User Guide

Sound System Design Reference Manual
Chapter 5: Fundamentals of Room Acoustics
5-1
Introduction
Most sound reinforcement systems are located
indoors, and the acoustical properties of the
enclosed space have a profound effect on the
system’s requirements and its performance. Our
study begins with a discussion of sound absorption
and reflection, the growth and decay of sound fields
in a room, reverberation, direct and reverberant
sound fields, critical distance, and room constant.
If analyzed in detail, any enclosed space is
quite complex acoustically. We will make many
simplifications as we construct “statistical” models of
rooms, our aim being to keep our calculations to a
minimum, while maintaining accuracy on the order of
10%, or ±1 dB.
Absorption and Reflection of Sound
Sound tends to “bend around” non-porous,
small obstacles. However, large surfaces such as the
boundaries of rooms are typically partially flexible
and partially porous. As a result, when sound strikes
such a surface, some of its energy is reflected, some
is absorbed, and some is transmitted through the
boundary and again propagated as sound waves on
the other side. See Figure 5-1.
All three effects may vary with frequency and
with the angle of incidence. In typical situations, they
do not vary with sound intensity. Over the range of
sound pressures commonly encountered in audio
work, most construction materials have the same
characteristics of reflection, absorption and
transmission whether struck by very weak or very
strong sound waves.
Figure 5-1. Sound impinging on a large boundary surface