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Auditory scene analysis 2
• This lecture:
• Gestalt organising principles
• Sequential grouping
Gestalt Principles of Organisation
• Gestalt psychology founded in the early 20th century• A group of psychologists: Max Wertheimer (1880-
1943), Wolfgang Köhler (1887-1967) and Kurt Koffka (1886-1941) formed the Gestalt school
• Gestalt theory – the perceptual whole is more than the sum of its parts
• Put forward a set of Gestalt grouping rules that describe which elements in an image belong together to form an object – mostly described in relation to vision
Gestalt Principles of Organisation
• These principles can also apply to hearing / auditory perception
• Application of these principles generally results in a grouping of the parts of the input sound that come from the same source - segregating those that don’t
• Discuss each of the principles separately• Important: These principles work together – to
arrive at a correct interpretation of the input sound – no single rule will always work
Gestalt Principles of Organisation
• Similarity
• Good Continuation
• Common Fate
• Closure
• The Figure-Ground phenomenon and Attention
Similarity
• Sounds are grouped into a single perceptual stream if they are similar in pitch, timbre, loudness or subjective location
• Demonstration 17: Failure of crossing trajectories to cross perceptually
• Falling and rising sequence are interleaved – tones from the rising and falling sequence are alternated in time.
• How easy is it to hear out each of the four standards• Grouping by timbre and frequency region
Good Continuation
• changes in frequency, intensity, location or spectrum within a single sound source tend to be smooth and continuous rather than abrupt
• Smooth change implies a change within a single source
• Abrupt change – new stream – new source• Asa demo 12 – Effects of connectedness on
segregation
Good Continuation
• In this example, the tendency of a sequence of high and low tones to split into two streams is reduced when successive tones are connected by frequency glides
• Hear H1 (2000 Hz), L1 (614 Hz), H2 (1600 Hz), L2 (400 Hz) tones
• Connecting the tones, through frequency glides, helps prevent the sequence from segregating into separate streams
• Continuity helps hold auditory sequences together• Streaming stronger in unconnected sequence
Common Fate
• Based on the fact that different frequency components arising from a single sound source usually vary in a highly coherent way.
• Tendency to start and finish together, change in intensity and frequency together
• Two or more frequency components in a complex sound are grouped together and perceived as part of the same source if they undergo the same kinds of changes at the same time.
Common fate
• For example, a group of frequency components in a complex sound that are frequency modulated at the same rate can be heard out as separate group from the other components
• Asa demo 24 Role of frequency micro-modulation in voice perception
• Frequency components in speech contain small fluctuations called micromodulation.
• These micromodulations are in all the harmonics of a vowel, and so they move in parallel
Common Fate
• Hear: pure tone, pure tone + harmonics (hear pure tone continuing on), micromodulation and vibrato is added to all harmonics causing them to fuse (pure tone not heard as a separate sound)
• Pure tone no longer heard as a separate sound – singing voice emerges
• This correlated change causes the harmonics to group into a coherent speech sound – example of common fate
Closure
• A sound may be temporarily masked by other sounds – a masked sound may be perceived as continuing behind the masker.
• Demonstrated through the continuity effect• Asa demo 28 – Apparent continuity – does the
tone appear to continue through the noise?• Asa demo 29 – Perceptual continuation of a
gliding tone through a noise burst
Perceptual organisation of sequences of sounds
• Sequential grouping (integration) – connecting over time – e.g. connecting of the notes of the same instrument together to create a melody – leads to the formation of auditory streams
• Stream segregation / fission – hear a rapid sequence of sounds – the sounds may be perceived as a single perceptual stream or they may split into a number of perceptual streams
• Streaming – to denote the processes determining whether one stream or multiple streams are heard.
Perceptual organisation of sequences of sounds
• Streaming can occur if the elements making up the sequence differ markedly in frequency, amplitude, location or spectrum.
• More difficult to judge the temporal order of a pair of elements when they are part of separate streams than when they are in the same stream.
• Asa demo 1 – Stream segregation in a cycle of six tones
• 3 high and 3 low tones – order: H1 (2500 Hz), L1 (350 Hz), H2 (2000 Hz), L2 (430 Hz), H3 (1600 Hz), L3 (550 Hz)
Stream segregation
• When the sequence of tones is played slowly we clearly hear the alternation of high and low tones – a single six note melody - easy to hear the temporal order of the tones
• When played fast we hear two streams, one high and one low – a pair of three note melodies - in this case it is more difficult to hear to order of the tones
• Stream segregation becomes stronger at faster tone rates – segregation affects the perceived melody
Stream segregation
• Asa demo 3 – Loss of rhythmic information as a result of stream segregation
• Triplets of tones separated by silences – HLH – HLH – HLH… - perception of a ‘galloping rhythm’
• Loss of ‘galloping rhythm’ when streaming occurs – each stream has its own separate melody and rhythm
• Illustrates the importance of speed and frequency separation of sounds in the formation of streams
Stream segregation
• For perceptual segregation of a sequence of tones – played at a fast rate and large separations between the frequencies of the high and low tones.
• No segregation at slow speeds • At high speeds there may be depending on
frequency separation• At high speeds – need large frequency separation
in order for the sequence to break into two separate streams
Stream segregation
• Asa demo 5 – Segregation of a melody from interfering tones – note when you can identify the melody
• Asa demo 10 – Stream segregation based on spectral peak position
• How timbre differences promote segregation
Stream segregation
• Two tones with the same fundamental but different positions of spectral peaks (i.e. where in the frequency range is the most energy) – difference in timbre
• Duller tone – spectral peak at 300 Hz• Brighter tone – spectral peak at 2000 Hz• Tones alternated in a galloping rhythm which
gradually speeds up• Hear separate streams of brighter and duller tones?
Figure-Ground phenomenon and Attention
• Generally don’t attend to every aspect of the auditory input – certain parts are selected for conscious analysis
• Complex sound analysed into streams – we attend to one stream at a time – attended stream stands out perceptually – rest of sound becomes less salient
• separation into attended and unattended streams – ‘figure-ground phenomenon’.
• Attend to one conversation at a time at a party – other conversations form a background
Figure-Ground phenomenon and Attention
• Possible to switch attention from one conversation / melody to another, and we may be aware of other sounds, but is seems that one stream at a time is selected for complete conscious analysis
• Importance of changes – the listeners’ attention is usually drawn to aspects of the sound that are changing – it becomes figure while the relatively unchanging part(s) become background