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 field.” “A sound field is said to be a free field if it is uniform, free from boundaries, and is undisturbed by other sources of sound. In practice, it is a field 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 first-hand the primary influence 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. Reflection by and diffraction around solid-objects.
4. Refraction and shadow formation by wind and temperature and wind variations.
5. Reflection and absorption by the ground surface itself.
Research in recent years has advanced the knowledge of atmospheric absorption significantly from the original base laid by Kneser, Knudsen, followed later by Harris, and, more recently, by the work of Sutherland, Piercy, Bass, and Evans (see Figure 3.1). This prediction graph is felt to be reliable within +5% for the temperature indicated (20°C) and 10% over a range of 0 to 40°C.
The June 1977 Journal of the Acoustical Society of America had an exceptional tutorial paper entitled “Review of Noise Propagation in the Atmosphere,” pages 1403–1418, and included a 96 reference bibliography.