30 SecTion | 1 Test and Measurement too close to the loudspeaker, the measured sound levels will be accurate for that position, but may not accurately extrapolate to greater distances with the inverse-square law. As the sound travels farther, the response at a remote listening position may bear little resemblance to the response at the near field microphone position. For this reason, it is usually desirable to place the microphone in the far free field of the loudspeaker--not too close and not too far away. The approximate extent of the near field can be determined by considering that the path length difference from the measure- ment position (assumed axial) and the edge of the sound radiator should be less than ¼ wavelength at the frequency of interest. This condition is easily met for a small loudspeaker that is radiat- ing low frequencies. Such devices closely approximate an ideal point source. As the frequency increases the condition becomes more difficult to satisfy, especially if the size of the radiator also increases. Large radiators (or groups of radiators) emitting high frequencies can extend the near field to very long distances. Line arrays make use of this principle to overcome the inverse-square law. In practice, small bookshelf loudspeakers can be accurately measured at a few meters. About 10 m is a common measurement distance for moderate-sized, full-range loudspeakers in a large space. Even greater distances are required for large devices radi- ating high frequencies. A general guideline is to not put the mic closer than three times the loudspeaker's longest dimension. 1.3.5.7 Estimate the Critical Distance D C Critical distance is easy to estimate. A quick method with adequate accuracy requires a sound level meter and noise source. Ideally, the noise source should be band limited, as critical distance is frequency-dependent. The 2 kHz octave band is a good place to start when measuring critical distance. Proceed as follows: 1. Energize the room with pink noise in the desired octave band from the sound source being measured. The level should be at least 25 dB higher than the background noise in the same octave band.