Audio compression - The MPEG - page 109

Figure 4.12: Contours of equal loudness showing that the frequency response of the ear is highly level dependent
(solid line, age 20; dashed line, age 60).
Usually, people's ears are at their most sensitive between about 2 kHz and 5 kHz, and although some people can
detect 20 kHz at high level, there is much evidence to suggest that most listeners cannot tell if the upper frequency
limit of sound is 20 kHz or 16 kHz. [ 4 ][ 5 ] For a long time it was thought that frequencies below about 40 Hz were
unimportant, but it is now clear that reproduction of frequencies down to 20 Hz improves reality and ambience. [ 6 ]
The generally accepted frequency range for high- quality audio is 20 Hz to 20 000 Hz, although for broadcasting an
upper limit of 15 000 Hz is often applied.
The most dramatic effect of the curves of Figure 4.12 is that the bass content of reproduced sound is
disproportionately reduced as the level is turned down. This would suggest that if a powerful yet high-quality
reproduction system is available the correct tonal balance when playing a good recording can be obtained simply
by setting the volume control to the correct level. This is indeed the case. A further consideration is that many
musical instruments and the human voice change timbre with level and there is only one level which sounds correct
for the timbre.
Oddly, there is as yet no standard linking the signal level in a transmission or recording system with the SPL at the
microphone, although with the advent of digital microphones this useful information could easily be sent as
metadata. Loudness is a subjective reaction and is almost impossible to measure. In addition to the level-
dependent frequency response problem, the listener uses the sound not for its own sake but to draw some
conclusion about the source. For example, most people hearing a distant motorcycle will describe it as being loud.
Clearly at the source, it is loud, but the listener has compensated for the distance. Paradoxically the same listener
may then use a motor mower without hearing protection.
The best that can be done is to make some compensation for the level- dependent response using weighting
curves . Ideally there should be many, but in practice the A, B and C weightings were chosen where the A curve is
based on the 40-phon response. The measured level after such a filter is in units of dBA. The A curve is almost
always used because it most nearly relates to the annoyance factor of distant noise sources. The use of A-
weighting at higher levels is highly questionable.
[ 4 ] Muraoka, T., Iwahara, M. and Yamada, Y., Examination of audio bandwidth requirements for optimum sound
signal transmission. J. Audio Eng. Soc. , 29 , 2-9 (1982)
[ 5 ] Muraoka, T., Yamada, Y. and Yamazaki, M., Sampling frequency considerations in digital audio. J. Audio Eng.
Soc. , 26 , 252-256 (1978)
[ 6 ] Fincham, L.R., The subjective importance of uniform group delay at low frequencies. Presented at the 74th Audio
Engineering Society Convention (New York, 1983), Preprint 2056(H-1)
4.7 Frequency discrimination
Figure 4.13 shows an uncoiled basilar membrane with the apex on the left so that the usual logarithmic frequency
scale can be applied. The envelope of displacement of the basilar membrane is shown for a single frequency at (a).
The vibration of the membrane in sympathy with a single frequency cannot be localized to an infinitely small area,
and nearby areas are forced to vibrate at the same frequency with an amplitude that decreases with distance. Note
that the envelope is asymmetrical because the membrane is tapering and because of frequency-dependent losses
in the propagation of vibrational energy down the cochlea. If the frequency is changed, as in (b), the position of