8 - Signal Processing Flashcards
When is reaching targets problematic?
Steeply sloping losses
What are 3 reasons why steeply sloping losses are problematic?
1) Achieving target gain does not make speech intelligibility better
2) Achieving target gain does not actually lead to audibility
3) Achieving gain leads to audible feedback
What is the problem with this picture?
- Patient isn’t hearing those gain targets in the high frequencies
- Clarity of speech, high frequency consonants, /s/, plurals
What is frequency lowering?
- Bring the high frequencies (speech signals preferably, and environmental sounds) to a lower frequency region so that they can be audible
- Shift the higher frequencies into a lower frequency range that is audible for the patient
Frequency lowering is ____ distortion
deliberate (we know the signal will be distorted)
What are the 3 types of frequency lowering used by manufacturers?
1) Frequency compression
2) Frequency transposition
3) Frequency translation (high frequency “reinforcement”)
What does frequency lowering do exactly?
Addresses dead regions in the cochlea
Hearing loss = ____ and ____ damage
OHC, IHC
Damage to OHC results in hearing loss up to ____ or ____
~50 dB HL (low frequencies), ~65 dB HL (high frequencies)
For loss greater than ____dB HL, there is likely also IHC damage
60-70
What is a cochlear dead region?
- A region in the cochlea where IHC are damaged, but a tone that produces peak vibration of the basilar membrane in that region will be detected at an adjacent location (if the vibration is great enough)
- IHC immediately adjacent to the dead region pick up the tone
What is used to diagnose dead regions?
- TEN test (Threshold-Equalizing Noise) may be used to diagnose dead region
- Measuring pure tone thresholds in TEN (threshold equalizing noise) masked noise
Where are dead regions most common?
More common in high frequencies than low frequencies
What does the prevalence of dead regions increase with?
Increasing degree of hearing loss
Prevalence of dead region at 4000 Hz is more than 50% when hearing thresholds exceed ____
70 dB HL
What 5 things does providing amplification to a dead region NOT do?
1) may not restore audibility
2) may not lead to better intelligibility
3) may not lead to ideal listening experience
4) make speech intelligibility worse
5) affect the potential for feedback
What 2 reasons are frequency lowering technologies recommended?
1) Dead regions
2) When high frequency targets cannot feasibly be met
What might patients with cochlear dead regions complain of?
- They aren’t hearing things well
- Things are mumbled
- Subtle differences in high frequencies
What are the 4 steps of verifying frequency lowering?
1) Conduct speech mapping as per standard guidelines with no active frequency lowering, with a full spectrum reference (speech), with patient thresholds present
2) Play high frequency filtered stimuli, or /s/ stimulus at 65 dB SPL (make sure noise reduction is off)
3) Does /s/ appear to be audible for the patient?
- If yes, frequency lowering may not be needed
- If no, frequency lowering may be warranted
Activate frequency lowering, replay the /s/ signal at 65 dB SPL
4) Have you observed a shift for improved audibility?
Frequency lowering: before and after (picture)
- /s/ not audible before frequency lowering
- /s/ is audible after frequency lowering
How does the frequency lowering technique; transposition work?
- High frequencies are “cut and pasted” to a lower frequency region; high frequency band eliminated
- The HA is making an assumption that at a particular area, there is going to be a region that is unaudible
- The HA will take the peak that was there and cut and paste it into a lower region of the audible spectrum
- Now we have frequencies that are stacked on top of one another
Who would do good with transposition and who would do poorly?
Good:
- This requires really good central auditory functioning so the patient’s brain can sort it out for distinction and meaning
Poor:
- APD and cognitive impairments won’t do as good with this
What is the benefit of transposition?
Giving accessibility (access to information they otherwise wouldn’t have access to)
How do you verify transposition? (picture)
How does the frequency lowering technique; frequency compression work? What are we reducing?
- High frequencies are “squeezed” to a lower frequency region, reducing the bandwidth and spectral details
- We are distorting the signal of the patient to give them access to these cues
- Other high frequency cues can be affected by this (can create some distortion for the patient)
- Processing has to be good in order to make good use of this
Frequency compression has a ____ and ____ associated with it.
CT, CR
What is the lower cutoff and upper cutoff indicating?
- The lower cut off = the first region is a conditional compression (the HA will be monitoring)
- The upper cut off = maximum audible bandwidth, maximum point of applying gain to the frequencies, always in compression
When we are moving towards audibility, we are…
…moving frequencies into a lower range
When we are moving towards distinction, we are…
…keeping frequency lowering to a minimum
Who is frequency lowering best for?
This is best for adults with an acquired hearing loss (getting the distinction of the /s/ back)
How does the frequency lowering technique; frequency translation work?
- High frequencies are not lowered.
- Instead, the hearing aid first identifies if high frequency energy is present (in a region that is probably not audible to a patient); if so, it replicates the envelope at a harmonically related lower frequency (where it is audible to a patient)
- 2 humps = 2 bands of energy that have been replicated
What is another name for frequency translation?
High frequency reinforcement
What manufacturers use the frequency lowering strategy of compression?
- Phonak: SoundRecover 2
- Unitron: Frequency Compression 2
- Signia/Sivantos: Frequency Compression
- Resound: Sound Shaper