ACAud Roadshow

ACAud Roadshow
Acoustics for Professionals
Dominic Power
Connect Hearing
University of Melbourne
• Acoustics
– What it is
– Why it is important
– Clinical examples and applications
Nature of sound
What is sound
How does it travel
How is it measured
Sound Pressure
Sound Intensity
What is sound?
• Propagation of vibrations through an elastic
– Air, water, steel etc
• Speed of propagation of sound is governed by
the properties of the medium, namely, the
elasticity and the mass of the medium.
• Longitudinal waveform.
• Pure tones, complex tones, complex aperiodic
How is it measured?
• Sound Level meters
• Measure sound pressure level.
– Pressure is the force per unit area that a sound
wave imparts (SPL)
– Intensity of a waveform is the energy that is
transmitted over a unit of area (IL)
– dB is a log ratio of an absolute measure of one
value compared to a reference value.
• dB SPL= 20logPx/Pr where Pr is 20uPa;
• A log ratio of two levels of sound.
– If the absolute value is larger than the reference value,
positive dB
– If absolute value is smaller than the reference value,
negative dB
– Standard reference value for dBSPL is 20uPa or 20
millionths of a Pa (1 Pascal is one newton of force spread
over an area of 1m2)
– 20uPa corresponds to the pressure that is just perceived
on the tympanic membrane, threshold of hearing.
– dBSPL = 20 log 20uPa/20uPa
= 20 log 1
= 0dBSPL
• dBSPL relates to a log of a ratio of two pressure
• What effect does doubling pressure have?
– dBSPL = 20 log 2/1
= 20 x 0.3
= 6dB increase
• What happens with a tenfold increase in
– dBSPL = 20 log 10/1
= 20 x 1
= 20dB
Typical Sound Pressures
Gain increase in prescriptions
• Doubling output of hearing aid
– 6dB increase in output
– Doubles pressure
– Decreases battery life
• Half gain, third gain
• NL1
• NL2 gain applied in 1/3 octave filters
• A filter is an acoustical system that changes the spectrum
of a sound (a ‘frequency-selective’ system)
• A filter has a frequency response (transfer function) that is
determined by the ratio of the amplitude coming out of the
filter divided by the amplitude going in at each frequency
Parameters of a Filter
Natural, or centre, frequency (fC)
2. Upper cutoff frequency (fU)
3. Lower cutoff frequency (fL)
4. Bandwidth ( f or BW)
5. Attenuation rate (in dB/octave)
1. Natural or Centre Frequency
• Frequency corresponding to maximum amplitude of
vibration, fC (depends on mass and elasticity of system)
• Compare two curves: fC (B) > fC (A)
2. Upper Cutoff Frequency
• The frequency
above fC for which
the amplitude of the
response is 3 dB
less than the
response at fC
• The 3dB down
3. Lower Cutoff Frequency
• The frequency
below fC for which
the amplitude of the
response is 3 dB
less than the
response at fC
• The 3dB down
4. Bandwidth
• The passband of
the system: the
range of
frequencies passed
by the filter
• f = fU - fL
• f quantifies how
narrowly or broadly
the filter is tuned
5. Attenuation Rate
• AKA roll-off rate, or
rejection rate
• The slope of the filter
curve, expressed in
• The rate at which
energy for frequencies
< fC or > fC is rejected
5. Attenuation Rate
• Filter A: 10 dB/octave
• Filter B: 15 dB/octave
• Attenuation rate
quantifies the
‘selectivity of a filter’
Acoustic Filters
1. Low-Pass
2. High-Pass
3. Band-Pass
4. Band-Reject (notch)
1. Low-Pass Filter
• Passes energy below
some fU; attenuates
energy above fU
• Two parameters:
• fU
• Attenuation rate
2. High-Pass Filter
• Passes energy above
some fL; attenuates
energy below fL
• Two parameters:
• fL
• Attenuation rate
3. Band-Pass Filter
• A combination of a
low-pass and a highpass filter connected
in series
4. Band-Reject Filter
• Rejects energy
between some fL and
• A combination of a
low pass and high pass
filter connected in
Idealised versus Realised Filters
• When we specify
parameters of a
real filter, we
describe it as if it
were an idealised
rectangular filter
• Specification of
attenuation rate
reveals how much
the realised filter
departs from the
idealised one
A Common Constant Percentage Bandwidth
Filter – The Octave Filter
• The Octave filter: f = 0.707 fC
•Thus the bandwidth of an octave filter is always
70.7% of the centre frequency (fC)
• Bandwidth (f) is given by fU-fL
• The centre frequency is the geometric mean of the
upper and lower cutoff frequencies, given by:
fC = √ fL x f U
Relative amplitude in dB
Real Filters…
Relative amplitude in dB
Real Filter Sets…
Slope of filter (attenuation rate): 24 dB per octave
Filter sets in hearing aids
• Filter = channel
– 4, 8, 12, 16, 20 etc
• More channels, narrower the filter
• Higher accuracy in matching output to
prescribed target.
• Broad channels pose problems in
manipulating output of aid.
• Understanding properties of filters can help
Filter sets in hearing aids
• Problem
– Adjusting one channel affects output of adjacent
– Properties of filter
• Bandwidth
• Slope of filter
– Solution
• Adjust adjacent channels
REM & filter sets
REM & filter sets idealised
REM & filter sets realised
REM & filter adjustments
REM & filter adjustments
REM and filter adjustments
The Inverse Square Law
• Inverse square law only holds strictly in free unbounded
medium with no obstacles
• If sound wave encounters obstacle, it will be:
• Reflected
• Absorbed
• Refracted
• Diffracted
• Law of reflection: angle of reflected path to the perpendicular
equals angle of the incident path to the perpendicular
Standing Waves
• Occur when two
progressive waves,
incident and reflected,
of same frequency and
amplitude, travel in
opposite directions in
or along a medium
• Nodes (points of no
vibration) and
antinodes (points of
maximum vibration)
Transverse wave motion
Standing Waves
• Panel B: incident and
reflected waves are in
phase – reinforcement
or constructive
• Panel C: waves are out
of phase - cancellation
or destructive
Longitudinal wave motion
• Opposition to sound transmission will exist at any boundary where
impedances differ
• If impedance is infinite, intensity of reflected wave will equal
intensity of incident wave
• Ir = Ii
• If impedance is not infinite, some sound energy will be absorbed
by new medium
• Intensity of reflected wave will be less than intensity of incident
• Ir < Ii
• When a wave encounters an obstacle offering large impedance,
the wave is reflected with no change in speed of propagation
• When a wave moves to another medium, or encounters a change
in the medium, the speed of propagation changes and rays are
Stick in the water: image is bent
because of change in speed of
propagation (Snell’s Law)
• The bending or scattering of a sound wave around an obstacle
• Long wavelengths (low frequencies) are more likely to be diffracted
around obstacles than shorter wavelengths
• Sounds with wavelengths longer than the diameter of obstacles it
encounters will diffract around or through the obstacle
Plane progressive sound waves
encountering a barrier (A) or
an opening in a wall (B). Some
energy is reflected back, the
wave fronts scatter or bend
around the obstacle, then the
wave fronts reform and
continue as plane wave fronts
Acoustic considerations for hearing aid
selection and optimisation
• Venting
– Size, length, type
– Acoustic mass Ma = 1.8(L/A)
• Dampers
– Mid frequency response smoothing
– Location/type
• Sound Bore
– Horns
– Aimed at matching impedance of receiver
with that of eardrum.
Effects of widening the vent
Ma = 1.8(L/A)
Mould selection
• Style of mould
– Material
• Length of mould
– Comfort and secure retention are primary concern
– High gain requirements
• Volume of canal (halve volume, double the pressure at
the eardrum , +6dBSPL )
– Mindful of medial canal skin thickness
Acoustics and Pathologies
• Middle ear Impedance
• Complex sum of Resistance and Reactance
– Resistance is friction – frequency independent
– Reactance  mass and stiffness
• Increasing stiffness in the middle ear system
results in decreased low frequency transmission
of sound
• Increasing the mass of the middle ear system
results in decreased high frequency transmission
of sound
Acoustics and Pathologies
• Audiogram plus middle ear measurement
gives diagnostic value in determining
aetiology and progression of pathology.
• Importance of good clinical history and
accurate testing methods
Case 1
• Short term (2 weeks) feeling of blockage in ears
associated with sinus inflammation and head
• Tympanometry shows -250daPa
• Audiometry reveals very mild low frequency
conductive hearing loss
– Retraction of TM increases stiffness of middle ear
system. This decreases low Hz transmission leaving
mid to high Hz relatively unaffected.
– Management – encourage Valsalva and retest in 6
Case 2
• Serial cotton bud abuser
• Recent discomfort and noticed that hearing is
• Otoscopy shows large wax plug on TM
• Audiogram shows mild high Hz CHL
• Tympanometry shows As (normal ECV)
• Mass loading of TM reduces high Hz
• Management – aural toilet under microscope
Case 3
Family history of hearing loss emerging in 20s
Audiogram showing symmetric low Hz CHL
Type A tympanograms.
Hearing not poor enough for amplification
Review in 12/12
Case 3
• Retested 12/12 later
• Feels hearing has dropped.
• Audiogram shows high Hz and worsening low
• Type A tympanograms.
• Audiogram, tympanogram and history
suggestive of otosclerosis.
Case 3
• Increased stiffening of the middle ear system
caused by ossicular fixation leads to low Hz
transmission being impaired
• Progression of condition increasing stiffness
deteriorate low Hz transmission, plus bony
deposits building up on stapes footplate
increases mass in the system impairing high
Hz transmission.
Case 3
• Management?
• Thank you

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