hearing aids exam 2

parts of a hearing aid

microphone, A/D converter, preamp, DSP, D/A converter, output stage amp, receiver

microphone

converts acoustic energy to electrical energy

analog to digital converter

converts electrical signal (analog) to numbers (digital)

preamplifier

boosts small signals from microphone

digital signal processor

applies prescription to digital signal

digital to analog converter

converts numbers back to acoustic signal

output stage amplifier

boosts small electrical signal

receiver

converts electrical energy to acoustic energy

electret condenser microphone (ECM)

sound enters chamber through inlet port (preamp integrated in chamber), sound enters into front volume that acts as diaphragm moving with sound waves, electret backplate behind diaphragm has permanent static charge that creates electric field between diaphragm and backplate, diaphragm movement —> systematic change in voltage that is amplified by preamp

ideal characteristics of ECM

broadband, flat frequency response, linear

broadband for ECM

captures large number of frequencies

flat frequency response for ECM

any frequency at the same level should have same electrical output

linear for ECM

each input level increase has an equal output level increase 

frequency response of ECM

low frequency roll off, peak comes from Helmholtz resonance (sound going through neck of mic to chamber)

advantages of ECM

represents all frequencies well, low noise

disadvantages of ECM

not so small, higher power consumption, frequency response can “drift” over time

micro-electromechanical systems (MEMS)

flexible diaphragm suspended over fixed backplate, fixed charge between the two and as sound comes through HOLES in the backplate the diaphragm moves in proportion to amplitude of peak/trough of pressure waves, capacitance change from diaphragm movement can be converted to electrical signal

advantages of MEMS

very precise and highly repeatable (performs like every single other element of circuit and all other mics made), excellent stability across temperature range, ultra-small, very low power consumption, very low equivalent input noise 

what is the most common configuration for microphones?

two electronically integrated omnidirectional mics, uses active time delay in directional mode

omnidirectional

equally sensitive to sound from all directions

directional

sensitivity to sound from specific directions (front)

what is essential for ITE/BTE microphones?

port alignment; one mic towards front and one towards back

what axis should the microphones be on?

horizontal

dual mic considerations

deeper faceplate = greater natural shielding effects of pinna and less directional advantage

mic vulnerabilities

moisture/debris, internal noise (electrical and acoustic), vibrations, wind noise, low frequency roll-off

electrical noise

random motion of electrons

acoustic noise

random motion of air molecules

internal noise if mainly for which microphone type?

ECM

wind noise

wind reaches mic and sets up turbulence at inlet port and mic converts it to electrical energy

HA susceptibility to wind depends on ____ and _____

location AND type

wind is a ______ problem

near-field

environmental noise

defined moment to moment, noise that is something you DON’T want to hear

solutions to manage environmental noise

reactive approach and proactive approach

reactive approach

reduce noise at level of signal processor AFTER it enters HA, requires noise and signal to differ from one another along an acoustic parameter

proactive approach

reduce effect of noise BEFORE enters HA, requires physical separation of noise and signal

what type of mic is good for the proactive approach?

directional microphones

operational assumptions to optimize directional mics

signals of interest and noise spatially separated, desired signal in front and relatively close, undesired signals to back and sides, HA user can position themselves between signal and noise

digital sampling

accuracy of representation depends on number of intervals that signal amplitude is sampled at

typical audio file sampling rate

44.1k Hz

HA sampling rate

20-33k Hz

Nyquist frequency

highest frequency that can be represented in signal is HALF sampling rate

pre-filtering/analysis filter bank

before acoustic input to HA amplified, first filtered into frequency-specific bands that are grouped into channels, signal processing features applied to each channels

types of signal processors

hard wired architecture, application-specific instruction-set processor (ASIP), ASIPs with hard wired accelerators

hard wired architecture

all processing components implemented by dedicated circuits, fundamental function is fixed and can only be changed before manufacturing

application specific instruction-set processor

algorithms can be modified/replaced by changing program code, power consumption higher and silicon chip size larger than hard-wired architecture

ASIPS with hard wired accelerators

process intensive computing tasks on accelerator while ASIP performs computations and controls accelerator processing

input

signal level entering aid (in dB SPL)

output

signal level leaving HA after processing

gain

amount of amp applied to input

ideal receivers are:

wideband, linear, smooth frequency response, large dynamic range

max potential influenced by:

size of receiver, quality of signal processing, output of battery

moving coil receiver

coil of wire around armature, as armature moves due to magnetic field the diaphragm moves and creates pressure waveforms that turn into acoustic signal

receiver frequency response for ITE

uses just a 2-cc coupler

receiver frequency response for BTE

uses 2-cc coupler with a tube that is 10 mm long and 1 mm in inner diameter

peak clipping

armature may hit magnets and limit movements with high output levels

harmonic distortion products

occurs at frequencies that are harmonics of input frequency

intermodulation distortion products

occur at frequencies that are combinations of harmonics (ex. 2f1-f2)

output limiting

max output of HA limited to levels below distortion

compression limiting

amp reduces gain as level nears max, reduces distortion and increases comfort

dynamic range

range of output levels from HA fits within range between hearing thresholds and upper limits of comfort

with high quality devices, input signal sampled ______

20k times per second

output as function of frequency

offers frequency specific measures of HA output relative to dynamic range

output as function of input level (I/O curves)

frequency specific measures of gain, show linearity or nonlinearity of signal processing

linear amplifier

uniform amount of gain applied to all input levels

nonlinear amplifier

one fixed amount of gain is NOT applied to all input levels

compression

decreasing gain with increasing input level

compression threshold

input levels in dB SPL where input/output relationship changes

compression ratio

input/output ratio

compression range

input range over which compression occurs

expansion

increasing gain with increasing input, helps keep low level sounds quiet and not putting TOO much gain

gain as function of frequency

gain frequency response of two hearing aids

gain as function of input level

gain input levels for two frequencies in the SAME hearing aid

real-ear measures

electroacoustic tests of HA output in patient’s ear

aspects of real-ear measures

quick, objective, sensitive, reliable, doesn’t require sound treated rooms

what are real-ear measures used for?

verify gain/output, shape of frequency response, max output, effects of venting/tubing/mics

parts of RE measurement systems

sound-field loudspeaker, reference/control mic, probe mic

sound field calibration

signal from control mic used to regulate sound level near ear to required level

what does the control mic need to be calibrated to?

within 2dB across frequencies

test signals based on

objectives (fitting vs. quality control), environment (test box vs. on-ear), type of processing (directional mics/noise reduction)

type 1 signals

pure tones

type 2 signals

steady-state composite or speech-shaped BBN, digital speech in noise, custom stimuli (carrot story)

real-ear unaided response/gain

natural resonance of unaided ear canal, no HA just probe tube and assembly and sound

REUR

absolute measure of unaided SPL in ear canal

REUG

difference in SPL at control mic from SPL measured in ear canal

external ear effect

response consists of head, torso, pinna, and ear canal effects (sound field transform)

real ear aided response

direct measure of aided SPL in ear canal, hearing aid with probe and assembly

REAR

absolute measure of aided SPL in ear canal

REAG

difference in SPL at control mic from SPL measured in ear canal

real ear saturation response

estimate of max output of HA while in patient’s ear

stimuli for RESR

short tone bursts at 90 dB SPL

factors that influence RE measures

depth of probe tube, position of loudspeaker, condition of probe tube

depth of probe tube

sounds reflected back from TM interferes with sound directed toward TM, standing waves cause nulls in response

optimal distance for probe from TM?

5-6 mm

position of loudspeaker

as distance of loudspeaker increases, this increases room effects AND as distance decreases, there is increase in head turn effects

how far away should patient be from loudspeaker?

2-3 feet away

condition of probe tube

poor positioning, squashing of tube, cerumen blockage, cracks or bends

real-ear conversions and transforms GOAL

individualized real-ear sound pressure levels

why were RE transforms first developed?

for infants and small children who cannot handle listening to certain stimuli (like carrot story) multiple times for a long time

first step in HA fitting

accurate and individualized estimate of hearing threshold in dB SPL

supra aural earphones use _____

6-cc coupler