Decibel A, B and C
The sound pressure is not equally sensed by the human ear at different frequencies. This can be compensated with the dB(A), dB(B) or dB(C) filters
Sponsored Links
The human ear is more sensitive to sound in the frequency range 1 kHz to 4 kHz than to sound at very low or high frequencies. A higher sound pressure is therefore acceptable at lower and higher frequencies.
This knowledge is important in acoustic design and sound measurement. To compensate for the human ear, sound meters are normally fitted with filters adapting the measured sound response to the human sense. Common filters are
- dB(A)
- dB(B)
- dB(C)
dB(A)
The decibel A filter is widely used. dB(A) roughly corresponds to the inverse of the 40 dB (at 1 kHz) equal-loudness curve for the human ear.
Using this filter, the sound level meter is less sensitive to very high and very low frequencies. Measurements made with this scale are expressed as dB(A).
dB(B) and dB(C)
The decibel C filter is practically linear over several octaves and is suitable for subjective measurements at very high sound pressure levels. The decibel B filter is between C and A. The B and C filters are seldom used.
Comparing dB(A), dB(B) and dB(C)
The decibel filters A, B and C are compared below:
| Relative Response (dB) | Frequency (Hz) | ||||||||
| 31.5 | 63 | 125 | 250 | 500 | 1000 | 2000 | 4000 | 8000 | |
| dB(A) | -39.4 | -26.2 | -16.1 | -8.6 | -3.2 | 0 | 1.2 | 1 | -1.1 |
| dB(B) | -17 | -9 | -4 | -1 | 0 | 0 | 0 | -1 | -3 |
| dB(C) | -3 | -0.8 | -0.2 | 0 | 0 | 0 | -0.2 | -0.8 | -3 |
Example - Measuring dB(A)
If sound pressure is measured at different octaves the resulting dB(A) sound pressure can be calculated by logarithmic addition.
| Octave | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
| Measured Sound Pressure Level (dB) |
54 | 60 | 64 | 53 | 48 | 43 | 39 | 32 |
| db(A) filter (dB) |
26 | 16 | 9 | 4 | 0 | -1 | -1 | 1 |
| Resulting Sound Pressure Level (dB) |
28 | 44 | 55 | 49 | 48 | 44 | 40 | 31 |
- Logarithmic adding decibels in octave 4 and 5 gives approximately 51.5 dB.
- Logarithmic adding decibels in octave 3 together with the sum from 4 and 5 (1) gives approximately 56.5 dB.
- The resulting sound pressure level in octave 1, 2, 6, 7 and 8 is low compared with (2) and can be neglected.
- The resulting sound pressure level can therefore be estimated to approximately 57 dB(A)
Sponsored Links
Sponsored Links
SketchUp Engineering ToolBox - Online 3D modeling!
Free SketchUp Engineering ToolBox - enabled for use with the amazing, fun and free Google SketchUp.
Search the ToolBox
© The Engineering ToolBox 2005
8 6 27
