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 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 high- frequency 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 (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.

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 (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.

Figure (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. Virtual Lab 12/6

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 (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 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

Figure “Three Blind Mice”: (a) jumping octave version; (b) normal version. Virtual Lab 12/11

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 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 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

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