Page 93 - The Master Handbook Of Acoustics

68 CHAPTER THREE

closed pointed to the
Sound source source of sound with
great accuracy, confirm-
ing the fact that two ears
function together in bin-
aural localization.
Two factors are
involved, the difference
in intensity and the dif-
ference in time of arrival
(phase) of the sound
falling on the two ears.
FIGURE 3-17
In Fig. 3-17 the ear near-
Our binaural directional sense is dependent in part est the source receives a
on the difference in intensity and phase of the sound
falling on two ears. greater intensity than the
far ear because the hard
skull casts a “sound shadow.” Because of the difference of distance to
the source, the far ear receives sound somewhat later than the near ear.
Below 1 kHz the phase (time) effect dominates while above 1 kHz the
intensity effect dominates. There is one localization blind spot. A lis-
tener cannot tell whether sounds are coming from directly in front or
from directly behind because the intensity of sound arriving at each
ear is the same and in the same phase.
Another method of perception of direction comes into play in a rel-
atively small room. The sound reaches the person over a direct path
followed by many reflections from many different directions. The
sound that arrives first creates in the hearer the main perception of
direction. This has been called the law of the first wavefront.

Aural Harmonics: Experiment #1
This experiment, suggested by Craig Stark, 16 can be performed easily
with your home high-fidelity system and two audio oscillators. Plug one
oscillator into the left channel and the other into the right channel, and
adjust both channels for an equal and comfortable level at some midband
frequency. Set one oscillator to 24 kHz and the other to 23 kHz without
changing the level settings. With either oscillator alone, nothing is heard
because the signal is outside the range of the ear. (He notes here, how-
ever, that the dog might leave the room in disgust!) When both oscillators