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Besides
the level, the right hand edge of a spectral notch, is shifted
toward the higher frequencies in the right ear, when the sound
source is moving clockwise. The working area of this cue is from 40
to 180 degrees. In the frontal section a double notch would work as
directional cue from -40 to +30 degrees. It is the same double notch
that detects the elevation in zero azimuth. In horizontal-plane
localization the concha plays the major role in the HRTF.
The
sensation of a background sound source can be achieve with
13.2...15.5 khz high-pass or 14.5 kHz band-pass filtered noise. In
the first case there is no elevation and in the latter one source is
elevated 30 degrees. The common denominator is towards the high
frequencies ascending spectral slope. In HRTF there is also, at 16
kHz, a deep notch that doesn't appear in the frontal section. Front
back discrimination appears to be based on a level difference at a
band in these frequencies. In the lower frequency band from
3.75...7.5 kHz, depending on the azimuth angle, there is a deep
notch with rear section sound sources but none with frontal sources.
The third mechanism to achieving rear sensation might be a boosted
frequency band between 7 and 12 kHz.
The
frequency bands affecting the sensation of front, back and over head
sound source with the stimulus of one third octave band noise are
presented in figure 6. Figure 7 shows the main pinna cues that
probably are active in detecting the elevation and azimuth of the
sound source. In these measurements a torso in an unechoic chamber
has been used. The microphone is mounted in the right ear of the
torso.
Perception
of distance
Human
hearing is not very accurate in detecting the distance of a sound
source. Loudness is maybe the most widely accepted cue for distance.
Loudness decreases inversely as the distance in unechoic conditions
and about 20 phon [MBGA69] is needed to perceive half the distance.
In reverberant condition the situation is a little different.The
decrease in loudness is not as large as in an unechoic condition and
the loudness difference needed to double the perception of distance
varies from 22 to 41 phon [SØNI93]. For accurate perception of
distance absolute loudness must be known. The distance of the source
is over estimated with low levels of sound but improves considerable
with an increase in the sound level. Variations between individuals
are large. In unechoic condition it is almost impossible to estimate
the distance without any other cue other than loudness.
The
shape of the frequency spectrum is another factor that may give a
cue for ascertaining the distance of a sound source. Air damping is
frequency dependent. The loss of high frequencies is greater than
the losses of low frequencies. Besides direct air damping, many
echoes in reverberant rooms cause absorption of reflections from
walls. The loss of loudness depends on the materials of the walls.
To estimate the distance of sound source requires knowledge about
the characteristics of room and the frequency spectrum of the
source.
The
ratio between direct sound, early reflections and reverberation also
offers a cue as to the distance of a sound source. In this case
known sound and room characteristics are required for good
perception. With a known source like the human voice the
reverberation ratio appears to be the most important [SØNI93]
factor in estimating the distance. Also binaural differences may
sometimes be a valuable cue. The significance of this factor is not
however very strong. Experiments have been also shown that the
distance of a source is over estimated when the source is at an
angle to the receiver as opposed to directly in front.
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