| Sound perception and A-weighting |
| Humans can hear sound from about 20 Hz up to about 20000 Hz. In the diagram the total audible range and the typical ranges for music and speech are indicted. Sound with frequencies below 20 Hz is called infra sound, sound above 20000 Hz=20 kHz is called ultra sound. The human ear perceives sound with different frequencies differently. The following diagram shows curves (called Phon-curves) for sinus tones, which results in the same perception as a 1000 Hz sinus tone at the given level (=Phon value) (here the simplyfied Fletcher-Munson curves from 1933 are shown). To adjust the perception of noises with different frequency distribution to the human ear response, a frequency weighting with the A-filter can be performed. This is usually done in octave bands or 1/3 octave bands. A similar weighting with the C-filter is some times used to emphasize low frequent noise components. The measured dB-values (either in octave bands or 1/3 octave bands) are corrected with the filter values according to the table, before the sum (logarithmic addition) is calculated.  Example for comparison of total sound energy, with human perception.   |
