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Decibels |
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| It is common to specify the noise level of sounds not by their actual
energy intensity, but by a unit of measurement called a decibel (dß),
named after Alexander Graham Bell.
Decibels are a subjective scale of measurement based upon the lower limit of human audibility, which is considered to be 0 dß (the actual energy intensity of a sound at this level is still measured as: 10¹² W/m²). Human auditory range extends up to approximately 120dß. Sound levels perceived correlate quite reasonably with our subjective sensations of loudness as represented in the comparison chart at the right: |
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| However, the response of the ear is actually not the same
at every frequency. Thresholds for hearing, and for feeling auditory
pain, are different for different frequencies. Human hearing is most
sensitive to very low dß sounds in the vicinity of 4000Hz.
The same dß level for sounds below 1100Hz and above 6000Hz remain
beyond detection by the human ear. The lowest frequency sounds we
can hear at any dß level are between 20 to 80Hz; the highest,
between 16-20MHz for an individual with perfect hearing. Audio
range
actually lies between these limits.
In general, 120dß for any audible frequency is the human threshold of pain. Further, dß noise levels dissipate very quickly over distance from the source of the sound. Inversely, the closer someone is to the source, the louder the noise. Intensity diminishes as a function of the surface area of a sphere at the radius from the noise source. Therefore, noise diminishes more quickly the farther away. Every 3dß lower actually represents a reduction of ½ the sound intensity. Also, perceived noise is relative to other noise in the environment. A sound mixed into the noise of a busy urban street is perceived very differently than the exact same sound heard in an otherwise quiet location late at night. In the same way, the shape and size of the location affects perceived noise. A confined location reflects and reverberates sounds, increasing the intensity of a noise. An open location allows maximum attenuation of the noise intensity, thereby best reducing its dß level at distance. The following is a chart of normal attenuation of dß noise levels over distance: |
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| dß | dß | dß | dß | dß | dß | dß | dß | distance (m) | area sphere m² ( a² ) | area ratio a¹:a² | |
| 92
86 80 |
91
85 79 |
90
84 78 |
89
83 77 |
87
81 75 |
86
80 74 |
85
79 73 |
80
74 68 |
1
2 4 |
12.57
50.27 201.06 |
0.0
0.1 0.3 |
.
. European std. |
| 75dß | 74dß | 73dß | 72dß | 70dß | 69dß | 68dß | 63dß | 7meters | 615.75 | base ( a¹ ) | Japanese std. |
| 72
66 62 58 52 46 38 32 28 27 18 12 |
71
65 61 57 51 45 37 31 27 26 17 11 |
70
64 60 56 50 44 36 30 26 25 16 10 |
69
63 59 55 49 43 35 29 25 24 15 9 |
67
61 57 53 47 41 33 27 23 22 13 7 |
66
60 56 52 46 40 32 26 22 21 12 6 |
65
59 55 51 45 39 31 25 21 20 11 5 |
60
54 50 46 40 34 26 20 16 15 6 0 |
10
20 30 50 100 200 500 1000 1609 1829 5000 10000 |
1256.64
5026.55 11309.73 31415.93 125663.71 502654.82 3141592.65 12566370.61 32546750.48 42028345.15 314159265.36 1256637061.44 |
2.0
8.2 18.4 51.0 204.1 816.3 5102.0 20408.2 52856.9 68255.3 510204.1 2040816.3 |
.
. . . . . . kilometer std. mile nautical mile . . |
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