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Chapter 7

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PSYC 212
Kevin Francis Casey

Musical instruments produce complex tones which conform to a harmonic series. These produce a single pitch perception. 1. Musical pitch and timbre Pitch processor in auditory system examines spatial arrangement of wave patterns on basilar membrane. Ex: string has fundamental frequency, but also vibrates at series of higher harmonic frequencies, each contributing to complex tone. Each harmonic component produces travelling wave in basilar membrane, all have fixed distance relationship to each other - Fundamental frequency = at apex - Each successive harmonic = more and more towards base Brain registers spatial relationship: spatial pattern of peak wave activity on basilar membrane formscritical factor in producing single pitch sensation by a complex musical tone (how the brain does this is unknown) 2:1 frequency ratio = fundamental interval of musical pitch, called octave - Move one octave higher = double frequency Musical pitch spans 7 octaves (though human hearing spans 10 octaves) Tones represented by alphabetic notation system - Repetitive - Musical pitch within octave forms series of discrete transitions o Each octave divided into small intervals dependent on culture o Tone at each interval represented by letter (C,D,E,F,G,A,B) - Tone chroma: Relative position of tones within an octave (represented in circular manner) - Solimization: European tradition: Do-re-mi-fa-sol-la-ti (do = C) - Equally tempered scale: Each octave broken into 12 divisions plotted on alog frequency scale o Each division = “semitone”, each can be divided into 100 cents (signifying alterations in pitch) o The octave division into 12 semitones is repeated for each octave = called tone height Different notes represent different musical pitch sensations Relative pitch: Human ability to accurately identify tonal intervals Perfect pitch: Ability to identify tone by name (very rare) - May be combination of both genetic and environmental factors (born with it + learned) All material is © Chaudhuri (2011). There is no claim to ownershipof any information used. The use of these notes is for personal studying only. What happens to complex tone if harmonics are altered but frequencies stay same? - Same pitch occurs (due to same pattern of activity on basilar membrane) but perceptual impression is not the same. - We can distinguish difference in tone regardless of similar pitch = this is timbre (arises from different intensity distributions of harmonics) o Different instruments produce different intensity distributions for same harmonics ▪ This difference in tone quality arises from differences in neural activations due to intensity differences among harmonics o 3 general categories under which instruments can be classified 1) Chordophones (string instruments) 2) Aerophones (wind instruments) 3) Percussions o How each instrument produces different sounds is complex; all mechanisms (i.e. shape of mouthpiece, material of instrument, how they’re played, etc.) responsible for specific timbre/range of pitches associated with each instrument ▪ For string instruments: fundamental frequency is proportional to tension and density of string, and inversely proportional to string length. ▪ Each instrument produces complex tones with fundamentals within a certain range. 2. Perceptual aspects of music No general view of evolution of music perception - One theory: music evolved with emotive processes, such as courtship function - Other theories: evolution of music is accidental (no biological use for it) Nothing is sure except this: music has profound ability to transform emotions Music perception relies on 2 things: 1) T ONAL SUPERPOSITIONS (different tones sounded together to produce complex sound) 2) T ONAL SEQUENCES (different or same tones sounded in rapid succession) Polyphonic tone: Music composed of superpositions of multiple tones Brain has remarkable ability to disentangle complex sound waveform of a chord - When fundamental frequency of one tone corresponds to harmonics of another, we are able to identify the presence of both tones, even though expected sensation would be a single tone with different timbre - When exactly same note is played on two different instruments, both tones produce the same harmonic frequency distribution with different intensity values, but we don’t hear a single note at higher intensity or different timbre; we correctly perceive two notes of same pitch but different timbre o Reason for this: timing of notes can never be exactly synchronized; small asynchrony in timing helps brain to distinguish harmonic components of both tones - When two notes producing different pitch are superimposed, each harmonic component produces peak wave in basilar membrane at corresponding point in tonotopic map; but nothing to bind components so once sequence should belong to a specific note. However, brain can still distinguish between two simultaneous but distinct pitch perceptions! All material is © Chaudhuri (2011). There is no claim to ownershipof any information used. The use of these notes is for personal studying only. Consonant: Two or more simultaneously played notes deemed as “pleasant”; calm, stable feelings - Occur when two or more tones contain harmonic frequencies that coincide/reinforce each other o This can happen at different ratio intervals (e.g.: at an interval of a fifth, fundamental frequencies of two tones have 3:2 ratio; all even # harmonics coincide, odd # don’t) o The more complex the ratio, the lower likelihood of consonance - Needs to have distinct and separable frequency pattern present among two or more tones that are played simultaneously Dissonant: Two or more simultaneously played notes deemed as “unpleasant”; tension, harsh feelings - Occurs if harmonic representations are too close; results in interference between their activations on basilar membrane, produces clashing/dissonant sensation Melody: A group of musical tones that form a sequence in time Frequency characteristics of an individual tone determine its pitch Melodic sequence harmonizes with emotional centers in brain to produce pleasure 2 key parameters: 1) Time o Tempo: perceived speed of tonal presentation in a melodic sequence o Rhythm: Perceptual organization of a melody into subunits based on temporal character of the sequence 2) Contour o The nature of pitch changes with time (sequential rises/drops in frequency that produce a pattern of sound) o A melody retains same perceptual identity even if played in a different octave, as long as transposition to different pitch is applied equally to all notes Grouping of musical tones into melodic structure occurs in higher cognitive centers of brain according to a set of principles, now known as Gestalt theory 1) P RINCIPLE OF PROXIMITY o Tones appearing close together in time = grouped as a perceptual unit 2) P RINCIPLE OF SIMILARITY o Similar pitch/timbre are perceptually bound together in the same group 3) P RINCIPLE OF COMMON FATE o Perceptual grouping is based on similarity of the pattern of change with time, with regard to pitch or intensity 3. Music and the brain Music evokes significant emotional responses; rhythmic sound pattern may resonate with some internal rhythm in the brain that leads to higher emotional centers. Specific brain areas are likely involved in processing pitch sequences - Planum temporale: Enlarged in left hemisphere of professional musicians & people with perfect pitch o Responsible for processing speech sounds and potentially responsible for analytical tasks common to both language and music perception All material is © Chaudhuri (2011). There is no claim to ownershipof any information used. The use of these notes is for personal studying only. - Brain imaging shows distinct pattern of neural activity is evident when humans listen to tonal melodies as opposed to non-melodic sequences o Other distinct patterns of neural activity occur during musical imagination, music score reading, and music performance Very strong evolutionary force (survival benefit through communication) Speech perception is broadly divided into three areas: 1) Speech production 2) Speech comprehension 3) Brain areas responsible for both of the above functions 1. Speech production Language evolved as a means of communication between humans. Over 7,000 different languages (half concentrated in Central Africa & Papua New Guinea). - Language = can encompass any form of communication (written, verbal, etc.) o Most common way of using language = articulation, using vocal apparatus - Speech = the output of articulation o Note: it is the only structured stimulus that humans produce to form an active sensory signal Vocal apparatus: made up of a number of structures, divided into 3 systems: 1) Subglottal system (trachea, lungs, & muscles which expand/contract chest that moves air in/out) 2) Larynx 3) Vocal tract (larynx, tongue, teeth, palate, lips, cavities in nasal/oral chambers) Air path: nasal/oral cavities through pharynx larynx trachea - As air passes through larynx, passes through glottis (opening in vocal folds) o Open during inhalation, rapid open/close during exhalation; vibrates vocal folds o Closing of vocal cords vibrates air in throat causes initial vocal sound o This is the sound that is modified by the vocal tract to produce speech sounds Different speech sounds produced by - Contorting structures in vocal tract, OR - Producing constriction along pathway to modulate movement of air Speech sounds characterized by particular pitch containing fundamental frequency of voice signal - Determined by rate at which glottis opens/closes; 125 pulses per second for males, 200 PPS for females, 300 PPS for children o Lower frequency for males due to thickening/lengthening of vocal cords at puberty (difference in pitch between men and women) Speech composed of words composed of syllables composed of phonemes - Smallest unit of speech. Basic speech sound which distinguishes one word element from another - Generally restricted to vowels/consonants o Vowels ▪ Produced by changes in shape of mouth/lips o Consonants ▪ Require either constriction or impediment in vocal tract (usually by tongue movement) All material is © Chaudhuri (2011). There is no claim to ownershipof any information used. The use of these notes is for personal studying only. ▪ Different consonants can be described on basis of two characteristics: 1) Place of articulation 2) Manner in which constriction is made - International Phonetic Alphabet (IPA): Standard set of symbols representing various phonemes Two ways of describing speech sounds: 1) By way of how sound is articulated (describes details of how it is made) 2) Through sound’s acoustic properties o Represented by way of its pressure waveform (pressure changes depicted over time) ▪ (+): Can be stored and later used to recreate exact sound through mechanical device ▪ (-): Do not reveal details of frequency content in stimulus • We therefore cannot determine Fourier components in such a speech signal; and it’s the individual frequency components that are important for how sounds are processed by the auditory system • Solution: Identify principle frequency components associated with various phonemes o Vowels = Clustered toward lower frequency range o Consonants = cover a much wider range of frequencies ▪ Also considerable variability among consonants (e.g. nasal stop consonants have lower frequency content than oral stops) Neither pressure waveforms nor frequency representations convey substantial information about sound patterns that change over time (PWs = no frequency content, FRs = no info as to how frequencies change w.r.t. time) More effective way to depict speech stimulus = sound spectrograph - Analyzes audio signals, determines distribution of sound f
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