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)
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)
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
