Lynge - Odeon - A Design Tool For Auditorium Acoustics, Noise Control And Loudspeaker Systems.pdf - Akustyka(1) - dzidzia2603

ODEON - A DESIGN TOOL FOR AUDITORIUM

ACOUSTICS, NOISE CONTROL AND LOUDSPEAKER

SYSTEMS

C. Lynge Chr. Acoustic Technology, Ørsted●DTU, Denmark. e-mail:clc@oersted.dtu.dk

1. INTRODUCTION

The ODEON software was originally developed for prediction of auditorium acoustics. However

current editions of the software are not limited to these fields, but also allow prediction in rooms

such as churches and mosques, interior noise control, design of room acoustics and sound

distribution systems in public rooms such as foyers, underground stations and airports. Some of the

features in ODEON 5.0 Combined are; two methods for global estimation of reverberation time,

various point response calculations providing decay curves, reflectograms, miscellaneous

parameter graphs, 3D maps, multi-source calculations including point, line and surface sources,

facilities for noise control calculations and multi-channel auralization using fully filtered BRIR’s.

2. MODELLING

2.1 Modelling Rooms

Room geometries to be used in ODEON can be

modelled in two ways, either the geometries

can be modelled in a CAD program such as

AutoCAD and imported into ODEON in the

DXF format or the geometries can be

scripted in ODEON’s parametric modelling

language. No matter the method of

modelling, a surface is obtained by connecting

points on surface's edge. Defining a

sequence of points will automatically define

both sides of the surface so there is no

need to worry about drawing the sequence

of points clock or counter-clock wise, even

surfaces with concave shapes are allowed, see figure 1. ODEON being fairly insensitive as to how

the user entered the model data is in particular an advantage when the geometry is supplied as a

DXF file by third parties, who can not be assumed to be aware of any such rules.

Figure 1 Example on complex surfaces. Left: A

circular surface with a hole in the middle. Right:

A surface with concavities on the edges.

2.2 Modelling In the Odeon Modelling Language

The modelling language available to the ODEON user for modelling geometries is a versatile

scripting format [8]. The format can be used for simply entering geometries point by point and

surface by surface. To support the advanced user however, it is also possible to use constants,

variables, coordinate transformations and even programmatically scripts in order to create flexible

room models at high speed.

ODEON - a design tool for auditorium acoustics, noise control and loudspeaker systems – C. L.

Christensen

2.3 Checking The Geometry Of A Room Model

Creating a suitable geometry for

room acoustics calculations can

be a lengthy process, which

doesn’t necessarily end when a

nice looking model has been

created. One of the common

problems is that geometries

should be watertight. Another

problem, which may be less

obvious, is that geometries should

be consistent. ODEON has

several tools for verification of

geometries. To help finding leaks

in geometries, ODEON can

highlight free edges, display a

rendered surface geometry of the

room and indeed the ray-tracing

process can be visualized in a 3D

display in order to reveal leaks in the room or inappropriate source positions. To reveal surfaces,

which are by accident duplicated, surfaces that are partly overlapping other surfaces or warped

surfaces, ODEON provides a built in utility.

5.00 10.00 15.00

20.00 metres

Figure 2 The 3DInvestigate Ray tracing display is one of

many tools that help the user to verify room geometries.

Following the interactive ray-tracing on screen makes it

easy so spot leaks in the geometry or inappropriate source

positions.

3. CALCULATIONS AND ALGORITHMS

ODEON Combined allows simulation of point, line and surface sources. Point sources are intended

for simulation of musical instruments, speakers, loudspeakers, small noise sources in industrial

environments etc. Line and surface sources are intended for the simulation of large vibrating noise

sources such as machinery in industrial environments. However, extended sources can also be

useful for simulation of sound transmitted into the room through windows, ventilation noise etc. Any

of the three source types can be used in all types of calculations available in ODEON.

3.1 Quick Estimate, Statistical formulas

Quick estimated reverberation times (classic)

For initial calculations e.g. while selecting appropriate

surface materials, the reverberation time can be

estimated using the Quick Estimate method, which

provides prompt estimates of the reverberation times

using statistical formulas. In order to estimate the

volume, ODEON runs a small ray tracing calculation,

from which the mean free path and thus the volume is

obtained. Finally ODEON calculates the reverberation

times using the Sabine, Eyring and Arau-Pachades

formulas. The user may as an option provide the

volume manually, if not satisfied with the estimate

derived from the mean free path.

1,8

1,6

1,4

1,2

0,8

0,6

0,4

0,2

T Sabine

T Ey ring

T Arau-Puchades

63 125

250

500 1000 2000 4000 8000

Frequency (Hz)

Figure 3 Quick Estimate provides

fast estimates of reverberation time

using different statistical formulas.

0.00

ODEON - a design tool for auditorium acoustics, noise control and loudspeaker systems – C. L.

Christensen

3.2 Global Estimate

To get a global estimate of

reverberation time in a room, ODEON

also offers another method, which is

ray-tracing based. This method takes

into account the shape of the room,

location of the absorbing materials,

scattering properties of the materials

etc. To calculate the decay, ODEON

emits rays from a source, and then

calculates how the rays decay (on

average) due to absorption at surfaces

and due to air absorption. Unlike the

statistical methods this method also has

the advantage that it does not depend

on an estimated volume, which may or may not be correct. For rooms such as concert halls and

auditoria that are not dominated by strong decoupling, the reverberation time is a global measure

and in this case the global decay method provides a good estimate on the decay time in the room.

Estimated global reverberation times (Source 1, 8934 rays used)

Estimated room volume:5626.58 m³

T30,63=1.78 s

T30,125=1.58 s

T30,250=2.14 s

T30,500=2.14 s

T30,1000=2.16 s

T30,2000=2.14 s

T30,4000=1.69 s

T30,8000=0.95 s

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-30

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-50

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0,4

0,6

0,8

1,2

1,4

1,6

1,8

Time (seconds)

Figure 2 Global decay curves estimated using

the Global Estimate method takes into account

none diffuse conditions.

3.3 Point Responses

3.3.1 Point Responses From Line And Surface Sources

To calculate the point response from a line or surface

source, ODEON applies a special ray tracing method [7].

Taking the surface source as the example this is how

the calculation method works; ODEON distributes a

number of secondary sources having a Lambert

directivity over (one of the sides) of a selected surface in

the room geometry, then a ray is radiated from each of

these secondary sources and reflected at the surfaces of

the room. The orientation of the reflected rays are

calculated as a weighted direction between a random

chosen direction (the random angles being distributed

according to the Lambert distribution [4]) and the

specular reflection direction, using the scattering

coefficient of the reflecting surface as the weighing

factor. Using the approach described each ray will

generate a number of secondary sources corresponding

to the number of times the ray was reflected plus one.

The last part of the calculation is related to a specific

receiver, at this point it is determined which of the

secondary sources are visible at the receiver point and the contribution of the visible secondary

sources are summed to the response of that receiver.

Surface

Source

Figure 3 Illustration of one out of

many rays radiated from a surface

source and the first reflections of that

ray. At each reflection point including

the start point, a secondary source is

generated.

3.3.2 Point Responses From A Point Source

To calculate the point response from a point source, ODEON can either use a hybrid calculation

method (combining the image source method with special ray-tracing methods), which has been

proven to work well in rooms such as auditoria [5] that are not dominated by curved surfaces, or a

ODEON - a design tool for auditorium acoustics, noise control and loudspeaker systems – C. L.

Christensen

ray tracing method which yields better results in rooms such as churches or mosques [9], that are

dominated by curved surfaces. No matter which method is being used, a point source is described

by position, orientation, delay equalisation and a directivity pattern, allowing modelling of natural

sound sources, noise sources as well as loudspeaker systems with multiple active sources.

The ray-tracing method applied for point source calculations are similar to the method used for

surface and line sources except that the direct sound are emitted from one point (source) instead of

a number of secondary source points.

It is possible to select whether Odeon should use the hybrid calculation method or a pure ray-

tracing method, by adjusting a transition order, which determines the reflection order, at which the

calculation method changes from a hybrid calculation method that includes generation of image

sources to the pure ray-tracing method. The hybrid calculation method applied in ODEON is

described in by Rindel & Naylor [1,2] except that the current version of ODEON also includes

scattering for reflections below the transition order. In short the hybrid calculation method works as

follows, rays are emitted from the point source and for low order reflections, below the transition

order, rays are used indirectly in order to detect image sources while the program keeps track on

the image sources detected in order only to get one contribution from each image source. Above

the transition order the ray-tracing method, which is used for the line and surface sources, is used.

By nature the image source method does not include scattering so in order to include scattering in

the early reflections, the early reflection calculations is in fact a hybrid method on its own. In short;

Each time Odeon detects an image source, an inner loop of (scatter) rays are started, taking care of

the scattered sound which is reflected from this image source /surface.

Example: If all scattering coefficients in a room are 0.5, then the specular energy of a first order

image source is multiplied by (1-0.5) - and the specular energy of a second order IMS is multiplied

by (1-0.5)*(1-0.5). The scattering rays handle the rest of the energy. The early scatter rays are

handled in a way, which is indeed inspired by the way in which surface sources are simulated,

actually each time an image source is detected, ODEON will simulate a surface source, which will

emit a number of early scatter rays. The early scatter rays will be traced from the current reflection

order and up to the transition order. At each reflection point of the early scattering rays, including

the starting point, a secondary scattering source is created.

The last part of the point response calculation for a point source is, just as for the line and surface

sources, to examine which of the generated image and secondary sources are visible at the

receiver position. For the secondary sources generated by early scattering rays or by late rays, a

contribution is added to the point response, if the source is visible from the receiver. For the image

sources generated, a contribution is added to the point response if the entire reflection path from

the source to the receiver is unobstructed. The contributions added to the point response takes into

account:

•

Directivity factor of the primary source in the relevant direction of radiation

•

Reflection coefficients of the walls involved in generating the reflection (taking into account

the angle of incidence for the reflection)

•

Air absorption due to the reflection path of the reflection

•

Distance damping

•

Diffraction damping due to limited size of the surfaces generating the reflection

Three different point responses are available in ODEON 5.0 Combined; Single Point , Multi Point

and Grid response. All three point response calculations share the calculation methods just

described and offer a number of calculated room acoustic parameters in receiver point(s) for a

given source configuration.

ODEON - a design tool for auditorium acoustics, noise control and loudspeaker systems – C. L.

Christensen

3.3.3 The Single Point Response Results

The Single Point response offers detailed results and auralization for a selected receiver point.

Elevation

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-5

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Azimuth

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-45

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0,01

0,03

0,04

0,05

0,06

0,07

0,08

0,1

0,11

0,12

-2,5

-3

-3,5

time (seconds rel. direct sound)

63 250 2000

0,02

0,09

0,13

Frequency (Hz)

Figure 3 A few examples of the Single Point results. Early reflection paths displayed along with

its associated refectogram in a situation where a flutter echo is present. Individual reflections or

groups of refelctions can be examined in depth. ODEON is capable of predicting as well

auralizing echo problems even for high order reflections.

The Single Point Results are:

•

Room acoustics parameters: EDT, T 30 , SPL, C 80 , D 50 , Ts, LF 80 , STI, A-Weighted Late

Lateral SPL(A), SPL(A), ST early , ST late and ST total .

•

Decay curves.

•

Reflectogram showing the early reflections, the strength per octave band, time of arrival,

azimuth and elevation angle. The reflectogram is directly coupled to the 3DReflection path

display.

•

3D Reflection paths display allows tracking down early reflections, which are calculated

using the image source method in a 3D display of the room geometry.

•

A zoom able graph displaying the calculated BRIR (Binaural Room Impulse Response).

3.3.4 The Multi point response result

The Multi point response calculation calculates point responses for a number of discrete receivers.

Apart from offering room acoustic parameters for these receivers, the Multi point response also

provides a number of graphs and tools making it useful in particular for environmental acoustics:

•

A noise control display, where the influence of the different active sources can be assessed

simultaneously at the different receiver positions.

•

A graph showing the simulated spatial decay curves which are useful for evaluation of the

acoustics in workrooms [10].

•

Graph showing parameter versus distance for a selected parameter.

•

A graph showing a selected parameter for all receiver positions and frequency bands.

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