Book - School of Science and Technology

Sound control 4613.02.52.0Exampleto add 80db and 86db86–80 = 6dbFrom chart add 1dbto higher value86+1 = 87db1.5Add to higher value1.00.90.80.70.60.50.40 1 2 3 4 5 6 7 8 9 10Difference between individual valuesFigure 16.22 Addition of decibel ratings. Sound pressure level (with reference to a datum of 2 10 5 Pa which corresponds to thereference intensity 10 12 watt)ˆ 20 log 10 P/(2 10 5 )dBwhere P ˆ actual sound pressure level.Decibels are not normally added arithmetically, but if two sounds of known decibelrating are taken separately, the decibel rating of the two taken together will be:ˆ 10 log 10 [antilog(A/10) ‡ antilog(B/10)]Use of a graph, Figure 16.22, simplifies the task of addition of two dB ratings.The phonThis unit is based on the subjective reaction of the ear to loudness and it may be related tosound pressure level in dB by means of data which have been derived by experiments onindividuals. Some values on this scale are:Threshold of hearing ˆ 0 phonWhispering ˆ 20 phonAverage room ˆ 40 phonBusy street ˆ 50 70 phonPneumatic drill ˆ 100 phon