Free Field
Home Up Nature of Sound Waves Simple Harmonic Motion Characteristics of Sound Spectrum and Octave Band Strength of Sound Sound Pressure Level Sound Power Level Sound Intensity Level Free Field Manipulation with Decibels

 

9. Free Field

When a point source (or any source that radiates equally in all directions) radiates into free space, the intensity of the sound varies as where r is the distance from the source. This may be understood as a given amount of sound power being distributed over the surface of an expanding sphere with area 4p r2 (see Fig. 5). Thus the intensity is given by :

(15)

 

Figure 5 Spherical Sound Waves in a Free Field

 

DiffuseF01.jpg (357362 bytes)

 

where W is the power of the source. An environment in which there are no reflections is called a free field. In a free field, the sound intensity level decreases by 6 dB each time the distance from the source is doubled. The sound intensity level (or sound pressure level) at a distance of one meter from a source in a free field is 11 dB less than the sound power level of the source. This is easily shown as follows:

 

(16)

 

Similarly, it can be shown that at a distance of two meters, Li is 17 dB less than Lw.