Auditory scene

1. Auditory scene

2. Music provides us with a complex, rapidly changing acoustic spectrum, often derived from the superposition of sounds from many different sources. • Our auditory system has the task of analyzing this spectrum as to reconstruct the originating sound events.

3. Two mechanisms • The first is a fast, involuntary process of auditory grouping • The second involves using schemas to guide the grouping and listening process. Schemas are higher-level hypotheses or expectations based upon our knowledge of familiar sound.

4. Grouping • Dimensions along which grouping principles operate: frequency, amplitude, temporal position, spatial location, timbre • Principles that govern grouping along these dimensions

5. Principles of auditory grouping • Similarity of timbre • Similarity of pitch • Temporal proximity • Good continuation • Location • Onset and offset • Experience

6. The Gestalt approach to perceptual organization: How elements are grouped: Goldstein, pp. 148 – 156 • Auditory scene analysis: Identifying sound sources: Goldstein, pp. 395 – 401 • CWE, 245 – 247, 411 – 417 • Shepard, R. Stream segregation and ambiguity in audition. In: Cook, Chapter 10 • Deutsch, D. Grouping mechanisms in music. In: D.Deutsch (Ed), The Psychology of Music, (1999), Chapter 9

7. Law of similarity Similar things appear to be grouped together.

8. Law of similarity

9. Law of similarity

10. Law of similarity

11. Similarity of timbre • Sounds that have the same timbre are often produced by the same source. • This principle means that we tend to group stimuli that sound similar together. • If pairs are alternately similar and dissimilar in timbre, they will group by similarity.

12. Similarity of timbre Auditory stream segregation

13. Example Grouping by timbral similarity: trumpet tones alternating with a vocal tone and a steel drum sound. Track 31

14. Similarity of pitch • Sounds with similar frequencies are often produced by the same source. • Imagine tones with different pitch. Those tones close in pitch will become grouped by similarity.

16. Example • High and low pitch played in succession; easy to follow as single line • High-low-high pattern repeated ever faster until there are two streams of pitch • Slow alternation between a minor second apart. Note that it is easy to think of the sound as a single line • Same as 3., but the interval is wider • Same interval as 3., but faster • Same interval as 4., but faster • Same as 3., even faster. Do you here one or two lines? • Same as 4., even faster. Do you here one or two lines? Track 32

17. Composers in the Baroque period (1600 – 1750) knew that if a single instrument plays notes that alternate rapidly between high and low tones, with the high notes perceived as being played by one instrument and the low notes as being played by another. This effect, which has been called implied polyphony or compound melodic line, is an example of auditory stream segregation.

18. Example of Gestalt grouping and stream segregation Track 37

19. Similarity of pitch J.S.Bach, Cello Suite I, Prelude

20. Law of proximity or nearness Things that are near to each other appear to be grouped together.

21. Law of proximity or nearness

22. Law of proximity or nearness

23. Left: Competition between grouping by proximity (columns) and grouping by similarity ( rows) Right: Demonstration of active grouping when multiple possibility are available

24. Temporal proximity • Sounds that occur in rapid progression tend to be produced by the same source. • Grouping a set of tones so that some are closer together than others are perceived as belonging together. • If steps are large and rapid (e.g., on a diatonic scale), melodies are not perceived as a stream but segregate into separate entities.

25. Example Yodeling: The singer alternates between the head and chest voice modes of production, and the leaps are large and rapid. The high note and low notes of yodeling are often perceived as two separate streams. Track 33

26. Interleaving two melodies • Another example that illustrates this phenomenon involves interleaving two melodies so that the notes of each melody alternate. • If the relative range of the melodies is such that the interleaved patterns contain small leaps, the individual melodies are not recognized. • As one of the melodies are transposed so that it is farther and farther from each other, a point it reached when both melodies are recognized. • The connection from one tone to the next is a function of proximity.

27. Overlapped melodies cannot be separated even though they alternate in time. Separating in pitch allows the two melodies to be easily identified. Different timbre or volume

28. Example • Melodies just seen • Same as in 1., increasing distance between melodies each time • Separation by volume • Separation by timbre • Two new melodies, increasing distance • Separation by volume • Separation by timbre Track 34

29. Law of good continuation Points that, when connected, result in straight or smoothly curving lines are seen as belonging together, and the lines tend to be seen in such a way as to follow the smoothest path.

30. Law of good continuation

31. Good continuation • Sounds that stay constant or that change smoothly are often produced by the same source. • Sound stimuli with the same frequency or smoothly changing frequencies are perceived as continuous even if they are interrupted by another stimulus.

32. Good continuation • Albinez • Perceived as two separate streams

34. Example • Sinusoid going up and down in pitch with pauses at the top • Gaps in previous example are filled with noise. Now the sine wave sounds continuous, but is just obscured by the noise burst. • Musical example with gaps • Musical example with gaps filled by noise. Track 5

35. http://www.ece.uvic.ca/~aupward/p/demos.htm • http://www.psych.mcgill.ca/labs/auditory/bregmancd.html