Laserfiche WebLink
<br />Human Perception of Sound <br /> <br />The threshold of perception of the human ear is approximately three decibels, and a five-decibel <br />change is considered to be clearly noticeable to the ear. This is primarily due to the logarithmic <br />measuring metric typically associated with decibels. As Figure 3.1 demonstrates, a IO-decibel <br />change would be perceived to be twice as loud. <br /> <br />Perceived Change in Decibel Levels <br /> <br />Perceived Change in Decibel level <br />Change in Sound level Perceived Change to the Human Ear <br />.:!: 1 dB ------------------------- Not Perceptible <br />.:!: 3 dB _____n__________________ Threshold of Perception <br />.:!: 5 dB ------------------------- Clearly Noticeable <br />.:!: 10 dB ------------------------ Twice (or Half) as Loud <br />.:!: 20 dB ------------------------ Fourfold (4x) Change <br /> <br />Figure 3.1 <br /> <br />Weighting Networks <br /> <br />Weighting networks are used in noise monitors to attenuate specific frequencies in the audio <br />spectrum to attempt to duplicate the response of the human ear. The graph in Figure 3.2 <br />represents the compensation of a C-weighting network, A-weighting network and the sensitivity <br />of the ear. This illustration is useful in understanding how the ear is inefficient in the detection <br />of lower frequencies and is very sensitivc to higher frequencies. <br /> <br />The C-weighting network is a linear network that does not noticeably vary in its amount of <br />compensation throughout the audio spectrum. For this reason the C-weighting network <br />represents the actual sound pressure level that is received by the sound level meter. C-weighting <br />is used during the calibration of sound level meters to insure that the sound level displayed on the <br />meter is invariant of the frequency of the calibrator. <br /> <br />The human ear can not detect or "hear" lower frequencies as well as higher frequencies. The A- <br />weighting network is used to duplicate the sensitivity of the human ear. At 100 Hertz, the A- <br />weighting network filters out approximately 20 dB from the incoming signal before it is <br />combined with the levels from the other frequency ranges to produce an A-weighted sound level. <br />On the A-weighting scale, at 1000 Hertz, nothing is subtracted. <br /> <br />9 <br /> <br />l_ <br />