16 Chapter 1
A further consequence of level-dependent hearing response is that recordings that are
mixed at an excessively high level will appear bass light when played back at a normal
level. Such recordings are more a product of self-indulgence than professionalism.
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.
The best that can be done is to make some compensation for the level-dependent response
usingweighting 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 fi lter 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.
1.8 Frequency Discrimination........................................................................................
Figure 1.10 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 Figure 1.10(a). The vibration of the membrane
in sympathy with a single frequency cannot be localized to an infi nitely 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 Figure 1.10(b) , the position of maximum
displacement will also change. As the basilar membrane is continuous, the position of
maximum displacement is infi nitely variable, allowing extremely good pitch discrimination
of about one-twelfth of a semitone, which is determined by the spacing of hair cells.
In the presence of a complex spectrum, the fi nite width of the vibration envelope means
that the ear fails to register energy in some bands when there is more energy in a nearby
band. Within those areas, other frequencies are mechanically excluded because their
amplitude is insuffi cient to dominate the local vibration of the membrane. Thus the Q
factor of the membrane is responsible for the degree of auditory masking, defi ned as
the decreased audibility of one sound in the presence of another. Masking is important
because audio compression relies heavily on it.