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Summary 2.4 Problem 8
Summary of p8 for perception
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Problem 8 – Goldstein, CorenSound:
- Physical definition: sound is pressure changes in the air or other medium
o Condensation: air molecules pushed together – increase in the density
of molecules
o Rarefaction: air molecules spread out – decrease in density of molecules
o This pattern of air pressure changes causes a sound wave
- Perceptual definition: sound is the experience we have when we hear
Pure tones: occur when pressure changes in the air occur in a pattern described by
a sine wave
Characteristics of pure tones
Amplitude: the size of the pressure change
o Associated with our experience of loudness, higher amplitudes are louder
o Decibel (dB): unit of sound which converts large range of sound pressure
into a manageable scale
o Db = 20 x log(p/po)
Frequency: the number of times per second that the pressure changes repeat
o Associated with our perception of pitch, higher frequencies have higher
pitches
o Hertz (Hz): unit for frequency in which 1 Hz is 1 cycle per second
o Humans can perceive from 20 Hz to 20,000 Hz
Complex tones
- periodic tone: when a waveform repeats
- Fundamental frequency: the repetition rate of a complex tone
- Periodic complex tones consist of a number of pure tones
- Additive synthesis: building a complex tone by adding a number of sine-wave components
- Frequency spectra: another way to represent harmonic components of a complex tone
Pitch
- Tone height: increasing pitch with increases in tone’s fundamental frequency
- The pitch is determined by information that indicates fundamental frequency e.g. spacing
of harmonics, repetition rate of waveform etc. – not by fundamental frequency itself
- Effect of missing fundamental: the constancy of pitch even when fundamental or other
harmonics are removed
- Periodicity pitch: the pitch we perceive in tones
Range of Hearing
- Audibility curve: indicates the threshold for hearing
determined by free-field presentation
- Auditory response area: (above the audibility curve till
below the threshold of feeling) we can hear tones that fall
within this area
- Threshold of feeling: tones with these high amplitudes, they
can become painful and can damage the auditory system
- Loudness of a tone depends on both sound pressure and frequency
, - Equal loudness curves: indicate a number of different frequencies that create the same
perception of loudness at different frequencies
Timbre: the quality that distinguishes between two tones that have the same loudness, pitch and
duration but still sound different
The Ear
- 3 tasks of the auditory system:
o Must deliver the sound stimulus to receptors
o Must transduce this stimulus from pressure changes into electrical signals
o Must process these electrical signals so they indicate quality of the sound
The outer ear:
- Sound waves pass through the outer ear: consists of the pinna and the auditory canal
- Pinnae: the structure of the ear that sticks out from the sides of the head
- Auditory canal/external auditory meatus: tube-like structure that protects the delicate
structures of the middle ear such as the eardrum/tympanic membrane that’s at the end of
the auditory canal
- The pressure of air on both sides of the eardrum is kept equal by the eustachian tube
- The outer ear also enhances the intensities of some sounds by resonance
- Resonance: occurs in the auditory canal when sounds waves that are reflected back from
the closed end interact with the sound waves that are entering the canal
o this interaction reinforces sound frequencies
o the most reinforced frequency is called resonant frequency of canal, it is
determined by the length of the canal
The Middle ear:
- the middle ear is a small cavity which separates the outer and inner ear
- it contains the ossicles: the 3 smallest bones in the body
- first bone malleus (or hammer) is set into vibration by the tympanic membrane, malleus
transmits the vibrations to the incus (or anvil) which in turn transmits vibrations in to the
stapes (or stirrup)
- the stapes then transmits vibrations to the inner ear by pushing on the membrane covering
the oval window
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