Transcript Loudness perception - CSIS - Department of Computer

• Pitch organisation in Western tonal music
Pitch in two dimensions
• Pitch perception in music is often thought of in
two dimensions, pitch height and pitch chroma
(Shepard, 1964). (asa trk 52)
• This is to account for the perceived similarity of
pitches that are separated by octaves.
• Pitch height is the low / high dimension of pitch.
• The relative position of a pitch within a given
octave is referred to as its chroma.
Pitch in two dimensions
• In Western music theory this is indicated by
the octave equivalent pitch classes, C, C#
etc.
• Music notes are identified first by their
position within the octave, their chroma,
and then by the octave in which they are
placed (e.g. G3, F#6).
Circularity in pitch
• Asa demo 27 - trk 52
• Complex tone with 10 partials spaced at octave
intervals
• Complex tone shifted up an octave – identical to
the original tone – complete octave equivalence
• Bell shaped spectral envelope
• Chromatic scale
• Each tone has clearly defined chroma but absolute
pitch height is ambiguous i.e. which octave
Circularity in pitch
Loudness perception
Objective
• loudness perception:
• the physical properties of sound that
contribute to our sensation of loudness
• How the loudness of sounds is represented
in our auditory system
• Hearing impairments
Loudness
• Formal definition:
• “that attribute of auditory sensation in terms
of which sounds can be ordered on a scale
extending from quiet to loud.”
• Loudness judgments are subjective
Measuring loudness
• Loudness can be measured in terms of intensity
• Intensity – sound energy transmitted per second
through a unit area in a sound field
• We can hear a large range of intensities (ratio of: 1
000 000 000 000 / 1; level of 120 dB above the
faintest sound)
• A logarithmic scale that expresses the ratio of two
intensities - decibel sound intensity (pressure)
level (dB SIL, dB SPL)
• dB SPL = 10 log10(I1/I0) = 20 log10(P1/P0)
Measuring loudness
• Reference intensity, I0 = 10-12W/m2 (watts per
square metre), P0=20μPa – gives 0 dB SPL –
average absolute threshold for humans at 1 kHz
• I1 is x dB in level above the reference intensity I0
• The word level refers to the magnitude of a sound
• Absolute threshold (for sound level) – minimum
detectable level of a sound in the absence of any
other external sounds
Thresholds of audibility
• Sensitivity of the ear varies with the
frequency of a sound
• Compare the thresholds of audibility of
tones with different frequencies
• Demonstration 6: Frequency response of the
ear: trk 17-18
• At what frequencies is it highest?
Equal-loudness contours
• One sound may be perceived as being louder than
another even though they are of the same intensity
• The perceived loudness of a sound depends on
frequency
• Indicated in the equal-loudness contours
• Equal-loudness contours – dB SPL plotted as a
function of frequency for which the listener
perceives equal loudness
Equal-loudness curves
• Listener to adjust the level of a test pure tone (of
variable frequency) to match the known level of a
1 kHz pure tone.
• The loudness level (in phons) of the 1 kHz tone is
equal to its sound pressure level in dB SPL
• Repeat for different frequencies of the test tone –
we get an equal loudness contour - labelled in
units of phons
• Q - Max hearing sensitivity at what frequency?
Why?
Equal-loudness curves
• e.g. 40 phon contour - find the level for each
frequency to sound as loud as the 1 kHz 40 dB
SPL sound.
• Audibility threshold curve – indicates the lowest
sound level that can be heard as a function of
frequency.
• Frequency range we are most sensitive to –
threshold is lowest – here we are less sensitive to
very low and very high frequencies – absolute
threshold depends on frequency
Equal-loudness curves
• Notice about these contours:
• flatter at high loudness levels – less of a
difference in perceived loudness as a
function of frequency
• rate of growth of loudness differs for tones
of different frequencies
• Remember these curves were constructed
for pure tones – single frequency sounds
Loudness and duration
• The absolute threshold for detecting sounds is
affected by duration
• Up to a few hundred milliseconds the threshold for
detecting sounds decreases (more sensitive) with
increasing duration.
• Effect of duration on loudness – variability in the
results
• Generally found that for a given intensity,
loudness increases with duration up to 100-200ms.
• Demonstration 8: temporal integration – trk 21
Loudness discrimination
• Smallest detectable change in sound level
• 0.3-2dB for a wide range of levels and types of
sound
• a value of 0.5-1dB for wideband noise – holds
from about 20dB to 100dB above threshold – JND
increases for sounds close to absolute threshold
• For pure tones the JND for loudness varies slightly
with frequency (best 1-4 kHz) and may improve at
higher sound levels.
dB scale
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Trk 8-11
Broadband noise reduced in 10 steps of 6dB
Reduced in 15 steps of 3 dB
Reduced in 20 steps of 1 dB
Loudness and the auditory
system
• Mechanisms related to the perception of
loudness:
• Increase in sound level - increased BM
movement – leads to increased firing rates
in the neurons of the auditory nerve
• spread of activity to adjacent neurons
• the summation of neural activity across
different frequency channels – critical bands
Loudness and the auditory
system
• Phase locking: more neurons may phase lock to a
tone as it is increased in intensity
• Pattern of phase locking may be important for
coding the relative levels of components in
complex sounds (spectral composition)
• when the level of a component is increased
relative to the other components, the degree of
phase locking to that component increases
Loudness of complex sounds
• The perceived loudness of a sound with a
certain amount of energy depends on the
frequency bandwidth of the sound –
whether narrow or spread over a wide range
of frequencies
• Related to the critical bandwidth
Loudness and the critical
bandwidth
• The effect of the critical bandwidth on loudness
• for a sound with a given amount of energy - louder
if its bandwidth exceeds one CB than if its
bandwidth is less than one CB.
• At low levels (10-20dB) - the loudness of a
complex sound is independent of bandwidth
• Spread of excitiation and loudness summation
across CFs
Hearing impairments
• Prolonged exposure to excessive sound
levels may cause hearing defects
• 2 main types of hearing loss: conductive and
sensorineural
Conductive hearing loss
• is where a defect in the outer or middle ear
prevents sound from passing efficiently to the
cochlea in the inner ear.
• Results in attenuation of the incoming sound
• some of the causes are wax build up in the ear
canal, immobilization of the stapes due to the
growth of bone over the oval window
(otosclerosis) or an infection in the middle ear due
to the build up of a viscous fluid (otitis media).
• May be corrected by a hearing aid or surgery
Sensorineural hearing loss
• generally occurs because of defects in the cochlea,
auditory nerve or higher centres in the auditory
system.
• may be caused by prolonged exposure to loud
noise, aging, illness, head trauma, toxic
medications or an inherited condition.
• Cochlear implants may be used to improve
hearing, but most sensorineural hearing losses
cannot be treated by surgery
• may have difficulty understanding speech,
especially in noisy environments
Reduced frequency selectivity
• Damage to the cochlea structures of the ear can
lead to reduced frequency selectivity (a loss in the
ability to resolve frequency components) and
loudness recruitment
• Reduced frequency selectivity may be reversed if
the cause (e.g. noise exposure, drugs/asprin) is
removed quickly enough.
• In an ear with reduced frequency selectivity the
auditory filters are broader than those in a healthy
ear.
Reduced frequency selectivity
• A perceptual consequence of reduced
frequency selectivity is the greater
susceptibility of a signal (e.g. speech or
music) to masking by interfering sounds.
Loudness recruitment
• refers to the condition in which the growth of
loudness with increasing level is more rapid than
normal.
• The absolute threshold of the ear is elevated, but
the level at which sounds become uncomfortably
loud may be normal.
• The effect of loudness recruitment is to reduce the
effective range of sound levels normally heard.
Loudness recruitment
• Loudness recruitment may be due to damage to
the active process in the cochlea that enhances
sensitivity at low sound levels – outer hair cells
• A hearing aid with automatic gain control that
amplifies low-level sounds more than high-level
sounds may be used to compensate for loudness
recruitment (Moore, 2003).
Cochlear implant
• For people who have a damaged cochlea in
particular the hair cells, but a healthy or partially
intact auditory nerve, a cochlear implant may be
used
• The cochlear implant is a system where an array
of electrodes is implanted in the cochlea.
• This makes it possible to selectively stimulate
groups of neurons within the auditory nerve.
• The sensation of sound is created by direct
electrical stimulation of the auditory nerve.
Hearing loss due to aging
• presbyacusis
• begins at the extremely high frequencies and
progresses to the lower frequencies with
increasing age.
• Yost (2000): a possible reason - sounds of all
frequencies cause the stereocilia of the hair cells at
the base of the cochlea to bend, while only low
frequencies stimulate the stereocilia at the apical
end and this may lead to the hair cells at the base
“wearing out” before those at the apex