Page 69 - The Master Handbook Of Acoustics

44 CHAPTER THREE

Directional Cues: An Experiment
If the equipment is available, a simple psychoacoustical experiment can
illustrate how subjective directional impressions result from simple
changes in sounds falling on the ear. Listen with a headphone on one ear
to an octave bandwidth of random noise centered on 8 kHz arranged
with an adjustable notch filter. Adjusting the filter to 7.2 kHz will cause
the noise to seem to come from a source on the level of the observer. With
the notch adjusted to 8 kHz the sound seems to come from above. With
the notch at 6.3 kHz the sound seems to come from below. This experi-
1
ment demonstrates that the human hearing system extracts directional
information from the shape of the sound spectra at the eardrum.

The Ear Canal
The ear canal also increases the loudness of the sounds traversing it. In
Fig. 3-2 the ear canal, with an average diameter of about 0.7 cm and
length of about 3 cm, is idealized by straightening and giving it a uni-
form diameter throughout its length.
Eardrum Acoustically, this is a reasonable approxi-
mation. It is a pipe-like duct, closed at the
inner end by the eardrum.
Organ pipes were studied intensely by
early investigators when the science of
acoustics was in its infancy. The acoustical
similarity of this ear canal to an organ pipe

4 was not lost on early workers in the field.
The resonance effect of the ear canal
increases sound pressure at the eardrum at
certain frequencies. The maximum is near
the frequency at which the 3-cm pipe is
Pressure one-quarter wavelength—about 3,000 Hz.
Figure 3-3 shows the increase in sound
pressure at the eardrum over that at the
Distance opening of the ear canal. A primary peak is
FIGURE 3-2 noted around 3,000 Hz caused by the quar-
ter-wave pipe resonance effect. The pri-
The auditory canal, closed at one end by the eardrum,
acts as a quarter-wavelength “organ pipe.” Reso- mary pipe resonance amplifies the sound
nance provides acoustic amplification for the impor- pressure at the eardrum approximately
tant voice frequencies.
12 dB at the major resonance at about