Lecture 8 Flashcards
What is gain?
- Gain refers to the difference between the input intensity (reaching the hearing aid microphone), and the output intensity (from the hearing aid receiver)
- Gain= Output - Input
What is frequency response?
Frequency response refers to the gain at various frequencies
Frequency response is ____ dependent
Frequency
What is saturation response?
- Maximum output that the hearing aid can produce. In a saturation response, a higher input does not translate to a higher output
- Top level volume
What is input-output function?
Visual representation of output as a function of input (the gain provided by the hearing aid will depend on the input level)
What is linear amplification?
- In linear amplification, the amount of gain applied is the same, regardless of the input. This is a 1:1 relationship
- Equal input and equal output
- Linear HAs also have a maximum output (the top level that the HA can produce)
- The way we get to the maximum output is by the gain and the limiting system of the HA
Linear gain is the ____ degree angle
45
What is non-linear amplification?
- In non-linear amplification, the amount of gain applied depends on the input (i.e. soft, moderate, or loud intensity sounds)
- As the input level gets higher, the gain will change to affect the output
Explain dynamic range in someone with SNHL (picture)
- SNHL has a reduced dynamic range (they need inputs to be brought up to a level of audibility, however, the loudness comfort of the patient doesn’t change that much)
- A: normal hearing
- B: SNHL (blue bar is showing the reduced dynamic range)
- C: Now this individual has linear amplification (need a mechanism in place to make loud sounds less loud)
- D: Now this individual has compression (placing all environmental sounds in that range)
- Provides comfort, audibility, and a difference between soft, medium, and loud sounds
What 4 things does compression do?
- Compression helps to preserve the waveform and maintains speech recognition ability
- Maintains a perception of “soft”, “moderate”, and “loud” sounds by fitting the sound into the residual dynamic range
- Maintains a comfortable listening levels for patients
- Reduces the adverse effects of loud inputs
What is compression ratio?
- The ratio of incremental change from input to output level
- Change in Input vs. Change in Output
- The reference for this is always to 1 dB change in output
- For example a 2:1 CR indicates that a change in input of 2 dB, results in 1 dB change in output
- CR = delta input/delta output
CR 1:1 and CR 2:1 (picture)
CR 1:1
- Linear amplification
- More gain for soft sounds
CR 2:1
- The HA is saying once I reach a certain intensity, our compression ratio will kick in
- Putting the breaks so sound isn’t too loud
When the curve flattens out we reach the MPO
- The maximum power output of the HA for safety and comfort
It is important to limit the ____ at ____
Output, high inputs (controlling the gain at high levels)
- may have a CR of 2:1 at high inputs
How do you control the gain at high levels?
- “Automatic Gain Control” (AGC); the gain is automatically adjusting based on the input level
- Compression may also be referred to as “Non-Linear Gain”
What happens without compression?
- Before compression was possible, an alternative to limiting the output could be achieved by Peak Clipping.
- Peak clipping results in distortion, and a poor speech perception, and degrades intelligibility
- The output is no longer a sine wave, as there is missing information to process from in the input signal (the top part of the amplitude of the signal is chopped off)
- However, this gets the job done and still creates a safe listening environment for the patient
What is the compression threshold (CT), or knee point?
- Compression Threshold or Kneepoint refers to the level set where compression will be activated. In an I/O graph, this is the “bend” deviating from the 45 degree (linear gain) angle.
- The knee point might be 40 dB (after an input of 40dB, the HA will start to compress that sound)
What is wide dynamic range compression (WDRC)?
- WDRC is compression activated across a wide range of inputs (i.e. a low knee-point or compression threshold (CT)
- A HA that is activating compression at a fairly low-level input
- Ex) 4:1 (4dB in and 1dB out)
Compression is not ____
Instantaneous
What is attack time?
Attack time refers to the time it takes to turn down the gain in response to an increase in the signal level. The attack times can vary (usually 1-20 ms)
What is release time?
Release time refers to the time it takes to turn up the gain in response to a decrease in the signal level. The release times can vary (20-30 ms up to approximately 5 seconds)
What happens if release time is too fast?
Slight distortion and “pumping” may be audible
What happens if release time is too slow?
If the delay in the gain restored slowly, there may be audibility gaps for our patients
How is compression controlled or set?
- Remember: we can adjust the amount of gain at soft, moderate, and loud inputs in the software BUT manufacturer’s can have different ways of making this adjustment
- Generally, we are not specifically selecting a compression knee point or choosing a ratio, but changing the gain at different inputs effects these two points.
What happens if we reduce the knee point?
If we reduce the knee point, it means that softer sounds are going to get more of a boost
What happens if we increase the knee point?
If we increase the knee point, it means that softer sounds are going to get less of a boost
Explain this picture
- Soft sounds are getting more gain then the loud sounds (a compression system)
- Dashed line is a feedback estimation line
- Very top line is the MPO of the HA (the HA cannot provide more gain than that)
If we increase sounds, we are ____ the compression ratio
Reducing
Ideally, we try to keep the compression ratios of ____ or less
2:1
higher compression is sound that isn’t as ____ to patients
Natural
What is headroom?
- Headroom refers to the dB range between the upper level of amplified speech and the hearing aid’s Maximum Power Output (MPO)
- Optimizing the headroom is ideal so that the peaks of speech can have fluctuation and variation while remaining comfortable
What is compression limiting?
Compression limiting occurs when the compression setting limits the output of the hearing aids
What are the two components of compression limiting?
Commonly the result of AGCo ,but can be AGCi
What is the AGCi?
- Automatic gain control input
- Input controlled compression systems detect the level input before the volume control of the hearing aid. The gain is reduced at the pre- amplifier of the hearing aid
- Sound comes into the HA and the level detector is happening before the pre-amplifier
What is the AGCo?
- Automatic gain control output
- Output Controlled Compression systems detect the level of input after the volume control of the hearing aid. The gain is reduced at the output amplifier of the hearing aid
- Gain comes into the HA and is modified and the compression system is reducing gain at the output past the pre-amplifier
Does the MPO change with AGCo?
The MPO doesn’t change no matter what position the patient has the volume in
Does the MPO change with AGCi?
Yes
What is expansion?
- Expansion is a technique used in hearing aids to make low level sounds softer
- Expansion reduces the gain for low-level sounds from the environment, and attempts to make the noise generated from the hearing aid lessened
- Expansion, like compression, can have a kneepoint, attack and release times, and a ratio
What is the expansion line?
- Below expansion knee point it is making low gain even softer (less gain for soft sounds)
- When you do REM, it will help confirm if the levels have been set correctly (gets it bang on for the patients)
Is there any downside to making soft sounds audible?
If expansion system is too aggressive, we can be affecting the audibility of the softest sounds (between the HF consonants /s, z/)
Explain the Barbie and compression anaolgy
- Think about compression like a car driving down a street
- Driving towards a stop sign
- Speed = gain
- Breaks = compression system
- Rapidly approaching the stop sign (a lot of gain), she slams on the breaks = linear processing with peak clipping
- This time she is driving and when she is about 15m away, she puts on the breaks and comes to an abrupt stop = linear amplification with a very high knee point
- Now, a km away from the stop sign and presses on the break and stops right at the stop sign = WDRC
- If pressing on the break as she slowly gets to the stop sign = WDRC with multiple knee points
