Page 60 - Master Handbook of Acoustics

CHAPTER 3

Sound in the Free Field

M any practical acoustical problems are invariably associated with structures such as rooms

and buildings, and vehicles such as automobiles and airplanes. These can generally be

classified as problems in physics. These acoustical problems can be very complex in a physical
sense; for example, a sound field might comprise thousands of reflected components, or temperature
gradients might bend sound in an unpredictable manner. In contrast to practical problems, sound can
be considered in a free field, where its behavior is very predictable. In a free field, analysis is
straightforward. This analysis is useful because it helps us understand the basic nature of sound
waves. Then, these basic characteristics can be adapted to more complex problems.

The Free Field

Simply put, a free field is an open space. Sound in a free field travels in straight lines and is
unimpeded. Unimpeded sound is not subject to the many influences that we will consider in later

chapters. Sound in a free field is unreflected, unabsorbed, undeflected, undiffracted, unrefracted,
undiffused, and not subjected to resonance effects. In most practical applications, these are all factors
that could (and do) affect sound leaving a source. An approximate free field can exist in anechoic
chambers, special rooms where all the interior surfaces are covered by sound absorbers. Also,
approximate free-field conditions can exist close to a sound source. But generally, a free field is a
theoretical invention, a free space that allows sound to travel without interference.

A free space must not be confused with cosmological space. Sound cannot travel in a vacuum; it
requires a medium such as air. Here, free space means any air space in which sound acts as though it
is in a theoretical free field. In this unique environment, we must consider how sound diverges from a
source, and how its intensity varies as a function of distance from the source.

Sound Divergence

Consider the point source of Fig. 3-1, radiating sound at a fixed power. The source can be considered
as a point because its largest dimension is small (perhaps one-fifth or less) compared to the distances
at which it is measured. For example, if the largest dimension of a source is 1 ft, it can be considered
as a point source when measured at 5 ft or farther. Looked at in another way, the farther we are from a

sound source, the more it behaves like a point source. In a free field, far from the influence of
reflecting objects, sound from a point source propagates spherically and uniformly in all directions.
In addition, as described in the following text, the intensity of sound decreases as the distance from
the source increases.