Welcome to Hearing Aid Compression, Digital Microphones & Noise - - PowerPoint PPT Presentation

Welcome to Hearing Aid Compression, Digital Microphones & Noise Reduction

Presenter:

Ted Venema

Audiologist, Speaker, Author Ted Annis Senior Marketing Specialist Carrie Pedersen Member Services Supervisor IHS Organizers:

Housekeeping

• This presentation is being recorded
• CE credit is available! Visit ihsinfo.org for

details

• Note taking handouts are available at

ihsinfo.org on the webinar page. Feel free to download now!

Agenda

• Linear Gain
• Compression in Analog Hearing Aids
• Compression for Severe-Profound HL
• Compression for Mild-Moderate SNHL
• Compression in Today’s Digital Hearing Aids
• Q&A (enter your questions in the

Question Box any time)

HEARING AID COMPRESSION:

IT’S A GAIN ISSUE NOT A SIGNAL-TO-NOISE RATIO THING

Hearing Aids Must Do Two Things:

• Provide gain for the HL
• Increase signal-to-noise ratio (SNR)

Compression is a gain issue:

• SNR is a Dmic & DNR issue
• To be covered in another webinar

Fitting the Eye Versus Fitting the Ear

=

Intact Retina Damaged Hair Cells

Fig 3-1, Venema, T. Compression for Clinicians 2nd edition, Cengage 2006

Fig 1-7 Venema, T. Compression for Clinicians 2nd edition, Cengage 2006

Normal Inner and Outer Hair Cells

Damaged Hair Cells (mostly outer)

Fig 1-8 Venema, T. Compression for Clinicians 2nd edition, Cengage 2006

Here’s a “Passive” Traveling Wave

Base Apex

Basilar Membrane

A wave without outer hair cells

Fig 1-4, Venema, T. Compression for Clinicians 2nd edition, Cengage 2006

Outer Hair Cells Amplify and Sharpen the Peak!

Base Apex

Basilar Membrane

They are the “muscles” of the cochlea They usually get damaged first

Lows Highs

The “sharpening” Done by OHCs Cannot be Restored

This is why hearing aids for ears aren’t like glasses for eyes

Natural shape of fluid wave:

2 peaks from 2 tones close in Hz

Hair cell damage results in:

smaller rounded peaks

Hearing aids make wave bigger:

but cannot sharpen it Fig 1-6, Venema, T. Compression for Clinicians 2nd edition Cengage 2006

Output Limiting WDRC

The Sharpened Traveling Waves…

• Increases Hz resolution…
• Our ability to distinguish between Hz’s close together
• Our ability to separate speech from background noise
• To compensate, we can increase signal-to-noise ratio (SNR)
• By means of D mics, etc.
• A topic for another webinar

Output Limiting WDRC

We Can However, Amplify Traveling Waves…

• In the same manner as is done by the OHCs
• Amplify soft sounds especially below 50 dB HL
• Focus amplification on soft inputs
• Gradually decrease gain as inputs increase
• The focus of Wide Dynamic Range Compression (WDRC)

Reduced Dynamic Range

Focus of All Fitting Methods and Reason for Compression in 1st Place

O O O O O O O X X X X X X X

MCL UCL MCL UCL

Dynamic Range Dynamic Range

Reduced Dynamic Range Means We: Cannot “Mirror” Audiogram with Average Speech Inputs X X X X X X X

{

{

{

• Soft inputs by

full degree of HL

• Average inputs by

½ degree of HL

• Loud inputs by

little or nothing at all

We can amplify: UCL MCL

Idea Behind ½ Gain Rule: Place Aided Speech into Dynamic Range

100 110 120 250 500 1000 2000 4000 8000 Hz 10 20 30 40 50 60 70 80 90

A A A A A A A A

10 25 25 30 30 30 30 15

LDL for Speech

Until Late 1980s: Almost All Hearing Aids Gave “Linear” Amplification

Input

0 20 40 60 80 100 60 80 100 120

Output

{

Peak Clipping

Venema, T. Compression for Clinicians 2nd edition, Thomson Delmar Learning 2006

Linear means same Gain for all inputs along 450 In the case shown here, for:

• Whisper
• Talking

Output = Input + Gain

Output Limiting Compression Came Next

Similar to linear gain, but limit MPO with compression

Output = input + gain

High knee point:

60 dB SPL input

Linear gain:

Left of knee-point Output increase = input increase

High compression ratio:

Right of knee-point

Input

0 20 40 60 80 100 60 80 100 120

Output

For inputs > 60 dB SPL compression dramatically limits MPO

I/O Function for 2000 Hz

Output Limiting WDRC

Loudness Growth Became Focus in 1990s

Normal Hearing vs Mild-Moderate SNHL

Very Soft Soft Comfortable Loud Too Loud

10 20 30 40 50 60 70 80 90 100

dB HL

Along with Loudness Growth Came WDRC

Focus on imitating OHC amplification of soft inputs Cochlea is WDRC amplifier! Knee point is lower:

40 dB SPL input

As with OLC, linear gain is:

Left of knee-point

Compression ratio also lower:

As inputs increase, Gain is slowly decreased

Input

0 20 40 60 80 100 60 80 100 120

Output

For inputs > 40 dB SPL compression slowly limits MPO

I/O Function for 2000 Hz

Output Output Input

2:1

0 20 40 60 80 100 60 80 100 120

Input

10:1

0 20 40 60 80 100 80 100 120 140

Limiting WDRC

OLC vs WDRC

Fig 5-7, Venema, T. Compression for Clinicians 2nd edition, Cengage 2006

OLC: High Knee-point High Compression ratio WDRC: Low Knee-point Low Compression ratio

OLC vs WDRC: Displayed as Frequency Responses

Frequency Gain 40 dB 60 dB 80 dB Frequency Gain 40 dB 60 dB 80 dB

OLC adjustment: Affects Knee-point and Output WDRC adjustment: Affects Knee-point and Gain

Output Input Output Input

OLC WDRC

max min max min

Different Ways of Adjusting Compression

Summary A Clinical “Spectrum” of Compression

Linear Limiting WDRC

High Compression ratio for:

• Loud inputs only

Adjusted by:

• MPO changes

Low Compression ratio for:

• Soft & medium inputs

Adjusted by:

• TK changes

Associated with:

• Peak clipping

Adjusted by:

• MPO changes

Summary: Clinical Application of Compression

100 110 250 500 1000 2000 4000 8000 Hz 10 20 30 40 50 60 70 80 90

WDRC

Mild-moderate SNHL

OLC

Severe-profound SNHL

Two (or more) Knee-points

Linear, WDRC, Output Limiting

Input Output

TK 1

Expansion

TK 2

Input Output

25 45 65 80

Expansion WDRC More compression ratio Linear gain Output limiting compression

Today’s Digital Hearing Aids

I/O Functions with Multiple Knee-Points

Note use of Linear gain for mid to slightly higher inputs

Is Linear Gain All That Bad?

• As long as it does not distort, it can sound “clean”
• That’s why HA manufacturers use it for some inputs
• Eg. average to slightly louder speech inputs

For the complaint:

• “I can hear others at tables further away better

than the person across the table!”

• ADRO (adaptive dynamic range optimization)
• Is an interesting departure

WDRC Shown On An Input/Output Function

Note linear gain only for very soft inputs up to 40 dB SPL; WDRC occurs for average and louder inputs. As inputs increase from 40 to 100 dB SPL, the outputs increase by

• nly half as much. This is a compression ratio of 2:1. This is how WDRC amplifies soft

sounds by a lot, and louder sounds by progressively less and less 40 dB SPL input + 50 dB gain 90 dB SPL output 60 dB SPL input + 40 dB gain 100 dB SPL output 80 dB SPL input + 30 dB gain 110 dB SPL output Etc.

Input

0 20 40 60 80 100 50 70 90 110

Output

130

Knee-point

ADRO Shown On An Input/Output Function

Greater and Lesser amounts of linear gain are provided, depending on the listener’s sound environment; in loud environments, less linear gain, in soft environments, more linear gain. The focus: listener’s comfort

Input

0 20 40 60 80 100 50 70 90 110

Output

{

Maximum Output

130

Note that a shift to the right actually shows a decrease in linear gain!

• Eg. a 40 dB SPL input results

in only a 70 dB SPL output, so the gain now is only 30 dB

WDRC: What it Does to Speech

Unaided Speech Aided Speech

The top sound wave represents an example of a sentence spoken at an average conversational loudness level. The peaks are the louder parts of speech; namely the

• vowels. The valleys are the softer parts; namely, the unvoiced consonants, like s, f, t,

ch, sh, etc. The bottom sound wave represents the same sentence amplified with WDRC. Note how the overall sound wave is amplified, but the peak-to-valley contrast is decreased. WDRC amplifies soft sounds by a lot, and louder sounds by less

Aided Speech with ADRO

The top sound wave represents the same sentence spoken at an average conversational loudness level. Remember, the peaks (vowels) are the louder parts of speech, while the valleys (sounds like s, f, t,ch, sh, etc., are the softer parts. The bottom sound wave represents the same sentence amplified with ADRO. Note how the overall sound wave is amplified, and the natural peak-to-valley contrast is preserved.

ADRO: What it Does to Speech

Unaided Speech

Expansion: Also Known As Internal Noise Squelch

0 20 40 60 80 100

Input

0 20 40 60 80 100 40 60 100

Output Input

20 40 60

Gain WDRC

Input/Output function

80

Input/Gain function

20

80 input = 100 output (gain = 20) 60 input = 90 output (gain = 30) 40 input = 80 output (gain = 40) 20 input = 40 output (gain = 20) 0 input = 0 output (gain = 0) Expansion: A greater-than 1:1 compression ratio Expansion with WDRC means greatest gain at (and only at) the kneepoint

Dynamic Compression Characteristics

Sudden changes to input intensity

• ver time

Attack Time Release Time

Hearing aid response to the changes

Types of Dynamic Compression

• Automatic Volume Control

Slow attack/release times

• Syllabic Compression

Fast attack/release times

• Average Detection

Attack/release times vary depending on input Transient inputs receive quick attack/release Slower inputs receive longer attack/release

• Digital Hearing Aids Today Default To:

Syllabic detection for low Hz’s Average detection for high Hz’s

Questions

Enter your question in the Question Box on your webinar dashboard

THANK YOU FOR ATTENDING!

Ted Venema tvenema@shaw.ca For more info on obtaining a CE credit for this webinar, visit www.ihsinfo.org