Handbook for Sound Engineers

Acoustical Modeling and Auralization 215

9.1 Introduction

As it is often the case with branches of engineering
dealing with the understanding and the prediction of
complex physical phenomena, modeling has rapidly
become an integral part of the acoustical design process.
When dealing with indoor sound propagation the use of
a suitable model may allow the designer to assess the
consequences that a change in parameters such as room
shape, material selection, or source placement will have
on variables like sound pressure, reverberation time, or
reflection ratios at specific points inside the room.
Acoustical models can also be developed for outdoor
sound studies as one may inquire what shape and height
a highway barrier needs to be in order to attenuate a
specific level of unwanted noise from highway traffic
over a given area. In these instances, the model is
expected to provide the designer with the answer to the
fundamental “what if?” question that is at the genesis
of an engineered design—i.e., one that leaves little to
chance in terms of the sensible selection of its parame-
ters in order to achieve a specific result. The answers to
the question allow the designer to assess the perfor-
mance or the cost-effectiveness of a design, based on a
specific set of criteria prior to committing to it.
Although an experienced designer may be able to
achieve a substantial understanding of the acoustics of a
given environment simply by looking at the data that
results from the modeling phase of the design, the
acoustical model may also be used to provide an audi-
tory picture of the data so that a qualitative evaluation
of the acoustics can be performed by trained and/or
untrained listeners. This phase of the design—known as
auralization—aims at doing for one’s ears what a
picture does for one’s eyes: present a description of an
environment in a way that is best suited to the most
appropriate sensor. In this instance, the basic goal is to
use sound to demonstrate what a specific environment
will sound like, just like the fundamental purpose of an
image is to illustrate what an environment may look
like. The challenges associated with virtual representa-
tion that exist in the visual world, such as accuracy,
context, and perception, are also present in the aural
environment, and the old engineering school adage that
“a picture is worth a thousand words, but only if it is a
good picture” rings also true in the world of acoustical
modeling and of auralization.
The aim of this chapter is to provide the reader with
a basic understanding of the various methodologies that
are used in acoustical modeling and auralization, and
the emphasis is on models that can be used for the eval-
uation of room acoustics. The reader is referred to the
bibliography for further in-depth reading on the topic.

9.2 Acoustical Modeling

This section will review various acoustical modeling

techniques from the perspective of theory, implementa-

tion, and usage.* The categorization and grouping of the

modeling techniques into three general families (physi-

cal models, computational models, and empirical mod-

els) and in further subgroups as presented in Fig. 9-1 is

provided as a means to identify the specific issues asso-

ciated with each technique in a fashion that is deemed

effective by the author from the standpoint of clarity. A

brief mention of hybrid models that combine various

techniques will also be introduced. The sections of this

chapter are written as independently from each other as

possible so the reader can go to topics of specific inter-

est without reading the preceding sections.

9.2.1 Physical Models

This class of model uses a scaling approach to yield a

3D representation of typically large acoustical environ-

ments like theater or auditoria for the purpose of evalua-

tion and testing. The models are constructed at

geometrical scales ranging from 1:8 to 1:40, and an

example of a 1:20 physical model is presented in Figs.

9-2 and 9-3. Physical modeling techniques became pop-

* Model (mäd’l) n. [Fr. modèle < It modello] 1. a) a

small copy or imitation of an existing object

made to scale b) a preliminary representation of

something, serving as the plan from which the

final, usually larger, object is to be constructed c)

archetype d) a hypothetical or stylized representa-

tion e) a generalized, hypothetical description,

often based on an analogy, used in analyzing or

explaining something.

From Webster New World College Dictionary. 4th

Edition (2000)

Figure 9-1. General classification of acoustical modeling

methodologies.

Acoustical Modeling

Methodologies

Physical

models

Computational

models

Empirical

models

Hybrid

models

Geometrical

approach

Wave-equation

approach

Statistical

approach

Image

method

Ray

tracing

Beam

tracing

Analytical

description

Numerical

description