Hearing Aid Characteristics

    Types of aids

    Basic components

   Options

    Electroacoustic characteristics

Types of Aids

 

   ___- Conduction

 

   Bone conduction

Bone Anchored Hearing Aid

 

Air-conduction

                                  Approx. Sales as of 2001

     Body                  .1%

     Ear-level             

          BTE              22%

          ___               40%

          ITC               23%

          CIC               15%

          (Eyeglass, rarely)

58% of aids were programmable

26% were digital

 


Bone-conduction

External receiver

 

Implantable receiver

 

 

Consider Differences Relative To

n    Microphone placement

n    Possible gain

n    Modification of output

n    _____________ of options

n    Ease of manipulation

n    Flexibility

n    Arrangements for repair

Basic Components

n    Microphone

n    Amplifier

n    ____________

n    Power Supply

Basic components, continued

n    Schematic drawing of a hearing aid

 

Microphone

Converts acoustic energy to ______________ energy which in turn is converted to electrical energy

Can be categorized by

    TYPE and by DIRECTIONALITY

 

Types: Carbon-early 1900’s

             Crystal-mid 1930’s

             ________ Microphones-as early as 1920’s

             Ceramic-1960’s

             Electret-1970’s

            Silicone-emerging

 

   

Electret Microphone
Commonly used today

 

Electret Microphone-
       has a permanent _______ across the         diaphragm

n    Diaphragm-thin film of metalized foil which will move as sound impinges on it

 

n    Electret-thin teflon material that maintains the electric charge

 

n    Metal _________-along with diaphragm forms a capacitor

 

Electret Microphone

Sound impinges on the diaphragm

 

Distance between diaphragm and backplate reduces with compression phase

 

Distance increases during rarefaction phase

Microphone Function

These changes alter the ability of the capacitor to store electrical charges

 

Thus, voltage at the plates changes as a function of ____________ movement

 

The voltage is delivered to a FET which causes current to flow

Electret Microphone Advantages

n      ­­­_____ frequency response

 

n      Small diaphragm’s mass results in excellent sensitivity

n      Resistant to shock damage

 

n      Limited ______ requirements…not as much shock absorbent material necessary

Advantages (cont.)

 

n    ___________ to temperature changes than magnetic microphones

 

n    Magnetic field not involved so mic and receiver can be mounted closer together

DIRECTIONALITY OF MICs

n    Omni directional

n    Directional

–  Single Microphone

–  ­­­­_____ Microphone

Omnidirecionl Mics

n    Equal ___________ to sound from all around

n    One Microphone Port

Directional Microphone

   More sensitive to sounds coming from the _________-Two mic ports separated by 4-12mm

 

Directional Microphone

   A sound entering from the rear mic is delayed so that……

   it reaches the ___________ about the same time as that sound reaches it via the front mic

   The delay is caused by an __________ damper or resistor

Dual Microphones

n      Also called  twin or two-microphone design

n      Two Omni-directional Mics, each with one port

n      Output of from second mic is ________delayed and subtracted from the first mic output

n      Phase cancellation occurs electrically rather than acoustically

n      Use can switch off one mic to return to Omni-mode

 

Objective measures of Directional Mics
are useful to compare across Mics

n    Directivity Index-

–  ________ of sensitivity for frontal sounds relative to sensitivity averaged across all other directions

–  May range from .5 to 2 dB

 

n    Polar Directivity Plots (PDP)

–  a representation of the output of the HA in response to input from at several points within an imaginary ________ around the HA relative to the output at 0 degrees azimuth

–  Could be measured in both vertical and horizontal planes, but because symmetry is assumed in the vertical plane, only the horizontal is addressed

–  Common to show plots for several key frequencies, ie 500, 1000, 2000, 4000 Hz

–  Four main patterns: Cardioid, ___________, Bi-directional, Supercardioid

Audio Demo of Directivity Patterns

DEMO By GENNUM  http://frontwave.gennum.com

 

n    Front-to-back ratio

–  the output of HA when speaker is _____________ azimuth, subtracted from the output of HA when speaker is at 0 degrees azimuth

–  can measure using HA test box or using real ear measures

–  May range from 10 to 30 dB

Dual Mics

n    Three Main factors-

–  _________ between the ports

–  Time Delay

–  Match between microphones

 

n    DEMO By GENNUM  http://frontwave.gennum.com

 

What is important Clinically???

n      To determine that directional mic is working…ie, that there is a directional effect!

n      If rear mic clogged or electronic delay does not occur, there may be ___________ from rear mic

n      Therefore, when you fit a HA with a directional mic you need to ___________the directionality via a simple front-to-back comparison

 

Amplifiers

Manipulate _____ signal

Deliver power to __________

Class _________ circuits

Class A Circuit

Simplest circuit (first amplifier)

Poor battery efficiency

Low distortion (within linear range)

Tradeoff between distortion & battery drain

Relatively limited max power output

Smallest amplifier

Class B Circuit (Push/Pull)

High power; for severe/profound hl

Larger circuit (2 class As)

Some _________ distortion

Push/pull on diaphragm

No _____ current needed

Class C Circuit

Used in radio transmitters

Primarily _____ frequency

Not for use in hearing aids

Class D Circuit

Eliminates crossover distortion of B circuit

___ kHz peak

Peak close to adult ear canal resonance

Improved sound quality

________ battery life

Class H Circuit

Simple circuit like Class A

_____ cost

Increased battery efficiency

_______ to severe hearing losses

New circuit

Amplifier

Consists of Integrated Circuits which are a series of _________ and resistors diffused on a small piece of silicon and encapsulated in plastic

Transistors

The transistors regulate current flow from the _______ to the microphone

Resistors and Capacitors

Resistors and capacitors are used to modify ______ response and gain

Push-Pull Amplifier

Has two transistors arranges so as operate out of phase….

The alternating signal arrives at one transistor

and pushes it while….

Push-Pull

It arrives at the other and pulls it…

Results in twice the power compared to single

output amplifiers….

OR an increase in pressure of ?? dB

Receiver (Earphone)

Converts electrical energy to _______ energy which in turn is converted to acoustic energy

Receiver Types

Air or Bone Magnet

Conduction Coiled conductor

Receiver has Diaphragm

Principle

Current flow through a conductor results in _____ field

 

Internal Receivers

Power Supply

Amplifies the weak signal from the ________ and supplies voltage to the amplifier

Electrodes and Electrolytes

Combining electrodes (metals) and electrolytes (acids) causes a chemical reaction in which electrodes are liberated

Common Battery Types

Primary Cells

wpe5.jpg (32891 bytes)

There is a large number of electrons at one point and fewer at the other

Electrons collect causing ________ charge

Side with fewer electrons has positive charge

Current Flow

When a conductor is connected between the electrodes, the free electrons at the negative electrode move to the ________ electrode… which results in current flow

Secondary Cells

wpe3.jpg (14418 bytes)

 

Require electrical charging before using

Those used in hearing aids known as Nickel-Cadmium (NiCad)

Accumulators

Electrical energy applied to the accumulator is converted to stored energy which causes __________ when a conductor is connected

Battery Capacity

Quantified in millamphere-hours (mAh)

Hours of life = (Capacity (mAh)) / (Current drain (mA))

Current Drain

Current drain depends on output level which

is not always consistent therefore can’t predict hours of life exactly

Hearing Aid Options

Directional microphone

Output controls

____ controls

Telecoil

Audio ______

 

Directional Microphone

More sensitive to sounds coming from the front

 

 

 

Output Controls (SSPL90)

Limit the output of the aid so that sounds are not dangerously intense or uncomfortable

Input

Output

Compression (AGC)

Level detecting device reducs gain when a certain level is reached

 

Input Level Detector

Level detector before the gain control

 

Output Level Detector

Level detector after the gain control

 

Compression Characteristics

Attack time-Time for gain to be _______

Release time-Time for gain to be returned to ______ level

 

Compression Characteristics

Kneepoint-Level at which compression starts

Ratio-Change in input over change in output

Input/Output Function

 

 

(graph)

 

 

Tone Controls

Variations are controlled by resistors and __________

Capacitors pass high frequencies more easily than low frequencies which result in

a High Frequency Emphasis Aid

Audio Input

Direct electrical input to the hearing aid from

 

Telecoils

Convert magnetic lines of force into electrical energy

Principles:

Magnetic fields are created around conductors carrying current

Magnetic fields can induce _______ to flow

 

Electroacoustic Evaluation

Instrumentation Needed

General Idea

Hearing Aid Settings

ANSI S3.22

Change from 1982 to 1987

Involves tolerances for custom aids and a new way to specify average response for high-frequency emphasis aids

Change from 1987 to 1996