Handbook for Sound Engineers

Acoustical Modeling and Auralization 235

ing of the physical parameters that define the space. A

diagram of the modeling and auralization process of this

system is presented in Fig. 9-27.

In this approach, known as dynamic auralization, the

presentation of the sound field can be done either via

binaural headphones or by multi-channel speaker tech-

niques and the auralization parameters must be updated

at a fast rate (typically more than ten times per second)

in order for the rendering to be of high quality. The

impulse response that is used for the convolutions can

be a combination of an accurate set of binaural

responses (that map head-tracking movements) to

account for the early portion of the reflections with a

simpler static impulse response that provides the foun-

dation for the calculation of the late part of the sound

field. This approach is moderately efficient in terms of

computational time and memory consumption and

recent developments^33 have been aimed at making use

of an efficient means to process the impulse response of

a space. Using an approach known as Ambisonics

B-format^34 the sound information is encoded into four

separate channels labeled W, X, Y and Z. The W

channel would be equivalent to the mono output from

an omnidirectional microphone while the X, Y and Z

channels are the directional components of the sound in

front-back (X), left-right (Y), and up-down (Z) direc-

tions. This allows a single B-format file to be stored for

each location to account for all head motions at this

specific location and to produce a realistic and fast

auralization as the user can move from one receiver

location to the other and experience a near-seamless

simulation even while turning his/her head in the virtual

model.

9.4 Conclusion

Acoustical modeling and auralization are topics of ongo-

ing research and development. Originally planned for

the evaluation of large rooms, the techniques have also

been used in small spaces^35 and in outdoor noise propa-

gation studies^36 and one can expect to witness the stan-

dard use of these representation tools in a wide range of

applications aimed at assessing complex acoustical

quantifiers. Even simple digital processing systems such

as those offered as plug-ins for audio workstations can

be used to illustrate the effect of frequency-dependent

transmission loss from various materials using simple

equalization and level settings corresponding to octave

or third-octave band reduction data.

Further work is needed in the representation and

modeling of complicated sources such as musical

instruments, automobiles, trains, and other forms of

transportation; work is also ongoing in the definition of

materials and surfaces so that the effect of vibrations

and stiffness is accounted for. Still, the models are

rapidly becoming both very accurate and very efficient

and they are demonstrating their adequacy at illustrating

the complicated issues that are associated with sound

propagation and, eventually, sound perception.

Figure 9-26. Speaker arrangement for multichannel presen-
tation. Adapted from Reference 29.

Figure 9-27. A real-time interactive modeling and aural-

ization system. Adapted from Reference 32.

Full range loudspeaker

one per channel

Anechoic

environment

Room Geometry

and Material Data

Real-time synthesis

Image-source

method

Difference

method Ray-tracing Measurements

Acoustical

attributes of the

room

Artificial late

reverberation

Direct sound

and early

reflections

Auralization

Nonreal-time analysis