Unlike a sound meter, which provides a repeatable, unbiased analysis of sound pressure, the sensitivity of our ears varies by frequency. Our ears are also attached to a highly arbitrary evaluation device, a.k.a. the brain. Table 2 identifies a number of factors that contribute to this subjectivity. It's this "wild card" that motivates - and frustrates! - efforts to devise a method for quantifying and specifying acoustical comfort.

As a selective sensory organ, the human ear is more sensitive to high frequencies than low ones. Its sensitivity at a particular frequency also changes with loudness. Figure 1 illustrates these traits using contours; each contour represents a specific loudness level across the frequency range shown. Notice that the contours for "quiet" (<90 dB) sounds slant downward as the frequency increases, indicating that our ears are less sensitive to low-frequency sounds. The contours flatten as the decibel level increases, indicating a more uniform response to "loud" (> 90 dB) sounds across the range of frequencies.

Tones, sounds that occur over a narrow frequency range, evoke a particularly strong response. Chalk squeaking on a blackboard, for example, produces a tone that is extremely irritating.

Continue on to Octave Bands and Decibels.