1. Compression 101

2. Goals
Achieve a basic understanding of various compression characteristics
Why is it important
Do I really need to commit this to memory?
Clinical impact
Adult versus pediatric considerations
Troubleshooting with compression

3. Linear
Definition: “A hearing aid circuit that provides the same amount gain for all input levels”
Textbook of Hearing Aid Amplification 2nd Edition. R.Sandlin

4. Compression
“Compression has many faces, there is no one simple way to describe it”
“To appreciate compression function, we can liken it to a piece of sculpture. We need to walk around and view it from several different angles to appreciate the entire piece of sculpture.”
Theodore Venema, PhD
“The major role of compression is to decrease the dynamic range of the signals in the environment so that all signals of interest can fit within the restricted dynamic range of a hearing-impaired person”
Harvey Dillon, Ph. D.

5. Compression Quiz Input
The signal in dB SPL coming into the microphone of the hearing aid.
Gain
Gain is the amount of amplification added to the input signal. Output minus input equals gain.
Output
The signal in dB SPL which is delivered into the patient’s ear.
Threshold Kneepoint
Point at which input-output function changes. (gain line takes a bend)
Compression Ratio
Function determining how much gain is provided by the hearing aid for the incoming sound.

6. Amplification Formula For Hearing Aids
Input +
Gain =
Output
/s/

7. Where is the limit?
Maximum Output (SSPL): The level beyond which the output can not exceed.
Measured when hearing aid is in saturation
OSPL 90
Importance of output limitation
Keep output of hearing aid from exceeding tolerance level of hearing aid user
Preference for limiting output without distortion

8. Methods of Output Limiting
Peak Clipping
Compression Limiting

9. Peak Clipping vs. Compression Limiting40 60 80
100 20
120
Input Intensity level, dB
Output Intensity level, dB
OSPL90
Basic graph showing the different input/output function of a linear and output compression circuit. Quick explanation of what happens to amplified sound when it comes up on the ceiling. Lead into the next slide

10. Peak Clipping
Graphic demonstration of a sound wave reaching saturation level and the distortion it creates. Santa is a different demonstration of the same concept.

11. Compression
Graphic demonstration of the same sound as the previous slide as it gets compressed to avoid distortion. Same for Santa example.

12. “Comparison of Sound Quality and Clarity with Asymmetrical Peak Clipping and Output Limiting Compression” Hawkins, D. Naidoo, S.: J Am Acad Audiol 4: (1993)
Figure from study showing the detrimental effect of peak clipping on a sound. This supports the previous three slides.

13. Two Basic Categories Of Hearing Aid Signal Processing
Linear
Unity gain until SSPL has been reached.
Results in peak clipping
Non-Linear
Gain changes as a function of input levels.
Amplification characteristics dependant on type of compression.
Summary of previous information

14. Non-Linear Amplification
Compression
1. Input/Output Compression
(all WDRC is input compression)
Compression Controls:
2. Regular vs Threshold Knee Points (TK’s)
3. Output Limiting vs Wide Dynamic Range Compression (WDRC)
(BILL/TILL: types of WDRC)

15. Examples of non-linear processing characteristics…
I/O functions
Compression Ratio
Output
Bands versus Channels

16. Basics of Compression – Input/Output functions40 60 80
100 20
120
Input Intensity level, dB
Output Intensity level, dB
OSPL90
TK = 50dB
10dB
50dB
20dB
Points to be made:
TK explanation
CR explanation
20dB
CR = 5:1
CR = 1:1

17. Input or Output Compression
Input Compression
VC located between the amplifier and the receiver
Output Compression
VC located between the mic and the amplifier
Ask group if they can differentiate between the two types of compression before showing the difference.

18. Input Compression - AGCi40 60 80
100 20
120
Input Intensity level, dB
Output Intensity level, dB
Amp VC
Rec
Mic
VC = Max
VC = Min
Graphical demonstration of i/o curves for input compression circuit and where the VC is placed in the amplification chain. Kneepoint stays at the same place for this type of compression since it has been set in the compressor/amplifier before the volume control can make changes. Important: gain above the kneepoint changes with the VC adjustments. MPO also changes.

19. Input Compression - AGCi
MPO dB
Continuation of previous slide. Since the VC is before the amplifier, the lowered voltage entering into the system results in a change in gain and MPO. This type of compression works well for mild to moderate hearing losses since the gain requirements are not that high, risking setting the MPO too loud.
Gain
Frequency

20. Output Compression - AGCo40 60 80
100 20
120
Input Intensity level, dB
Output Intensity level, dB
Mic
Amp VC
Rec
VC = Max
VC = Min
Output compression depiction with VC before the amplifier, resulting in changes in kneepoint location. VC Min has TK at 75dB resulting in 95dB output; therefore 20dB gain. VCMax has TK at 40dB and output at 90dB; therefore 50dB gain. Used for more severe hearing losses where a more linear approach provides greater gain for all inputs and MPO is the same.

21. Output Compression - AGCo
MPO dB
Gain
Frequency

22. Output Limiting vs. WDRC40 60 80
100 20
120
Input Intensity level, dB
Output Intensity level, dB
WDRC
CR < 5:1
TK <~ 55dB
Output Limiting
CR > 5:1
TK >~ 55dB
Differences in TK and CR that define WDRC and Output Limiting compression.

23. Bands and Channels
Bands are distinct frequency regions that can be shaped to change the dynamics, however compression characteristics cannot be altered.
Channels allow for adjustments to be made to the dynamics and also the compression characteristics.
Basic definitions of the differences between channels and Bands. There will be a visual explanation of multi-channel compression later in the ppt.

24. Advanced Compression functions
Expansion
Dynamic Compression characteristics
Multi-channel hearing aids

25. Expansion 120 100 Output Intensity level, dB 80 Additional TK 60 40
Input Intensity level, dB
Output Intensity level, dB
Additional TK
Graphic demonstration of the changes that occur to the gain of a hearing aid when the CR and TK are changed at the soft end of the amplification spectrum. As the TK is raised the gain is reduced for sounds below that TK. This will attenuate gain for soft inputs without affecting gain for moderate and loud levels. TK adjustments the same concept as output compression, just at a very low level.
Additional CR

26. Dynamic Aspects of Compression
Unlike static features like TK and CR, dynamic aspects of compression deal with the constantly changing intensities of our environment.
They are:
Attack and Release times

27. Attack/Release times
Attack times are usually very short (<50ms) in order to quickly react to the changed SPL.
Release times can vary from fast (50ms) to very slow (several seconds).
“Syllabic compression” seems to be a popular model since the attack and release times fall within the time of average syllable length – ms

28. Attack/Release times – cont’d
Possible pitfalls:
Release time too fast: “pumping” sound is heard.
Attack time too slow (150ms) the device will not react quickly enough to dynamic changes in environment.
Release time too slow may not allow amplification to return to higher gain levels once louder sound has ceased, thereby under-amplifying softer consonant following louder vowels.

29. Multi-Channel Compression
4000Hz
2000Hz
1000Hz
500Hz
250Hz
Graphic demonstration of the various compression characteristics that multiple channels can have in order to provide more precise amplification for the varying hearing loss thresholds. This means more appropriate amplification for all frequencies compared to a hearing aid that has fewer or only a single channel. Some channels may appear more WDRC and others more like output limiting. Significant benefit of multi-channel digital amplification to have this flexibility.

30. Channels and Compression
Compression can vary in multi-channel hearing aids.
In order to maintain a smooth response (no distortion) averaging of compression characteristics may occur.
Averaged levels are less precise than actual levels.
CR = 8:1
CR = 1.5:1
CR = ?
The risk of having significantly different compression characteristics in neighboring channels is that since they overlap, certain inputs need to be averaged rather than getting two significantly different commands, which would result in worse sound quality.

31. Multi-Channel Compression Concerns
Based only on the output graph, it is unclear what the compression ratios are for the varying channels.
This is a less than ideal compression situation!
Due to flexibility of multi-channel amplification, care must be taken to provide appropriate compression.
“:any multi-channel compression device risks distorting the normal loudness relationships internal to phonemic speech elements, relationships which provide cues for phoneme identification.”
Mead C. Killion, Ph.D.

32. Rationales for Compression
Avoid Distortion and Discomfort
Decrease inter-syllabic level differences
Increasing sound comfort
Normalizing Loudness

33. Why So Many Different Types? Isn’t “Digital” all we need today?
Digital hearing aid have _______________ type of signal processing
A conventional hearing aid (yes the ones with trim pots) are always linear? (True/False)
In terms of fitting adults or pediatrics, signal processing is not very important. (True/False)

34. Things to consider with adult fittings…
Amplification history
Length of time using previous technology
Goals for new hearing aids
Counseling, realistic expectations, acclimatization
Choice of hearing aids
Severity of loss; does it matter?

35. Things to consider with pediatric fittings…
Amplification history?
WDRC IS the gold standard
Yes digital is ideal
Maximizing the fit to ensure Audibility above all (DSL: Does this ring a bell?)
Older pediatric patients with a history of linear amplification. What to do?
Severity of loss does it matter?

36. Studies show that…
…the choice of the preferred signal processing is influenced by:
Hearing loss (degree and configuration)
Listening situations
Previous experience with amplification
Acclimatization to amplification
We know that the choice of the best signal processing strategy goes beyond the client’s hearing loss. It should also take into account personal sound preferences, the sound environments the wearer is in and their previous amplification history.
A ‘one size fits all’ signal processing concept simply does not address the varying needs and experiences of hearing impaired clients, let alone their various hearing loss configurations. The use of different processing and performance enhancer settings results in greater user benefit and speech understanding in different listening environments.
For more detail on this, see the Valeo SoundSelect Manager background story.

37. Troubleshooting with Compression – Part 140 60 80
100 20
120
Input Intensity level, dB
Output Intensity level, dB
Doctor,
Traffic noise is too loud, but speech is just right! What can you do for that?
Increase CR .
Result:
Loud sounds more comfortable, without affecting speech!
Δ TK to lower level.
Result:
Loud sounds more comfortable, but speech is softer!

38. Troubleshooting with Compression – Part 240 60 80
100 20
120
Input Intensity level, dB
Output Intensity level, dB
Doctor,
Whenever the fridge runs it sounds too loud. Plus, I don’t seem to hear conversation loud enough!
In this example the important point is the priority of the complaints. Since hearing speech is of greater importance it should be handled first; then the second complaint can be tackled with expansion controls. If the clinician focuses on the noise complaint first, the focus is away from the primary reason of the instrument. Stress that making changes with the amplification should occur only after the full picture of the situation is known.

39. Take Home…
Keep asking questions when the patient makes statements about problem situations. It may uncover different, more appropriate, solutions.
Know your programming software. Know the rational for the changes you are making.
Understand the concept, and you will be able to apply it to the fittings and adjustments.

40. That was it! Nikolas Klakow, AuD Northwest Customer Trainer
Questions to: