Sound and Hearing

The Relationship

Between Sound

and Hearing

Sound is created by mechanical vibrations that displace air molecules to create repetitive changes in air pressure. The ear detects these changes in air pressure, with the magnitude of the pressure perceived as loudness and the frequency of the changes perceived as pitch.

Due to the physiology of the ear, sound pressure does not correlate directly to the perceived loudness over all SPL and frequencies. The ear is most sensitive to frequencies between

3 to 5 kHz and much less sensitive to low frequencies. For a low frequency tone to be perceived as loud as a high frequency sound, it must have a much higher SPL. In addition, the ear’s sensitivity to the low frequencies also depends on the SPL. At high sound volumes, the loudness difference between the most sensitive frequencies and low frequencies is reduced.

The non-linear nature of the ear’s response to frequencies and loudness is well documented in the Fletcher-Munson equal loudness curves, updated in the Robinson and Dadson equal loudness curves that were adopted in the ISO (International Standards Organization) Recommendation R-226.

Note: The MAF Curve in Figure 2-1

represents the “Minimum

Audible Field” Curve.

Figure 2-1. Robinson and Dadson Equal Loudness Curves

The equal loudness curves are used when sound levels are measured with a sound level meter. If the meter has a “flat” response, then the displayed result shows a larger than perceived level when sounds with significant low frequencies are measured. For this reason, sound level meters have a correction or “weighting” filter built-in. This filter can more closely match the displayed reading with the ear’s response. The most widely used weighting curve (and the one required by NFPA 72) is the “A” weighted curve, which is approximately the inverse of the 40 phon equal loudness curve. Meters configured with the “A” weighted filter read out in units of dBA, short for “A” weighted decibels.

Other common weighting curves are the “B” and “C” curves, which approximate the ear’s response at higher decibel levels. From a practical standpoint these curves are useful for estimating the frequency content of the background noise during a room survey, but cannot be used to validate the audibility of an emergency voice/alarm communications signal.

Note: The ear is capable of perceiving a difference in the sound level only when the sound level has roughly doubled or halved. The dBA scale is a logarithmic scale, so a doubling of sound power represents a 3 dBA increase in the SPL of the sound. Therefore, most listeners can not perceive changes in SPL of less than 3 dBA. For a sound to be perceived as twice as loud, the power must be increased 10 fold.

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