Acoustics of Music Dr Ian Drumm

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Acoustics of MusicDr Ian Drumm

• http//www.acoustics.salford.ac.uk/
• web page via student_area

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Acoustics of Music Week1 S2Physics,
Perception, Art

• Aims
• To outline how the physics of sound relates to
  its perception and the art of making music
• Learning Outcomes
• Appropriate definitions of sound and music
• Classification of musical instruments
• Understanding of the mechanisms of perception

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Musical Acoustics

• Questions to ask
• What makes musical sounds an appealing means of
  expression?
• How do instruments make musical sounds?
• How can we make instruments sound better?
• How can we imitate musical instruments and make
  new musical sounds?

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Definitions of sound

• a wave motion in air or other elastic media
• the excitation of the hearing mechanism that
  results in the perception of sound

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• Sound is an oscillation
• (stiffness and inertia)
• Hearing is
• frequency dependant (place theory)
• sensitive - can detect ear drum motions 10-10m
• has vast dynamic range 0dB to gt120dB
• Signals processed by brain

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Definition of music

• The art of combining sounds so as to express
  thought or feeling to effect the emotions
• All types of sound potentially musical hence
  catholic definition.

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loud with discordant feedback for added
expression
melodic, ordered, refined sounds
Electronic synthesised sounds
(percussive, rhythmic sounds)
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Experimental
Stockhausen
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Classifying Musical Instruments

• How can we build a taxonomy of musical
  instruments?
• How do they vary?
• What key features do they have in common?
• How can this be a template to study, improve or
  emulate musical instruments?

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Stringed Instruments

• guitar, bass, violin, cello, sitar, etc
• receives energy from say a pluck or bow
• string oscillates
• vibration tonally sculptured and amplified by a
  sound box

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Wind Instruments

• clarinet, oboe, trumpet, flute, etc
• receives energy from moving air stream
• modulated by the vibration or a read or lips
• Air column oscillates
• induces resonance's in pipe and bell

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Physical Parameters and Perception
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Definition of Pitch
Definition of Frequency

• The number of complete cycles of a periodic
  process (e.g. waveform) occurring per unit time.

• That attribute of auditory sensation in terms of
  which sounds may be ordered on musical scales

• We can
• distinguish between frequencies (JND)
• associate pitch with harmonically complex sounds
• describe discrete packets of sound as notes for
  sharing

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Pitch / frequency dependence

• Strongly correlated (Mels against Hz)
• Though not linear or direct
• lt 20Hz pulses
• 27Hz to 5kHz musically useful
• Double Bass (41.2Hz)
• Middle C Piano (261.63Hz)
• Piccolo (4725Hz)
• gt10kHz whistle

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Just Noticeable Difference in Frequency

• depends on frequency, intensity and duration and
  person
• Consider two tones of similar frequency played
  together, presented to
• Separate ears JND 0.5
• Same ears ( gt25 1/3 octave)
• Why is this the case?

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Place Theory

• Sound creates Standing Wave in Basilar Membrane
• Bands of hair stimulated to varying extents
• Brain interprets resulting nerve signals
• Hence ears act as frequency meter and spectrum
  analyser

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Critical Bands
Equivalent Rectangular Bandwidth
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Loudness/level dependence

• Depends on frequency, duration and spectral
  balance
• Dynamic range (Orchestra 70dB, CD 16bit 96dB)

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Timbre / waveform dependence

• Quasi-periodic bursts of acoustic pressure
• Periodic complex waveforms can be deconstructed
  into simple harmonics (sine waves)
• Weak harmonics - smooth, pure, flute-like
• Strong harmonics - brash full and brassy
• A-periodicity - non-harmonic overtones
  (cymbals,etc)

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Problems with defining timbre

• Each frequency harmonic component varies with
  time
• Each harmonic component has its own pitch and
  loudness
• The sensation of these components together highly
  subjective
• (physical, anatomical, neurological,
  psychological)