Digital Signal Processing Reference
In-Depth Information
Level
Threshold of pain
120 phon
120
dB
100
80 phon
80
60
40 phon
40
20
Threshold of audibility
f
63
31 ,5
125
250
500
1000
2000
4000
8000
16000 Hz
Illustration 246:
Auditory sensation area with audibility threshold
The so-called auditory sensation area is indicated by hatching. Note the logarithmic scaling on both axes.
An increase of 20 decibels implies a tenfold increase in the signal level. In acoustics it is both useful and
appropriate to use a logarithmic measure for the frequency axis: here the frequency doubles from marking
to marking.
What is important in connection with encoding/compression is the frequency dependency of the sensitivity
of our hearing organs. It is greatest around 4 kHz.
Conclusion: weak tones in the immediate vicinity of loud tones do
not need to be transmitted as they cannot be heard anyway.
Illustration 248 looks at masking from a different angle. The more inaccurate the quanti-
zation in the A/D conversion the more unpleasantly loud the quantisation noise. In the
immediate vicinity of loud tones “coarser” quantisation - i.e. with fewer bits - is possible
than outside masking areas. Thus, considerably more quantisation noise could occur
within masking areas than outside.
