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Human Perception of Sound <br />Sounds qualitative aspects that can be described with adjectives, and quantitative aspects that can be described <br />with measurements. Sound can be perceived as pleasant or annoying, and as loudness, in terms of decibels. <br />Changes in loudness are described on a logarithmic scale because the human ear can hear such a wide variety <br />of sound levels. The human ear can usually tell the difference when sound changes by 3 dBA, and a 5 dBA <br />change is clearly noticeable. Because of the logarithmic scale, an increase of 10 dBA sounds twice as loud. <br />Change in decibel level and perceived change in loudness <br />+ 1 dBA Not Noticeable <br />+ 3 dBA Threshold of Perception <br />+ 5 dBA Noticeable Change <br />+ 10 dBA Twice (Half) As Loud <br />+ 20 dBA Four Times (One Fourth) As Loud <br />Figure 7 <br />Compared to the example of addition or subtraction of sources, doubling sources yielded an increase of 3 dBA, <br />which is a change that is just perceptible. <br />Weighting networks <br />Sound level meters used for monitoring can pick up sounds as a perfect computer, but the human ear is not so <br />precise. The human ear cannot hear lower frequencies as well as higher frequencies. <br />Weighting networks are used in noise monitors to attenuate specific frequencies in the audio spectrum to <br />attempt to duplicate the response of the human ear. The graph in Figure 8 represents the compensation of a C- <br />weighting network, A- weighting network and the sensitivity of the ear. This illustration is useful in <br />understanding how the ear is inefficient in the detection of lower frequencies and is very sensitive to higher <br />frequencies. <br />The C- weighting network represents the actual sound pressure level that is received by the sound level meter, <br />and does not noticeably vary in its amount of compensation throughout the audio spectrum. C- weighting is <br />used during the calibration of sound level meters to ensure that the sound level displayed on the meter is <br />invariant of the frequency of the calibrator. <br />The A- weighting network is used to duplicate the sensitivity of the human ear. At 100 Hertz, the A- weighting <br />network filters out approximately 20 dB from the incoming signal before it is combined with the levels from <br />the other frequency ranges to produce an A- weighted sound level. <br />10 <br />- 10 <br />Relative - 20 <br />Response <br />(dB) - 30 <br />- 40 <br />- 50 <br />A Guide to Noise Control in <br />Minnesota • October 2008 <br />G- Weighting or Linear <br />Weighting networks <br />ghting <br />Figure 8 <br />7 <br />Ear <br />20 50 100 200 500 1000 2000 5000 10,000 20,000 <br />Frequency (Hz) <br />Minnesota Pollution Control Agency <br />