# ACAud Roadshow Acoustics for Professionals Dominic Power Connect Hearing University of Melbourne.

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ACAud Roadshow Acoustics for Professionals Dominic Power Connect Hearing University of Melbourne

Objectives 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 Decibels

What is sound? Propagation of vibrations through an elastic medium – 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 waveforms.

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;

deciBels 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

deciBels dBSPL relates to a log of a ratio of two pressure values. What effect does doubling pressure have? – dBSPL = 20 log 2/1 = 20 x 0.3 = 6dB increase What happens with a tenfold increase in pressure? – 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

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 1. Natural, or centre, frequency (f C ) 2. Upper cutoff frequency (f U ) 3. Lower cutoff frequency (f L ) 4. Bandwidth (  f or BW) 5. Attenuation rate (in dB/octave)

1. Natural or Centre Frequency Frequency corresponding to maximum amplitude of vibration, f C (depends on mass and elasticity of system) Compare two curves: f C (B) > f C (A)

2. Upper Cutoff Frequency The frequency above f C for which the amplitude of the response is 3 dB less than the response at f C The 3dB down point

3. Lower Cutoff Frequency The frequency below f C for which the amplitude of the response is 3 dB less than the response at f C The 3dB down point

4. Bandwidth The passband of the system: the range of frequencies passed by the filter  f = f U - f L  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 dB/octave The rate at which energy for frequencies f C is rejected (attenuated)

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 f U ; attenuates energy above f U Two parameters: f U Attenuation rate

2. High-Pass Filter Passes energy above some f L ; attenuates energy below f L Two parameters: f L Attenuation rate

3. Band-Pass Filter A combination of a low-pass and a high- pass filter connected in series Signal LP HP

4. Band-Reject Filter Rejects energy between some fL and fU A combination of a low pass and high pass filter connected in parallel LP HP Signal

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 f C Thus the bandwidth of an octave filter is always 70.7% of the centre frequency (f C ) Bandwidth (  f) is given by f U -f L The centre frequency is the geometric mean of the upper and lower cutoff frequencies, given by: f C = √ f L x f U

Real Filters… 0 -3 Relative amplitude in dB Frequency fLfL fUfU fCfC

Real Filter Sets… Relative amplitude in dB Frequency 0 -3 fLfL fUfU fCfC -15 fCfC 2 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 channel? – Properties of filter Bandwidth Slope of filter – Solution Adjust adjacent channels

REM & filter sets

REM & filter sets idealised

REM & filter sets realised

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

1. REFLECTION 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 Loop

Standing Waves Panel B: incident and reflected waves are in phase – reinforcement or constructive interference Panel C: waves are out of phase - cancellation or destructive interference Longitudinal wave motion

2. ABSORPTION 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 I r = I i 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 wave I r < I i

3. REFRACTION 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 bent Stick in the water: image is bent because of change in speed of propagation (Snell’s Law)

4. DIFFRACTION 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 M a = 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 www.hearingreview.com M a = 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 cold. 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 weeks

Case 2 Serial cotton bud abuser Recent discomfort and noticed that hearing is “dull” 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 transmission. 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 Hz CHL 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?

Questions Thank you