Decibel A, B and C
Sound pressure is not equally sensed by human ear at different frequencies - compensated with dB(A), dB(B) or dB(C) filters
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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. Higher sound pressures are therefore acceptable at lower and higher frequencies than in the mid range.
The knowledge about human ear is important in acoustic design and sound measurement. To compensate, sound meters are normally fitted with filters adapting the measured sound response to the human sense of sound. 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 the dBA-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)
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