Chapter 11: Hearing
Sound is:
• (Physical:) pressure changes in the air or other medium.
• (Perceptual:) the experience we have when we hear.
A sound stimulus occurs when the movement or vibrations of an object cause pressure
changes in a medium that can transmit vibrations.
Compression is when air molecules are pushed together (increased pressure). Air
molecules spreading out to fill the increased space is called refraction (decreased
pressure). A pattern of air pressure changes is called a sound wave.
A pure tone occurs when changes in air pressure occur in a sine wave. These are
occasionally found in the environment. The frequency (Hz) is the number of cycles per
second that the pressure changes repeat, and the amplitude is the size of the pressure
change. Humans can perceive frequencies ranging from 20 to 20000 Hz, with higher
frequencies associated with higher pitches. Amplitude is associated with loudness. The
following equation is used for transforming sound pressure level into decibels: dB = 20 x
logarithm^10 (p/p0). When specifying the sound pressure in decibels, the notation SPL
for sound pressure level is added to indicate that decibels were determined using the
standard pressure p0 of 20 micropascals. In referring to the sound pressure of a sound
stimulus in decibels, the term level or sound level is usually used.
A periodic waveform repeats. The repetition rate is called the fundamental frequency of
the tone.
Complex tones are made up of pure tones (harmonics) added together. The first
harmonic is a pure tone with frequency equal to the fundamental frequency:
fundamental of the tone. Higher harmonics are pure tones with frequencies that are
whole number multiples of the fundamental frequencies. Harmonic components can be
represented by frequency spectra. Not all harmonics need to be present for the
repetition rate to stay the same, because the spacing remains, so there is still
information in the waveform indicating the frequency of the fundamental. The pitch also
stays the same -> effect of the missing fundamental. Timbre does change.
The audibility curve illustrates the threshold of hearing to every frequency. The area
above the curve is the auditory response area because we can hear tones that fall here.
The upper boundary of the a.r.a. is the threshold of feeling, where tones become painful.
The equal loudness curves indicate the sound levels that create the same perception of
loudness at different frequencies. At threshold. The level can differ for different
frequencies, but at some level above threshold, difference frequencies can have a
similar loudness at the same decibel level.
Pitch (which is perceptual) is most closely related to the fundamental frequency. It
cannot be measured in a physical way. The perceptual experience of increasing pitch
that accompanies increases in a tone’s fundamental frequency is called tone height.
1
, Notes on the piano with the same letter have the same tone chroma. Notes with the
same chroma have fundamental frequencies that are separated by a multiple of two.
The intervals between letters are called octaves.
Theory
Evidence
What is…
How does … contribute.
Explain the … effect
Timbre is the quality that distinguishes between two tones that have the same loudness,
pitch and duration, but still sound different. It is closely related to harmonic structure. It
also depends on the attack (buildup of sound at the beginning of the tone) and decay
(decrease in sound at the end of the tone). Playing a piano note backwards will therefore
sound like an organ.
Examples of aperiodic sounds: door slamming slut, people talking simultaneously,
static. Only periodic sounds can create a perception of pitch.
The outer ear consists of the pinnae and the auditory canal. The canal protects the
tympanic membrane (eardrum) and helps keeping the inner structures at a constant
temperature. It also enhances the intensity of sound using resonance (waves that are
reflected back interact with newly entering waves). The most reinforced frequency is
called the resonant frequency of the canal.
Sound waves make the tympanic membrane vibrate. The middle ear is a cavity that
separates the outer and inner ear and contains the three ossicles. The tympanic
membrane makes the malleus vibrate, which transmits it to the incus, which transmits it
to the stapes, which pushes on the membrane covering the oval window. The inner ear
contains liquid, which is denser than air, so only 1% of vibrations would be transmitted.
The ossicles solve this problem by:
1. concentrating the vibration onto the stapes, which increase the pressure by a
factor of 20.
2. Being hinged to create a lever action: increasing sound pressure.
Fish obviously don’t need this so they don’t have an outer or middle ear.
The middle ear muscles are attached to the ossicles. At high sound levels they contract
to dampen the ossicles’ vibration, which reduces the transmission of low-frequency
sounds. This may prevent low-frequency component to interfere with our perception of
high-frequencies (f.e. chewing).
2
Sound is:
• (Physical:) pressure changes in the air or other medium.
• (Perceptual:) the experience we have when we hear.
A sound stimulus occurs when the movement or vibrations of an object cause pressure
changes in a medium that can transmit vibrations.
Compression is when air molecules are pushed together (increased pressure). Air
molecules spreading out to fill the increased space is called refraction (decreased
pressure). A pattern of air pressure changes is called a sound wave.
A pure tone occurs when changes in air pressure occur in a sine wave. These are
occasionally found in the environment. The frequency (Hz) is the number of cycles per
second that the pressure changes repeat, and the amplitude is the size of the pressure
change. Humans can perceive frequencies ranging from 20 to 20000 Hz, with higher
frequencies associated with higher pitches. Amplitude is associated with loudness. The
following equation is used for transforming sound pressure level into decibels: dB = 20 x
logarithm^10 (p/p0). When specifying the sound pressure in decibels, the notation SPL
for sound pressure level is added to indicate that decibels were determined using the
standard pressure p0 of 20 micropascals. In referring to the sound pressure of a sound
stimulus in decibels, the term level or sound level is usually used.
A periodic waveform repeats. The repetition rate is called the fundamental frequency of
the tone.
Complex tones are made up of pure tones (harmonics) added together. The first
harmonic is a pure tone with frequency equal to the fundamental frequency:
fundamental of the tone. Higher harmonics are pure tones with frequencies that are
whole number multiples of the fundamental frequencies. Harmonic components can be
represented by frequency spectra. Not all harmonics need to be present for the
repetition rate to stay the same, because the spacing remains, so there is still
information in the waveform indicating the frequency of the fundamental. The pitch also
stays the same -> effect of the missing fundamental. Timbre does change.
The audibility curve illustrates the threshold of hearing to every frequency. The area
above the curve is the auditory response area because we can hear tones that fall here.
The upper boundary of the a.r.a. is the threshold of feeling, where tones become painful.
The equal loudness curves indicate the sound levels that create the same perception of
loudness at different frequencies. At threshold. The level can differ for different
frequencies, but at some level above threshold, difference frequencies can have a
similar loudness at the same decibel level.
Pitch (which is perceptual) is most closely related to the fundamental frequency. It
cannot be measured in a physical way. The perceptual experience of increasing pitch
that accompanies increases in a tone’s fundamental frequency is called tone height.
1
, Notes on the piano with the same letter have the same tone chroma. Notes with the
same chroma have fundamental frequencies that are separated by a multiple of two.
The intervals between letters are called octaves.
Theory
Evidence
What is…
How does … contribute.
Explain the … effect
Timbre is the quality that distinguishes between two tones that have the same loudness,
pitch and duration, but still sound different. It is closely related to harmonic structure. It
also depends on the attack (buildup of sound at the beginning of the tone) and decay
(decrease in sound at the end of the tone). Playing a piano note backwards will therefore
sound like an organ.
Examples of aperiodic sounds: door slamming slut, people talking simultaneously,
static. Only periodic sounds can create a perception of pitch.
The outer ear consists of the pinnae and the auditory canal. The canal protects the
tympanic membrane (eardrum) and helps keeping the inner structures at a constant
temperature. It also enhances the intensity of sound using resonance (waves that are
reflected back interact with newly entering waves). The most reinforced frequency is
called the resonant frequency of the canal.
Sound waves make the tympanic membrane vibrate. The middle ear is a cavity that
separates the outer and inner ear and contains the three ossicles. The tympanic
membrane makes the malleus vibrate, which transmits it to the incus, which transmits it
to the stapes, which pushes on the membrane covering the oval window. The inner ear
contains liquid, which is denser than air, so only 1% of vibrations would be transmitted.
The ossicles solve this problem by:
1. concentrating the vibration onto the stapes, which increase the pressure by a
factor of 20.
2. Being hinged to create a lever action: increasing sound pressure.
Fish obviously don’t need this so they don’t have an outer or middle ear.
The middle ear muscles are attached to the ossicles. At high sound levels they contract
to dampen the ossicles’ vibration, which reduces the transmission of low-frequency
sounds. This may prevent low-frequency component to interfere with our perception of
high-frequencies (f.e. chewing).
2
