1
Q
Define gain
A
Gain refers to the difference between the input intensity (reaching the HA microphone), and the output intensity (from the HA reciever)
Gain = output - input
2
Q
Define frequency response
A
The gain at various frequencies
3
Q
Define saturation response
A
- Maximum output that the HA can produce
- In a saturation response, a higher input does not translate to a higher output
4
Q
Define input-output function
A
- Output as a function of input (the gain provided by the hearing aid will depend on the input level)
- Depending on the input, we will see a certain level of output (on the graph, at 45 dB, input and output are equal which is a 1:1 compression ratio)
5
Q
How does the sound signal get into the HA?
A
Microphones convert acoustic energy into electrical energy and deliver sound into the HA
6
Q
Do microphones have to be protected? If so, what from and how?
A
- Microphones of HAs are “electret” microphones, and must be protected in the devices themselves and from electromagnetic interference
- A microphone is typically surrounded by metal (microphone can)
7
Q
What makes the microphone work?
A
- Backplate amplifier
- FET
8
Q
What is the frequency range of microphones?
A
HA electret microphones have a wide frequency response (very suited for speech and environmental signals)
9
Q
What two things are HAs sensitive to?
A
- Moisture
- Humidity
10
Q
What is essential to microphones?
A
Highly important to keep the microphones clear of debris (which can block the mic port or damage internal components)
11
Q
Do microphones create noise? How much?
A
Yes, usually between 25-30 dB SPL
12
Q
What are some ways to keep a HA dry?
A
- Desiccant bead kit
- Electrical dryer
- Built in dryer to charger
13
Q
Where are the microphones located on a custom HA?
A
On the face plate
14
Q
The microphone is on the ____ of the HA
A
Outside
15
Q
What is telling the HA how to work and function?
A
The DSP (digital signal processing chip)
16
Q
What 5 things does the DSP chip do?
A
- User settings (volume control, audiogram)
- The processing algorithms (how to treat sound based on the environment)
- Instruction sets (battery usage, on and off)
- Timing and sequencing information (how quickly things happen, different environments)
- Memory (manual programs, data logging)
17
Q
What do hearing aids have to do first in order to process sound?
A
Convert the signal!
18
Q
What are the steps of signal conversion?
A
- Mic picks up sound
- Conversation from analogue to digital
- DSP chip does the processing of the signal
- Sound is changed back to acoustic energy and is delivered back to the ear through the receiver
19
Q
How does the sound get delivered?
A
- Though the receiver
- Receivers are the speakers of the HA: processed sound exits the HA to be delivered to the ear
20
Q
Do receivers vary in gain?
A
The smaller receivers have less gain potential than the larger receivers
21
Q
Where is the receiver in a custom HA?
A
Sound outlet
22
Q
Where is the receiver in a RIC?
A
Sitting in the canal covered by a custom piece or dome
23
Q
Where is the receiver in a BTE?
A
outside of ear delivered into ear by ear hook
24
Q
Explain wax guards
A
- Wax filters should be changed at least once a month
- If HA is sounding muffled, change the wax filter
25
Where is a wax guard in a custom HA?
Right at the end of the device
26
Where is a wax guard in a RIC?
When you take the dome off, the wax guard is there
27
What are 4 different options to change the volume on a HA?
1. Toggle/rocker switch (indents when you push it)
2. Wheel (on custom HAs, rolling forwards turns it up, rolling backwards turns it down)
3. Push button (great for 2 HAs: R HA turns both up, L HA turns both down)
4. Touch-sensitive surface (not the best because no tactile response)
28
What are some off-ear devices to adjust volume on HAs?
- Connection through app
- Remote control
29
Can you manually change program buttons?
- Changing between automatic and manual program stored in the HA
- Programs may also be changes by remote or app
- If there is a single button, change can be determined by how long you press the button
30
What are telecoils (t-coils)?
- An alternative way to introduce a sound signal to a HA wirelessly
- Picks up sound through an electromagnetic induction signal to provide a wireless signal direct to the HA
- Generally in a public venue with connection to landline phones (phones have a T-rating)
- Different models will denote if a t-coil is available (look for "T" on the model description)
31
How do t-coils work in public venues?
- Loop systems have great benefits when competing with background noise or distance
- various configurations to the HA microphones selected in the program settings (T, MT, mT)
32
What can t-coils be affected by?
Interference: fluorescent lights and electrical wires
33
What is a T tele-coil
Microphone muted, full induction signal (just telecoil)
34
What is an MT tele-coil
We want the microphone and telecoil signal to be equal (people get a boost from the telecoil, but still have access to the microphone)
35
What is an mT telecoil?
Reduce microphone quite a bit, but the telecoil is dominant
36
Explain assistive listening devices?
FM systems & DAIs
37
How does an FM system work? How is the sound recieved?
- FM receivers bring frequency-modulated sound from one source, directly into the HA
- The signal is received through a direct-audio input device, such as a "boot" or "Shoe"
38
What is the benefit of an FM system?
Improved SNR, and overcome distance
39
Who benefits most from FM system?
Do a SIN test to see who will benefit most from FM (these are ideal candidates)
40
How does wireless streaming work?
- Bluetooth allows the wireless streaming of audio signal from one device to another
- Bluetooth connections can be from several devices
41
Explain ingress protection
- How HA components are protected
- IP ratings suggest how well HAs can stand up to debris and moisture
- First digit: solid material debris/dust (1-6)
- Wax, dust, skin
- Second digit: moisture (1-8)
- Ex) IP65
42
How does a HA perform compared to how it should perform?
- It is imperative that prior to fitting a HA for a patient, we first are certain it has passed a level of quality control, which can be done:
- With the manufacturer: when aids arrive, there should be documentation that determines if it has passed
- In clinic
43
What do we do in clinic to see if a HA is performing well?
- We can perform an electroacoustic analysis (ANSI) of hearing instruments with the verification equipment to determine if it is performing well
- Verifit
44
What is ANSI?
- The american national standards institute
- Sets the standards for hearing instruments
- Defines parameters and the allowable tolerances for hearing instrument function (within specs)
- Quantifying if a HA is weak, distorted, noisy (measured in a test box)
- If a device does not meet specifications, it may indicate a need for repair
45
What does our hearing instrument analyzer (test box) do?
- We set the aid in the test box for ANSI testing, connect to the programming software to test mode
- We will determine if the HA is set to FULL ON GAIN (FOG) or REFERENCE TEST SETTINGS (RTS)
- All adaptive features of the HA should be turned off (feedback managers, noise reduction features, and compression setting)
46
ANSI testing - output sound pressure level with 90 dB input (OSPL90)
- What are we measuring?
- How much power output is the HA capable of?
- How are we measuring?
- A 90 dB stimulus, across a frequency range is presented, and the output is measured in the coupler
- Tolerance: within 3 dB of specifications
- Note: this is different than setting the MPO of the HA in fitting verification
47
ANSI testing - high frequency average output sound pressure level 90 dB input (HFA-OSPL90)
- What are we measuring?
- What is the average output of the frequencies of 1000, 1600, and 2500 Hz (unless a manufacturer sets a different set of frequencies)
- How are we measuring it?
- A 90 dB stimulus, across a frequency range is presented, and the output is measured, and the average of the frequencies aforementioned
- tolerances: within 4 dB of specifications
48
ANSI testing - response curve full-on gain with 50 dB inputs (FOG)
- What are we measuring?
- What is the output of the HA when HA gain is set to maximum, with a softer input?
- How are we measuring it?
- A 50 dB stimulus, across a frequency range is presented, and the frequency response output is measured
- Tolerances: within 5 dB of specifications set by the manufacturer
49
ANSI testing - high frequency average full-on-gain (HFA-FOG)
- What are we measuring?
- What is the average output with the 50 dB input of the frequencies of 1000, 1600, and 2500 Hz (unless the manufacturer sets a different set of frequencies)
- How are we measuring it?
- A 50 dB stimulus, across a frequency range is presented, and the output is measured, and the average of the frequencies aforementioned
- Tolerances: within 5 dB of specifications set by the manufacturer
50
ANSI testing - reference test gain (RTG)
- What are we measuring?
- The response of the HA at an appropriate user setting with a softer input (near average speech)
- How are we measuring it?
- A stimulus is presented (usually at 60 dB) across a wide range of frequencies, the response is measured with the volume control at a normal level (60 dB input = speech)
- Tolerances: informational purposes only, no tolerances specified by ANSI
51
ANSI testing - total harmonic distortion (THD)
- What are we measuring?
- Is there good fidelity of the signal from input to output? Are there any other frequencies or harmonics generated?
- How are we measuring it?
- Distortion is measured in percentages
- Tolerances: no more than 3% compared to manufacturing specifications
52
ANSI testing - equivalent input noise (EIN)
- What are we measuring?
- How much internal noise is the HA itself creating
- How are we measuring it?
- Calculation of the noise creating by the HA with no signal present
- Tolerances: no more than 3 dB above the manufacturers specifications
53
Where is noise created in a HA?
Microphone
54
What is expansion?
- Reduce the output for low-level inputs
- Can effect this measurement and the specifications sheets must note if expansion was used during measurement
55
ANSI testing - battery current drain
- What are we measuring?
- If the drain of the battery is reasonable, and is measured in mA
- How are we measuring it?
- Testing the draw of the battery in the following setting
- Quiescent, with 65 dB input, and 90 dB inputs
- Tolerances: should not exceed 20% of the maximum value specified by the manufacturer
56
What can battery drain depend on?
- Where the volume control is set
- The inputs
- The use of streaming
57
When is it a good idea to have ANSI testing performed?
Before the devices are set for patients
58
What are the 8 ANSI tests?
1. Output SPL with 90dB input (OSPL90)
2. High frequency average output 90dB input (HFA-OSPL90)
3. Response curve full on gain with 50dB input (FOG)
4. High frequency average full on gain (HFA-FOG)
5. Reference test gain (RTG)
6. Total harmonic distortion (THD)
7. Equivalent input noise (EIN)
8. Battery current drain
Amplification I
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