It is not a mistake to believe that sound outdoors is simply lost in the open air. Sound leaves a source and travels outward, unimpeded. This free-field condition exists when sound is not reflected from a surface. Under this condition, sound from a point source radiates spherically outward and its sound-pressure level theoretically decreases by 6 dB with each doubling of distance from the source. For example, if the sound-pressure level of a source measures 80 dB at 10 ft, it will theoretically measure 74 dB at 20 ft. Note that this refers to sound pressure; sound intensity behaves differently. This exact theoretical result is rarely encountered in practice, but it is a handy rule for estimating sound changes with distance. Also, a point source is mainly a theoretically concept but easily approximated in practice; a source can be considered as a point if its largest dimension is small compared to the distance from it. For example, a source that is 1 ft across will act as point when measured from 5 ft or further.
Sound from a continuous line of vehicle traffic behaves somewhat differently than a point source. We assume that the sound radiates outward cylindrically (not spherically); thus its sound- pressure level decreases only 3 dB for every doubling of distance from the source. (In a continuous line, the traffic point sources reinforce each other.) This may explain why the traffic sound of the highway near your home is so annoying.
We usually, and correctly, visualize free-field conditions in an open space, but a free-field condition can also exist in an anechoic (without echoes) chamber, a specially built room with sufficient absorption to effectively absorb all energy from the source. In practice, anechoic chambers cannot quite accomplish this at low frequencies.
Most of the acoustical properties of a room are a direct result of the effects caused by the surfaces of the room. Sound energy radiated from a source will travel outward in different directions. Some sound energy returns from the surfaces in reflected patterns. In some cases, sound is reflected in a more complex way known as diffusion. In either case, the returned energy comes together in a complicated way to form the sound field of the room. From a perceptual standpoint, the sound field comprises the intricate, detailed fluctuations of sound pressure at the ears of a listener. Such fluctuations test the limits of the human ear’s sensitivity to sound intensity, pitch, and timbre. In addition, some sound will be absorbed by the surfaces of a room. The more energy that is absorbed by the room surfaces, the lower the sound level in the room.
Direct Sound and Indirect Sound
As noted, sound behaves differently depending on whether you are outdoors or indoors. A closer
look at sound indoors shows that it exhibits both outdoor and indoor characteristics, depending on your distance from the sound source. A near-field condition exists very close to the source; sources cannot be modeled as point sources; sound decreases 12 dB for every doubling of distance. This near field should not be confused with near-field or close-field monitoring.
Slightly farther from the source (perhaps 5 ft away in a small room) the sound-pressure level decreases by about 6 dB in a free-field condition; this is direct sound. Farther than that, the sound- pressure level remains constant at any distance away; this is indirect sound. The sound-pressure level is constant because the reverberant field is constant everywhere in the room. There is a transition region between the direct and reverberant sound fields. The more absorptive the room, the lower the level of the indirect reverberant sound field. Thus, in absorptive rooms, the free-field condition extends somewhat farther from the source. In the limiting case, an anechoic chamber, the only sound is direct sound so the free-field condition extends throughout the room. The relationship between direct and indirect sound in a room is shown in Fig.