Audio Engineering

Acoustic Environment

Don Davis and Eugene Patronis

3.1 The Acoustic Environment ......................................................................................

We are concerned about the effect the acoustic environment has on sound. We need to
know the effect of a particular acoustic environment on the unaided talker or musician, on
the sound system, if installed, and on unwanted sounds (noise) that may be present in the
same environment.

An outdoor environment can often be a “ free fi eld. ” “ A sound fi eld is said to be a free
fi eld if it is uniform, free from boundaries, and is undisturbed by other sources of sound.
In practice, it is a fi eld where the effects of the boundaries are negligible over the region
of interest. ” (From the GenRad instruction manual for their precision microphones.)

“ Free from boundaries ” is the catch phrase here. Anyone who has designed a sound
system into a football stadium, a replica of a Greek theater, or a major motor racing
course knows fi rst-hand the primary infl uence of a boundary.

We must also consider:

1. Inverse-square-law level change.


2. Excess attenuation by frequency because of humidity and related factors.

Other factors that can materially affect sound outdoors include:

3. R e fl ection by and diffraction around solid-objects.

4. Refraction and shadow formation by wind and temperature and wind
   variations.


5. R e fl ection and absorption by the ground surface itself.

CHAPTER 3