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Hearing 1
Hearing
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Sources: Rossing. (1990). ”The science of sound”. Chapters 5–7.
Karjalainen. (1999). ”Kommunikaatioakustiikka”.
Pulkki, Karjalainen (2015). ”Communication acoustics”
Moore. (1997). ”An introduction to the psychology of hearing”.
Hearing 2
1 Introduction
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Auditory system can be divided in two parts
–  Peripheral auditory system (outer, middle, and inner ear)
–  Auditory nervous system (in the brain)
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Contents:
1. 
Introduction
2. 
Ear physiology
3. 
Loudness
4. 
Masking
5. 
Pitch
6. 
Spatial hearing
Ear physiology studies the peripheral system
Psychoacoustics studies the entire sensation:
relationships between sound stimuli and the subjective
sensation
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1.1 Auditory system
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Dynamic range of hearing is wide
–  ratio of a very loud to a barely audible sound pressure level is
1:106 (120 dB)
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Frequency range of hearing varies a lot between
individuals
–  only few can hear from 20 Hz to 20 kHz
–  sensitivity to low sounds (< 100Hz) is not very good
–  sensitivity to high sounds (> 12 kHz) decreases along with age
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Selectivity of hearing
–  listener can pick an instrument from among an orchestra
–  listener can follow a speaker at a cocktail party
–  One can sleep in background noise but still wake up to an
abnormal sound
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1.2 Psychoacoustics
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Perception involves information processing in the brain
–  Information about the brain is limited
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Psychoacoustics studies the relationships between sound
stimuli and the resulting sensations
–  Attempt to model the process of perception
–  For example trying to predict the perceived loudness / pitch /
timbre from the acoustic properties of the sound signal
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In a psychoacoustic listening test
–  Test subject listens to sounds
–  Questions are made or the subject is asked to describe her
sensasions
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2 Ear physiology
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2.1 Outer ear
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Outer ear consists of:
–  pinna – gathers sound; direction-dependent response
–  auditory canal (ear canal) - conveys sound to middle ear, and acts
as a resonator at 3-4 kHz amplifying sound +10 dB
The human ear consists of three main parts:
(1) outer ear, (2) middle ear, (3) inner ear
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Outer ear is passive and linear, and its behavior can be
completely described by laws of acoustic wave
propagation.
Nerve
signal
to brain
[Chittka05]
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2.2 Middle ear
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The ossicles transmit eardrum vibrations (in air) to the
oval window of the inner ear (filled with fluid)
Acoustic reflex:
–  when sound
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pressure level exceeds
50-60 dB, eardrum tension
increases and staples is
removed from oval window
–  Protects the inner ear
from damage
2.3 Inner ear, cochlea
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Middle ear contains
–  Eardrum that transforms sound waves into mechanic vibration
–  Tiny audtory bones (ossicles): 1 Malleus (resting against the
eardrum, see figure), 2 Incus and 3 Staples
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The inner ear contains the cochlea: a fluid-filled organ where
vibrations are converted into nerve impulses to the brain.
Cochlea = Greek: “snail shell”.
Spiral tube: When stretched out, approximately 35 millimeters long.
Vibrations on the cochlea’s oval window cause hydraulic pressure
waves inside the cochlea
Inside the cochlea there is
the basilar membrane,
On the basilar membrane
there is the organ of Corti
with nerve cells that are
sensitive to vibration
Nerve cells transform
movement information into
neural impulses in the
auditory nerve
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2.4 Basilar membrane
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Figure: cochlea stretched out for illustration purposes
–  Basilar membrane divides the fluid of the cochlea into separate
tunnels
–  When hydraulic pressure waves travel along the cochlea, they
move the basilar membrane from oval window to round window.
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Basilar membrane
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Different frequencies produce highest amplitude at different sites
Preliminary frequency analysis happens on the basilar membrane
Travelling waves:
apex
base
apex
Best freq (Hz)
base
2.5 Sensory hair cells
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Distributed along the basilar membrane are sensory hair cells that
transform membrane movement into neural impulses
When a hair cell bends, it generates neural impulses
–  Impulse rate depends on vibration amplitude and frequency
Depending on the position on the basilar membrane, each nerve cell
has a characteristic frequency to which it is most responsive to (right
fig.: tuning curves of 6 different cells from different positions)
Left fig: Varying the amplitude of stimulus (dB) broadens the range of
frequencies that the hair cell reacts to
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3 Loudness
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Loudness describes the subjective level of sound
–  Perception of loudness is relatively complex, but
–  consistent phenomenon and
–  one of the central parts of psychoacoustics
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The loudness of a sound can be compared to a
standardized reference tone, for example 1000 Hz
sinusoidal tone
–  Loudness level (phon) is defined to be the sound pressure level
(dB) of a 1000 Hz sinusoidal, that has the the same subjective
loudness as the target sound
–  For example if the heard sound is perceived as equally loud as 40
dB 1kHz sinusoidal, is the loudness level 40 phons
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3.1 Equal-loudness curves
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3.2 Critical bands
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Comparing to two band-limited noises with same center
frequencies and increasing the other’s bandwidth, the perceived
loudness increases only after a critical bandwidth
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Ear analyzes sound at critical band resolution. Each critical band
contributes to the overall loudness level
Each center frequency fc has a critical bandwidth Δf (Bark
bandwidths)
Equivalent rectangular bandwidth
(ERB) is a different type of
procedure to measure a rectangular
bandwidth of auditory filters.
Both bandwidths can be used to
define a frequency Bark and ERB scales
Loudness level (phons)
Sound pressure level (dB)
–  Figure: 1 kHz @ 60 dB, Critical bandwidth is 160 Hz at 1 kHz
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Frequency (Hz)
–  Stack bandwidths on top of each other
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3.3 Perceptually-motivated frequency scales
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3.4 Loudness of a complex sound
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mm. on basilar membrane
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frequency / kHz
frequency / mel
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frequency / Bark
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A broadband sound is perceived louder than a
narrowband sound of equal SPL, as shown by the critical
bandwidth experiment.
Loudness of a complex sound is calculated by using socalled specific loudness of a critical band as intermediate
unit
Specific loudness is roughly proportional to the log-power
of the signal at the band (weighted according to sensitivity
of hearing and spread slightly by convolving over bands)
Overall loudness is obtained by summing up specific
loudness values over critical band
3.5 Measuring sound level
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SPL does not work well as a perceptual metric of sound
loudness.
Different frequency weighting curves (A,B,C,D) can be
applied to measure sound level, which is more related to
frequency sensitivity of hearing
A-weighting is most common,
wikipedia
referred as dB(A), it roughly
resembles the inverse of
the hearing threshold curve.
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4 Masking
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Masking describes the situation where a weaker but
clearly audible signal (maskee, test tone) becomes
inaudible in the presence of a louder signal (masker)
Masking depends on both the spectral structure of the
sounds and their variation over time
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4.1 Masking in frequency domain
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Model of the frequency analysis in the auditory system
–  subdivision of the frequency axis into critical bands
–  frequency components within a same critical band mask each
other easily
–  Bark & ERB scales: frequency scales that are derived by
mapping frequencies to critical band numbers
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Narrowband noise masks a tone (sinusoidal) easier than
a tone masks noise
Masked threshold refers to the raised threshold of
audibility caused by the masker
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sounds with a level below the masked threshold are inaudible
masked threshold in quiet = threshold of hearing in quiet
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Masking in frequency domain
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Figure: masked thresholds [Herre95]
–  masker: narrowband noise around 250 Hz, 1 kHz, 4 kHz
–  spreading function: the effect of masking extends to the spectral
vicinity of the masker (spreads more towards high freqencies)
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Additivity of masking: joint masked thresh is approximately
(but slightly more than) sum of the components
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4.2 Masking in time domain
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5 Pitch
Forward masking
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–  masking effect extends to times after the masker is switched off
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Backwards masking
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Forward/backward masking does not extend far in time
Pitch
–  Subjective attribute of sounds that enables us to arrange them on
a frequency-related scale ranging from low to high
–  Sound has a certain pitch if human listaners can consistently
match the frequency of a sinusoidal tone to the pitch of the sound
–  masking extends to times before the masker is been switched on
" simultaneous masking is more important phenomenon
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backward
masking
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Fundamental frequency vs. pitch
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forward
masking
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Fundamental frequency is a physical attribute
Pitch is a perceptual attribute
Both are measured in Hertz (Hz)
In practise, perceived pitch ≈ fundamental frequency
"Perfect pitch" or "absolute pitch" - ability to recognize the
pitch of a musical note without any reference
–  Minority of the population can do that
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5.1 Harmonic sound
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5.2 Pitch perception
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For a sinusoidal tone
Pitch perception has been tried to explain using two
competing theories
–  Place theory: “Peak activity along the basilar membrane
determines pitch” (fails to explain missing fundamental)
–  Periodicity theory: “Pitch depends on rate, not place, of response.”
Neurons fire in sync with signals
–  Fundamental frequency = sinusoidal frequency
–  Pitch ≈ sinusoidal frequency
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Harmonic sound
Trumpet sound:
* Fundamental
frequency
F = 262 Hz
* Wavelength
1/F = 3.8 ms
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The real mechanism
is a combination of
the above
–  Sound is subdivided into
subbands (critical bands)
–  Periodicity of the
amplitude envelope
(see lowest panel) is
analyzed within bands
–  Results are combined
across bands
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6 Spatial hearing
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The most important auditory cues for localizing a sound
sources in space are
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6.1 Monaural source localization
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Is relatively constant with frequency
2.  Interaural intensity difference (ILD)
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Increases with frequency
3.  Direction-dependent filtering of the sound spectrum by head and
pinnae
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Diretional hearing works to some extent even with one ear
Head and pinna form a direction-dependent filter
–  Direction-dependent changes in the spectrum of the sound arriving in the
ear can be described with HRTFs
–  HRTF = head-related transfer function
1.  Interaural time difference (ITD)
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HRTFs are crucial
for localizing
sources in the
median plane
(vertical localization)
Terms
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Monaural : with one ear
Binaural : with two ears
Interaural : between the ears (interaural time difference etc)
Lateralization : localizing a source in horizontal plane
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Monaural source localization
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HRTFs can be measured by recording
–  Sound emitted by a source
–  Sounds arriving to the auditory canal or eardrum (transfer function
of the auditory canal does not vary along with direction)
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In practice
–  left: microphone in the ear of a test subject,
OR
–  right:
head and torso
simulator
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6.2 Localizing a sinusoidal
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Experimenting with sinusoidal tones helps to understand
the localization of more complex sounds
Angle-of-arrival perception for sinusoids below 750 Hz is
based mainly on interaural time difference
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Localizing a sinusoidal
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Interaural time difference is useful only up to 750 Hz
–  Above that, the time difference is ambiguous, since there are
several wavelengths within the time difference
–  Moving the head (or source movement) helps: can be done up to
1500 Hz
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6.3 Localizing complex sounds
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Complex sounds refer to sounds that
–  involve a number of different frequency components and
–  vary over time
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At higher frequencies
(> 750 Hz) the auditory system
utilizes interaural intensity
difference
Localizing sound sources is typically a result of
combining all the above-described mechanisms
1.  Interaural time difference (most important)
2.  Interaural intensity difference
3.  HRTFs
–  Head causes and acoustic
”shadow” (sound level is
lower behind the head)
–  Works especially at
high frequencies with
shorter wavelenghts compared
to head dimensions
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Wideband noise: directional hearing works well
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6.4 Lateralization in headphone listening
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When listening with headphones, the sounds are often localized
inside the head, on the axis between the ears
–  Sound does not seem to come from outside the head because the
diffraction caused by pinnae and head is missing
–  If the sounds are processed with HRTFs carefully, they move outside the
head
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6.5 Precedence effect
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The sound in indoors that reaches the listener contains the direct
path, early reflections, and late reverberation.
The brain suppresses early reflected sounds to aid in source
direction perception
–  Early reflections are not heard as separate sounds.
–  Instead, they amplify the loudness of direct sound
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Precedence effect works when
1.  Early reflections arrive less than ~ 35ms after direct sound.
2.  The spectrum of a reflection is similar to that of the direct sound
3.  Reflections are lower or at least not signigicantly louder than direct
sound
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Even a sound with +10 dB SPL than direct sound can not be heard