Download Hearing and the environment

SOUND IN THE
WORLD AROUND US
OVERVIEW OF QUESTIONS
What makes it possible to tell where a sound
is coming from in space?
When we are listening to a number of
musical instruments playing at the same
time, how can we perceptually separate
the sounds coming from the different
instruments?
How do we find that phone lost in our room?
The three directions
used for studying
sound localization:
azimuth (left-right),
Distance and
elevation (up-down)
FINDING THE PHONE : AUDITORY LOCALIZATION
Auditory space - surrounds an observer and
exists wherever there is sound
Researchers study how sounds are
localized in space by using:
Azimuth coordinates - position left to right
Elevation coordinates - position up and down
Distance coordinates - position from observer
AUDITORY LOCALIZATION - CONTINUED
On average, people can localize sounds
Directly in front of them most accurately
To the sides and behind their heads least
accurately.
Location cues are not contained in the receptor
cells like on the retina in vision; thus, location
for sounds must be calculated.
Comparing location information for vision and
hearing.
Vision: The bird and the cat are at different locations
and are imaged on different places on the retina.
Hearing: The frequencies in the sounds from the bird
and the cat are spread out over the cochlea, with no
regard to the locations of the bird and the cat.
CUES FOR SOUND LOCATION
Binaural cues - location cues based on the
comparison of the signals received by the
left and right ears
Interaural time difference (ITD)- difference
between the times sounds reach the two ears
When distance to each ear is the same, there
are no differences in time.
When the source is to the side of the
observer, the times will differ.
The principle behind interaural time difference (ITD).
The tone directly in front of the listener, at A, reaches
the left and the right ears at the same time.
However, when the tone is off to the side, at B, it
reaches the listener’s right before it reaches the left
ear.
BINAURAL CUES
Interaural level difference (ILD)- difference in
sound pressure level reaching the two ears
Reduction in intensity occurs for high
frequency sounds for the far ear.
The head casts an acoustic shadow.
This effect doesn’t occur for low frequency
(pitch) sounds.
Figure 12.5. Why interaural level difference (ILD)
occurs for high frequencies (high pitch) but not for
low frequencies. When water ripples are small
compared to an object, such as this boat, they are
stopped by the object. The spaces between highfrequency sound waves is small compared to the
head. The head interferes with the sound waves,
creating an acoustic shadow on the other side of the
head.
Figure 12.5. (b) The same ripples are large compared
to the single cattail, so they are unaffected by it. (d)
The spacing between low-frequency (low pitch) sound
waves is large compared to the person’s head, so the
sound is unaffected by the head.
IDENTIFYING SOUND SOURCES
You see three musicians.
Locate each because
image falls on different
part of retina.
You hear three musicians.
Sounds hitting your ear at same
time. How do you identify which
musician is producing which
sound?
AUDITORY SCENE ANALYSIS
Auditory Scene - the array of all sound
sources in the environment
Auditory Scene Analysis - process by which
sound sources in the auditory scene are
separated into individual perceptions.
This does not happen at the cochlea since
simultaneous sounds are together in the
pattern of vibration of the basilar
membrane.
PRINCIPLES OF AUDITORY GROUPING 1
Heuristics that help to perceptually organize
stimuli.
Ex. Marching band vs fire truck
Onset time - sounds that start at different times are
likely to come from different sources.
Location - a single sound source tends to come
from one location and to move continuously.
Similarity of timbre and pitch - similar sounds are
grouped together.
AUDITORY SCENE ANALYSIS
MARCHING BAND
FIRE TRUCK
AUDITORY STREAM SEGREGATION
Compound melodic line in music is an
example of auditory stream segregation.
Experiment by Bregman and Campbell
 Stimuli were alternating high and low tones
 When stimuli played slowly, the perception is hearing
high and low tones alternating.
 When the stimuli are played quickly, the listener hears
two streams; one high and one low.
 Gestalt principles in action.
http://www.youtube.com/watch?v=0vRfRuNZdHs
Four measures of a composition by J. S. Bach
(Chorale Prelude on Jesus Christus unser Heiland,
1739). When played rapidly, the upper notes become
perceptually grouped and the lower notes become
perceptually grouped, a phenomenon called auditory
stream segregation.
Figure 12.15 (a) When high and low tones are
alternated slowly, auditory stream segregation does
not occur, so the listener perceives alternating high
and low tones.
(b) Faster alternation results in segregation into high
and low streams.
Virtual Lab
12/6
Figure 12.17 (a) Two sequences of stimuli: a series of similar
notes (red), and a scale (blue).
(b) Perception of these stimuli: Separate streams are perceived
when they are far apart in frequency, but when the frequencies
are in the same range, the tones appear to jump back and forth
between stimuli.
AUDITORY STREAM SEGREGATION
Experiment by Deutsch - the scale illusion
or melodic channeling
Stimuli were two sequences alternating
between the right and left ears.
Listeners perceive two smooth sequences by
grouping the sounds by similarity in pitch.
This demonstrates the perceptual heuristic that
sounds with the same frequency come from
the same source, which is usually true in the
environment.
Figure 12.18 (a) These
stimuli were presented to a
listener’s left ear (blue) and
right ear (red) in Deutsch’s
(1975) scale illusion
experiment. Notice how the
notes presented to each ear
jump up and down.
(b) What the listener hears.
Although the notes in each
ear jump up and down, the
listener perceived a smooth
sequence of notes. This
effect is called the scale
illusion, or melodic
channeling. (Adapted from
Deutch, 1975).
PRINCIPLES OF AUDITORY GROUPING 2
Proximity in time - sounds that occur in
rapid succession usually come from the
same source.
This principle was illustrated in auditory
streaming.
Auditory continuity - sounds that stay
constant or change smoothly are usually
from the same source.
GOOD CONTINUATION
Experiment by Warren et al.
 Tones were presented interrupted by gaps of silence
or by noise.
 In the silence condition, listeners perceived that the
sound stopped during the gaps.
 In the noise condition, the perception was that the
sound continued behind the noise.
Virtual Lab 12/9
Figure 12.19 A demonstration of auditory continuity, using tones.
PRINCIPLES OF AUDITORY GROUPING 3
Effect of past experience
 Experiment by Dowling
Melody “Three Blind Mice” is played with notes
alternating between octaves
Listeners find it difficult to identify the song
But after they hear the normal melody, they can
then hear it in the modified version using melody
schema
Virtual Lab 12/11
Figure 12.20 “Three Blind Mice”:
(a) jumping octave version; (b) normal version.
ACOUSTICS: HEARING INSIDE ROOMS
Direct sound - sound that reaches the
listener’s ears straight from the source
Indirect sound - sound that is reflected off of
environmental surfaces and then to the
listener
When a listener is outside, most sound is
direct; however inside a building, there is
direct and indirect sound.
(a) When you hear a sound outside, you hear mainly direct
sound (path a).
(b) When you hear a sound inside a room, you hear both direct
(a) and indirect sound (b, c, and d) that is reflected from the
walls, floor, and ceiling of the room.
ARCHITECTURAL ACOUSTICS
The study of how sounds are reflected in rooms.
Factors that affect perception in concert halls.
Reverberation time - the time is takes sound to
decrease by 1/1000th of its original pressure
If it is too long, sounds are “muddled.”
If it is too short, sounds are “dead.”
Ideal times are around two seconds.
FACTORS THAT AFFECT PERCEPTION IN
CONCERT HALLS
 Intimacy time - time between when sound leaves its
source and when the first reflection arrives.
Best time is around 20 ms.
 Bass ratio - ratio of low to middle frequencies reflected
from surfaces.
High bass ratios are best.
 Spaciousness factor - fraction of all the sound received
by listener that is indirect.
High spaciousness factors are best.
ACOUSTICS IN CLASSROOMS
Ideal reverberation time in classrooms is
.4 to .6 second for small classrooms.
1.0 to 1.5 seconds for auditoriums.
These maximize ability to hear voices.
Most classrooms have times of one second or
more.
Background noise is also problematic.
INTERACTIONS BETWEEN
VISION AND SOUND
Visual capture or the ventriloquist effect - an
observer perceives the sound as coming
from the visual location rather than the
source for the sound
Experiment by Sekuler et al.
Balls moving without sound appeared to move
past each other.
Balls with an added “click” appeared to collide.
Figure 12.24 Two conditions in the Sekuler et al.
(1999) experiment showing successive positions of
two balls that were presented so they appeared to be
moving. (a) No sound condition: the two balls were
perceived to pass each other and continue moving in
a straight-line motion.
Figure 12.24 Two conditions in the Sekuler et al.
(1999) experiment showing successive positions of
two balls that were presented so they appeared to be
moving. (b) Click added condition: Observers were
likely to see the balls as colliding.
Virtual Lab 12/15
WOULD YOU LIKE TO BE A DUMMY?
http://www.youtube.com/watch?v=htU6qYsLsEE