SPECIFIC FEATURES OF

HEARING AID

PRESENTED BY:Prabuddha BhataraiBASLP 2nd Year

Outline:

1. Concept regarding Routing of signals, Head shadow/Baffle/Diffraction effects

2. Output Limiting: Peak Clipping, Compression

3. Automatic Signal Processing

4. Programmable and Digital Hearing Aids

5. Signal Enhancing Technology

Routing Of Signals

1.IROS

2.CROS

3.BICROS

4.Stereo CROS ( CRIS-CROS)

5. Transcranial CROS

IROS

Ipsilateral routing of signal is merely a

conventional monoaural or binaural fitting.

The microphone mounted on the ear with decreased hearing feeds its output to the amplifier

on the same ear.

Simple CROS aidsThe microphone mounted on

the ear with the worse hearing ,feeds its output to the amplifier and receiver mounted on opposite side of the head.The receiver is coupled to the

ear using an open earmold , so that the unamplified sound can

also directly enter the better ear.

Bilateral CROS aids ( BICROS)

The sound is picked up by both microphones and

amplified signal is fed to one side

The BICROS fitting is useful where one ear has

substantially greater degree of hearing loss or worse

discrimination than the other ear or where chronic drainage percludes using an earmold or

even tubing earpiece.

Stereo CROS( CRIS-CROS)

The left microphone feeds the receiver on the right side and the right microphone feeds the receiver on the left side.

Invented with an aim of achieving high gain, combined with an open fitting , in both ears.

The hope was that because each microphone is separated by head from receiver to which it is directly connected,the feedback path would be weak.

Transcranial CROS aids

Also known as Power CROS/ Internal CROS

Transmits signal from one side of head to the other using bone conducted sound.

Intended for a person with no useable hearing in ear but who has to listen to

sounds arriving from that same side of the head.

Head Shadow Effect

Refers to how we perceive the difference of sounds volume between

ears which allows us to localize where the sounds are coming from.

Individuals with hearing loss in one ear have difficulty distinguishing

where sounds are coming because they can’t utilize head shadow effect to localize the different high pitches

sound.

Diffraction Effect

Diffraction is the bending of sound waves in the presence of obstacles in its path.

Dependent on the wavelength of sound and the size of the obstacles.

A small obstacle does not produce any hindrance in the path of sound waves or does not leave any shadow

When an obstacle which is much larger than the wavelength of sound wave, diffraction occurs.

Head Baffle

Refers to the fact that sound waves impinging on the

surface such as head will be reflected.

The incident and reflected waves may then combine

near the side of the head to create greater sound

pressure than that without the head.

Peak clipping

If the output signal is near the maximum and either the input level or gain of amplifier is increased, then amplifier will clip

(remove) the peaks of signal.

The deviation from output occurs when signal gets too large for an amplifier to handle properly.

An ideal amplifier would have no noise internally and would not distort the signal, no matter how large is the input signal.

All amplifiers create some distortion but

amplifier create large amount of distortion if

signal is peak clipped.

Distortion degrades quality of speech

when present in large amount.

Compression Amplifier

A compression amplifier is nothing more than an amplifier that turns down its own gain as

input to( or output from) the amplifier increases.

The major role of compression is to decrease the range of sound

levels in the environment to better match the dynamic range

of hearing impaired person

• Compression threshold : Defined as the input SPL which when applied to the hearing aid, gives a reduction in the gain of 2(+- 0.5)dBwith respect to gain in the linear mode.

• Threshold Kneepoint: The point at which the slope of I/O function changes is referred to as the threshold kneepoint(TK).

• Compression Ratio: It relates the change in input level to change in the output level.

• CR=ΔInput/ΔOutput

Automatic Signal Processing

Automatic Signal Processing refers to any circuit that automatically changes the way incoming signals are processed as a result of their characteristics.

Fixed Frequency Response

FFR systems maintain a constant frequency response shape regardless of input level, since all frequencies are compressed more or less equally

Traditional compression or AGC instruments fall under this category.

Types: 1.Compession Limiting 2. WDR compression

Compression Limiting

Compression limiting are

used to avoid peak

clipping problems

associated with linear

instruments.

Provides unity gain (1:1) until

kneepoint is exceeded

and compression

activated.

They tend to have high kneepoints

of compression

and high compression

ratios.

Above Figure has Knee point of 80 dBSPL and Compression Ratio of 5:1

WDR Compression Circuits

Dynamic Range: Person’s range of usable hearing.i.e UCL-Threshold of Hearing

WDR compression circuits has a low knee point with a compression ratios.

WDR circuits are most effective with persons with marked reduction in dynamic range.

• Above figure has a low knee point of 65 dBSPL• Compression ratio 2:1

Level Dependent Frequency Response Systems

LDFR systems provide frequency response shapes that vary depending on the input level as a result of a combination of compression and filtering.

BILL

Provides relatively broad frequency response in quiet setting

More bass response is provided for low level inputs than for high.

When input level increases a low-cut filter is activated and bass response is decreased.

Based on belief that reducing low frequency energy (background noise) will improve speech recognition ability in

unfavourable listening situation.

Manhattan III is an example of popular BILL device.

TILL

Provides greater treble response for

low level inputs than for high.

Provides significant high frequency

gain for soft sound with minimal high frequency gain for

loud sound.

The K-Amp is most commonly used

TILL Circuit.

PILL

Most ve

rsatile

type

Can provide either a BILL or a TILL

response.The instruments

must have minimum

of two channel

compression to

achieve PILL

circuitry.

Types Of Hearing Aids

Analog Hearing Aids

Programmable Hearing

Aids

Digital Hearing Aids

Analog Hearing Aids

Amplify the continuous sound waves by simply making it

larger.

Use physical Components such as microphone , an amplifier, a receiver , filters and resistors

that are designed to alter acoustic signal by changing the

signal electronically.

Block Diagram of Analog Hearing Aids

Programmable Hearing Aids

Have analog processing that is under digital control, allowing them to be programmable under computer control.

Sophisticated programmable devices have numerous frequency channels with non linear compression circuitry

Can be adjusted in all or some channels as well as several memories for storing responses programmed for different listening situations.

Flexibility and precision of electroacoustic adjustments can be made to fit an individual’s particular hearing loss configuration.

Digital Hearing Aids

Differs from other types that the analog signals from microphone are converted to digital form by analog to

digital converter.

In digital form, the signals are manipulated by sophisticated processing algorithms and then converted

back to analog form by digital to analog converters.

Along with flexibility inherent in advanced programmability, modern devices provide more precise

and flexible frequency shaping, more sophisticated compression algorithms, better acoustic feedback

reduction and enhanced noise reduction algorithms.

• Block Diagram of Digital Hearing Aids

Signal Enhancing Technology

There are various features of hearing aids that helps the listener to help improve

communication in difficult listening situation.

Channels

Separates sound signal into discrete

sections for processing.

With more channels, sound environment is

more accurate.

However, with some hearing losses , hearing aids with

many channels may not have significant improvement over hearing aids with fewer channels.

Directional Microphone SystemsDesigned to boost to sounds coming from the front of wearer and reduce sounds coming from behind

the wearer.

Improves speech understanding in

presence of background noise.

Digital noise reduction

Analyze the signal to determine if it contains unwanted noise.

If unwanted noise is detected , the system reduces the level of noise.

Makes the background or environmental noise less annoying for the hearing aid wearer and increases their listening comfort.

Impulse noise reduction

Detects any transient loud noises, such as car keys rattling , typing on

keyboard or dishes rattling and soothes them out so that they don’t

annoy the hearing aid wearer.

Digital Feedback ReductionFeedback management systems are used to combat the inevitable combat loops that occur when microphone and speaker are kept in close proximity.

Feedback loops create an annoying whistling sounds that can get in the way of listeners comfort.

Basic feedback management systems may reduce the overall amplification in order to reduce whistling.

Advanced feedback management systems reduce or eliminate whistling without affecting overall amplification of hearing aid.

TelecoilIs a wireless feature that picks up

electromagnetic signals from compatible telephones or looped room.

Telecoils can remove the signal to noise ratio because it directs signal directly to hearing aid processor without directing

it to microphone.

Public performances , tours and worships services are commonly made

accessible to individual with hearing loss via telecoil.

Bluetooth Compatibility

Enables hearing aids to connect to mobile phones

and other devices that uses bluetooth, often through

intermediary device.

Improves signal to noise ratio and eliminate

feedback from microphone.

Wind Noise Reduction

Is an advanced hearing aid technology that detects the

impact of wind blowing across the hearing aid

microphones and reduces or avoids amplification of it.

Especially helpful for people with hobby or jobs in those environment –

examples: Golfers or Boaters.

DISCUSSION

References• Hearing Aids; Harvey Dillon• The Hearing Aid; Kenneth W. Berger• Practical Hearing Aid Selection and Fitting; Henry Tobin• Clinical Audiology; Brad A. Stach• http://hearing.wustl.edu/HearingAids/

FeaturesofHearingAidTechnology.aspx• http://www.healthyhearing.com/help/hearing-aids/

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