.

2. METHODOLOGY
The methodology used for creating auralisations has been:
1.
2.
3.
4.
5.

Create noise source(s).
Create a propagations model for the sound in the wanted scenario.
Filter the sound source(s) by the calculated sound propagation transfer functions.
Mix the filtered sounds together.
Adding a soundscape providing a realistic and natural sound of the surroundings.

These five steps are described in the following paragraphs and illustrated in Figure 1. For roadnoise auralisations, the procedure is also described in detail in (1).
Choice oflocation / situation

Number ofvehicles

Noise prediction
softwareTTransfer
ffunctions

Single noise
source recording

SSoundLibrary((recordings)

AAuralisation
ssoftware3

T
TrafficmixmA
AddingSSoundscape

Finalaauralisation

Choice ofSoundscapeSingle soundscaperecordingS
SoundscapeLibrary((recordings)

Figure 1 – Diagram of auralisation work flow3 (roads)
2.1 Creating a Source
The creation of noise sources has been accomplished by using a HATS (head-and-torso simulator)
to make binaural recordings of single noise sources, such as a car passing by or a wind turbine. Thebenefit of this has been a high-fidelity sound recording of the source that sounds like the real source(in opposition to synthetic generated sound source). The challenges with this method are primarily
background noise during the recording of the sound sources and limitations due to the recording
position.
Due to background noise it is necessary to be very selective, when it comes to choosing the places
to carry out the recordings. Avoiding background noise from other roads, singing birds etc. is essential,and therefore recordings have to be carried out relatively close to the road, i.e. 25 m, at evening andnight time. Furthermore, there has to be low down wind speeds, i.e. from the noise source toward theHATS location (Figure 2).
The recording position poses a couple of limitations to the auralisation. The HATS has been setup
pointing towards the noise source. When listening to the recorded signal, it appears to be located in
front of the listener. Without using HRTF filtering it is not possible to virtually move the head around.
The relative closeness to the road also sets a limit for, how far away from the road the auralisations
sound realistic.

3The auralisation software (AuralizationLAB) is developed in-house with LabVIEW and takes care of thefiltering of the noise.

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Figure 2 – Photo of HATS location close to a motorway
Finally, it is only possible to auralise existing and available noise sources.
Recording time has been between 30 seconds – 2 minutes, depending on the noise source.
For vehicles that pass by, the needed recording time depends on the speed of the vehicle and thebackground noise in the auralised situation. The noise from the vehicle must have time to fallsufficiently below the noise from other noise sources (other vehicles, other types of noise sources orthe soundscape noise).
For stationary noise sources such as wind turbines, the recording time has been as long as the final
auralisation in order to avoid any discontinuities. Trying to loop a stationary (spatially stationary)sound source, where generated sound and the meteorological conditions continually shift, has provenit difficult to accomplish without discontinuities.
2.2 Sound Propagation
The propagation of sound from source to receiver is carried out twice: First for the recording
situation and then for the auralised situation. The first propagation is only carried out once for eachrecording. The second propagation is carried out for each auralisation situation.
The propagation is carried out using a noise modelling software to create a computer model of thereal situation and using Nord2000 (2) noise propagation method, the noise is propagated from source
to receiver. In the situation with a moving sound source, the propagation is carried out for several
point sources along the sound source path, i.e. each 10 m. The result of this is a 1/3 octave band
spectrum for each source receiver position.
For each recording and new auralisation situation, the spectrum difference between auralisation
situation and recording is obtained as the input to the next step.
2.3 Filtering Sound
The sound source is filtered using the calculated spectrum differences. In the case of stationary
sources, this is quite trivial, since the whole sound is filtered with the same spectrum difference
between auralised and recording situations.
In the case of moving sound sources, the binaurally recorded sound is split up into several soundclips corresponding to the time difference between source points (speed dependant) and carefully
matched against the correct source receiver position. These sound clips are then filtered with thespectrum difference between the sound propagation spectrum for that point and the spectrum for thecorresponding point in the recording situation. Finally, the filtered sound clips are put together again.
Therefore, it is crucial that the position of the vehicle relative to the HATS position (in time) in therecording is known.
The result is realistic; An auralised sound file for each single source.
2.4 Mixing filtered Sounds
When auralising a road or a combination of sound sources, several files with auralised single
sources are mixed together to create the situation aimed at e.g. busy afternoon traffic.
This is an important step, as it adds context to the sensation of “being there”.

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For a situation with vehicles on a road, a semi-random, but somewhat even distribution of auralised
single source files are mixed together. The mixing is performed by adding files with a specific timedelay for each file. The time delay depends on what traffic flow, the auralisation has to illustrate. A
somewhat random experience is achieved, as none of the recorded vehicles drives at identical speeds.
2.5 Adding the Soundscape
Finally, a suitable binaural soundscape with ambient noise that matches the situation that theauralisation is aimed at is added. Different soundscapes have been obtained using HATS at various
places and with different noise events. One example was a situation, where a planned motorway was
to be passing residential houses, and there was already a railway track nearby with passing trains.
Using HATS recordings of the ambient sound, a very realistic binaural soundscape with natural
ambient sounds was recorded with a train passing in the distance behind the HATS. This was added
to a mix of vehicles passing by on the planned road in front of the auralised position. The result wasan auralisation with recognizable and realistic ambient noise making the presentation trustworthy forthe residents.

3. SOURCES
This auralisation methodology/technique has been used with different sources:
Road vehicles, such as cars, busses, trucks etc. ha ve been used extensively in auralisations.
Numerous recordings are performed on single vehicles passing by the HATS.
Trains, including normal trains and trams ha ve also been used in a similar way that road vehicles
have been recorded and used in auralisations.
Wind turbines are perhaps easiest to record, but only in one position and wind speed at a time.
Since they are stationary, any change in relative angle between the direction of the nacelle and theauralisation point must be recorded separately.

4. EXPERIENCE
For years now, we have made auralisations used in the planning new roads in collaboration with
the Danish Road Directorate, which is responsible for the major roads like motorways andinterregional roads in Denmark. The auralisations have aimed at:
1) Presenting realistic and trustworthy noise levels for specific places in the surroundings like theresident’s garden or terrace, so one can hear, how the road will actually sound at one’s home.
2) Presenting the noise levels at specific distances using different noise reducing measures likebarriers, special pavement etc.
The auralisations have been used at public hearings using calibrated headsets. Pictures of thelandscape were presented on a screen simultaneous to simulate the context, i.e. the listener should
imagine being at the exact location when hearing the auralisations. This was done using the screenwith relevant pictures for each location (Figure 3).

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Figure 3 – Auralisations presented at a public hearing using calibrated headphones
Another example was used at the public hearing, where there was a “noise café”. Here, the public
could – by themselves – activate the presentation using calibrated headsets and a video screen tovisualise the situations.

Figure 4 – Auralisations presented at a public hearing. Photo from Danish Road Directorate
Finally, due to the great interest, we created a website with generic auralisations. Here thelisteners could use it from their homes with their own equipment. This of course, makes it
impossible to have full control over the calibration level and the quality of the playback system.
However, we made a calibration procedure, allowing the listener to adjust the volume according to aspeech signal, i.e. the listener should “adjust the volume so it sounds like a person speaking 1 meterin front of you”. The website can be found here: https://acoustics.madebydelta.com/viden/auraliseringfor-vejdirektoratet-2/road-noise-auralisations-2/

5. FUTURE WORK
There are several future developments that would expand the possibilities and improve themethodology. The ultimate scenario would be a very realistic VR-experience with both visualisation
and auralisation combined. Before that goal can be reached, there are several challenges to overcome.
One major improvement that is being investigated is synthetic sources. The possibility to createhigh-fidelity synthetic noise sources would eliminate some of the limitations of using binauralrecordings. There would be no issue with background noise and no limitations to the position of theauralised position. (i.e. we could auralise the sound from a future road in Australia in Denmark (if itwas loud enough)). However, this requires the use of HRTF (head related transfer function). Synthetic
sources with directivity would also allow for sources being placed in any direction relative to thereceiver.
Using HRTF would make it possible to turn the listening position freely around, relative to thesound source and soundscape.
There are also possibilities to automate some of the (manual) steps in the process.

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ACKNOWLEDGEMENTS
The authors wish to thank the Danish Agency for Institutions and Educational Grants for financial
support of the work. The authors also wish to thank the Danish Road Directorate for financing and forbeing a part of the development of road noise auralisations used at public hearings concerning theplanning of new roads.

REFERENCES
1. Finne P., Fryd J.: "Road Noise Auralisation for Planning New Roads", Proceedings of INTER-NOISE,
Hamburg, 2016.
2. Plovsing B. "Proposal for Nordtest Method: Nord2000 - Prediction of Outdoor Sound Propagation"
DELTA Report AV 1106/07 revision 4.

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