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Chapter 11

PS262 Chapter Notes - Chapter 11: Stapes, Lever Action, Tectorial Membrane

Course Code
Elizabeth Olds

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PS262 Chapter 11 Sound, the Auditory System and Pitch Perception
The Sound Stimulus
Physical definition: sound is pressure changes in the air or other medium
o EX// the sound levels was 10 decibels
Perceptual definition: sound is the experience we have when we hear
o EX// the sound of the trumpet pierced the air
Sound as Pressure Changes
A sound stimulus occurs when the movements or vibrations of an object cause pressure changes
in the air, water or any elastic medium that surrounds the object
When the diaphragm of the speaker moves out, it pushes the surrounding air molecules
together a process called condensation which causes a slight increase in the density of
molecules near the diaphragm
o This increased density results in a local increase in the air pressure that is super imposed
on the atmospheric pressure
When the speaker diaphragm moves back, air molecules spread out to fill in the increased
space, a process called rarefaction
o The decreased density of air molecules caused by rarefaction causes a slight decrease in
air pressure
When air pressure changes, which travels through the air at 340 meters per second it is called a
sound wave.
Although air pressure changes move outward from the speaker, the air molecules at each
location move back and forth but stay in about the same place
Pressure Changes: Pure Tones
A pure tone occurs when pressure changes in the air occur in a pattern described by a
mathematical function called a sine wave
o A person whistling or the high-pitched notes produced by a flute are close to pure tones
Amplitude the size of the pressure change and the frequency the number of times per
second that the pressure changes repeat
An unit of sound, decibel which converts the large range of sound pressure into a more
manageable scale
SPL = sound pressure level
Frequency, the number of cycles per second the change in pressure repeats, is the physical
measure associated with our perception of pitch, with higher frequencies associated with higher
o Frequency is indicated in units called Hertz (Hz)
Pure tones are important because they are simple and because they have been used extensively
in auditory research

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Pressure Changes: Complex Tones
The repetition rate of a complex tone is called a fundamental frequency of the tone
An important property of periodic complex tones is that they consist of a number of pure tones
We can build a complex tone by using a technique called addictive synthesis in which a number
of sine-wave components are added together to create the complex tone
A way to represent the harmonic components of a complex tone is by frequency spectra
o The position of each line on the horizontal axis indicates the harmonics frequency and
the height of the line indicates the harmonics amplitude
o Frequency spectra provides a way of indicating a complex tones fundamental frequency
and harmonics without drawing the tone’s waveform
Note: that removing a harmonic changes the tone’s waveform but that the repetition rates
remain the same
The distance between harmonics equals the fundamental frequency
o When the fundamental is removed, this spacing remains, so there is still information in
the waveform indicating the frequency of the fundamental
Perceiving Sound
Loudness is the quality most closely relate to the amplitude or sound pressure which is also
called the level of auditory stimulus
o Decibels are often associated with loudness
o Decibels are a physical measure whereas loudness is psychological
Pitch is defined as the attribute of auditory sensation in terms of which sounds may ordered on
a musical scale
Tone height is the perceptual experience of increasing pitch that accompanies increases in a
tone’s fundamental frequency
o In addition to the increase in tone height that occurs, we move from low to the high end
of the piano keyboard, something else happens: the letters of the notes A, B, C, D, E, F
and G repeat and we notice that notes with the same letter sound similar
o Notes with the same letter have the same tone chroma
o Every time we pass the same letter on a keyboard we have gone up a interval called
The pitch is determined not by the presence of the fundamental frequency but by information
such as spacing of the harmonics and the repetition rate of the waveform that indicates the
fundamental frequency
The constancy of pitch is called the effect of the missing fundamental and the pitch that we
perceive in tones and that has had harmonics removed is called periodicity pitch
We hear sound only within a specific range of frequencies called range of hearing
The audibility curve indicates the threshold for hearing determined by free-field presentation
versus frequency
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