Hearing Aid Components
Describe Class B amplifiers
- 2 amplifiers (1 for positive phase of input signal and 1 for negative phase of signal) - minimal current drain when idle (2 separate amplifiers) - higher saturation level and more symmetrical peak clipping than Class A an - Cross over distortion b/c of the 2 amplifiers at the activation of one amplifier and the deactivation of the other amplifier - good for high power HAs - larger - more expensive than Class A
Describe a dual mic directional system.
- 2 omni directional mics, each with 1 port - made directional by introducing a delay - output from mic #2 is electronically delayed and subtracted from mic #1 - electronic allows us the flexibility to have delay ON or OFF (omni, rear mic off)
Explain the advantages of digital filtering over analog filtering.
- Digital filter is more precise, more complex shapes possible b/c of ability to change gain for more specific frequency regions, but depends on the the manufacturer controls allowed to you (i.e., how many channels they give access to manipulate) - Analog filtering has less precise frequency shaping; passive tone control (shallow slope, gradual transition b/w attenuated and passed frequencies, requires no power); active tone contrl (steeper slopes possible, cleaner transition, consumes small amount of power) - Digital better than passive or active analog filtering
Describe the limitations of a single-mic dual port directional mic system.
- Directivity may be limited by low pass filter cutoff (might not decrease all the noise b/c low pass filter cutoff is too low) - Low frequency cut in gain (b/c noise is LF then you try to cut some of the low frequency gain, but sometimes you need more low frequency gain - may need to boost LF gain, but then the mic noise will increase also) - May need omni directional microphone in some situations (this is always directional)
Describe the difference between first order and second order directional microphones. Which system provides better directionality?
- First order = 1 difference (b/w 2 mics or 2 ports) - Second order = combines the signal from either 2 directional mics or 3 omni directional mics - Second order could provide a little more than a 3 dB improvement in directivity compared to first order
What are the types of distortion?
- Harmonic distortion - Intermodulation distortion - [Cross over distortion] - [Overload distortion ]
What is the difference between internal and external feedback?
- Internal feedback: happens in the HA itself; plug the out of the HA (sound cannot escape) and you still hear whistling; indicates that a seal inside the HA (between mic and receiver) is broken - External feedback: most common; sound out leaves the HA and is picked back up by the mic
What is the difference between a "low-cut" and a "high-cut" tone control?
- Low-cut (i.e., high pass filter) = pass the highs, cutting the lows - High-cut (i.e., low pass filter) = pass the lows, cut the high s
What type of saturation level do you want for a HA?
- a higher saturation level = hearing aid less likely to distort - saturation level = the level at which an increase in input no longer results in an increase in output (exceed = distortion)
Define distortion.
- a signal that contains additional components that were not present in the original signal (distortion products)
Describe the function, basic characteristics and limitations of the HA transducer microphone.
- acoustic to electrical; input transducer - electret microphone; essentially flat frequency response;variation is frequency response possible
Describe Class H amplifiers
- aka "sliding class A" - like Class A but with variable bias current (strong signal = put more current in, weak signal = put less current in) - more efficient than A - adds distortion to signal if slides to fast or too slow (fast = change is audible, slow = peak clipping)
Describe the basics of a directional microphone system.
- depends on external (b/w 2 ports) and internal (damper effect) time delays - damper acts as a low pass filter and those sounds that are passed are amplified with some delay - sounds entering the rear ports are delayed relative to the front port (external) - OVERALL: damper in rear port acts like a low pass filter and delays the sound coming in from the back leading to partial cancellation
Describe intermodulation distortion.
- distortion products are the combinations of the frequency components of the signal - can be sum or difference tones - only occurs for complex signals
Describe harmonic distortion.
- distortion products harmonically related to the frequency components of the signal - can occur for pure tones and complex signals - signal drives amplifier into saturation and you will get distortion products that are harmonically related to the signal; distorts the waveform when they combine
Describe the function/basic characteristics of the HA transducer receiver.
- electrical energy to acoustic energy - frequency response: one major peak (receivers resonance), smaller peak (acoustic path), extra peaks (BTE = other resonances, tubing)
Why does feedback sometimes occur with a microphone but not with a telecoil?
- feedback primarily is caused by sound coming out of the HA receiver and the HA mic picking that sound back up - when the HA is in telecoil mode it using a magnetic signal, not an acoustic signal so the HA mic is turned off and feedback is not possible
What does the term frequency shaping mean? When is this done?
- filtering the amplified signal entering the HA so that the output of the frequency response is different that then input frequency response - this is done b/c most people dont have equal hearing loss across frequencies, so they will not need equal amplification in all areas; need to "shape" the frequency response of the HA based on the patients specific hearing loss and provide appropriate gain and specific frequencies
Describe the primary characteristics of a hearing aid receiver's frequency response.
- frequency response = amplitude of sound coming from hearing aid plotted for different frequencies - Major peak = receiver's resonance - Smaller peak = acoustic path - Other resonances (= BTE, tubing)
Describe the limitations of the HA transducer telecoil.
- frequency response: 6 dB/octave cut off in low frequency (may not be enough low frequency for some people) - electromagnetic interference (present in some environments and can create a buzzing or clicking that interferes with the sound quality of the signal) - telecoil is directional: telephone use = horizontal wires is best; loop system = vertical wires best
Explain how reverberation affects directional microphone performance.
- front signals can reverberate off back walls and vice versa - directional mics work by delays so if noise from the back and bounces off a wall and is then picked up by the front mic it will get amplified
Describe what a "filter (tone control)" does.
- implements frequency shaping - changes the gain in different frequency regions
Describe polar plots.
- indicates directional sensitivity of a microphone at one frequency - response shape dependent on ration of internal to external delay - show what happens when HA is dangling in space, doesn't account for head affects - gives a 360 view of how the microphone responds and the amount of attenuation relative to the front at different locations - Limitations: doesnt account for the head effects; need for different frequencies; difficult to summarize on spec sheet
Describe Class A amplifiers.
- low battery life (continuous drain) - if you improve battery life, you lower saturation level = saturation distortion occurring earlier = asymmetrical peak clipping - inexpensive
Describe the function/basic characteristics of the HA transducer telecoil.
- magnetic to electrical; input transducer - wire coil produces voltage in response to a magnetic field produced by telephones; converts the field to electrical energy - composed of a bar of metal wound with many turns of fine wire; electromagnetic wave induces a voltage at the 2 terminas of the telecoil; voltage is processed by the amp
Describe the limitations of the HA transducer microphone.
- major limitation = complete failure due to exposure to chemicals/moisture/debris/dust and everyday wear and tear - random electrical noise; sensitivity to vibrations - wind noise (esp on BTE, can create LF sound - [internal feedback - not completely mics fault due to broken seal]
Describe a microphone array directional system.
- more than 2 mics - more mics = more directivity (space limitations) = more mic noise - an accessory or built into HA - rely on internal and external time delays
What is the primary difference between omni and directional microphones?
- omni = equally sensitive to sounds coming from all directions - directional = suppresses sound coming from some direction more than sounds in the other direction; dependent on delays between HA front and back (sensitivity dictated by polar plots and directivity index)
Describe a single mic - dual port directional microphone system.
- one microphone, 2 openings (front and rear) - acoustic damper in the rear port - sounds entering the rear port delayed (and partially cancelled)
Describe the limitations of the HA transducer receiver.
- output is related to receiver size (larger receiver = larger output it can produce); small HA can only fit a small receiver so limited in output range - susceptibility to wax: #1 reason for HA "malfunction"; primarily with custom products and RICs - unwanted resonances (peaks in frequency response - BTE aids) - produces harsh quality and can cause feedback problems
Describe the Directivity Index (DI)
- single number characterization - ratio of sensitivity for front sounds re: sounds from all directions - want number to be positive - most popular single number measure - EX: if you have a DI of 3 dB = sound coming from the front is 3 dB higher than the sound coming from all other angles at the same time
Describe the Front-to-Back ratio (FBR)
- single number characterization - used in research, a little less realistic - ratio of sensitivity for front sounds re: sounds from the back - Limitation: doesnt consider a diffuse noise environment; similar to what is done in test box, not very real world
Explain what is meant by "acoustic feedback" and how does this occur.
- sound coming out of the HA travels out of the aid and is picked back up by the HA microphone - generally a high pitch squealing - can have internal and external feedback
Describe the applications of the telecoil
- telephone (eliminates feedback b/c mic is off): can hear on phone w/o interference from outside acoustic sources or can have combo of both - Induction loop: listening in rooms where you need some amplification; sound sources has a mic which leads to amp which sends current to a wire loop around the room. The person inside the loop turns to T mode and then they amplified signal is sent to their HA
Describe the directional sensitivity of telecoils including the best orientation for telephone and induction room loop applications.
- telephone use = wires horizontal - loop use = wires vertical - custom produces you can decide which way, most cases go halfway between those so you can get the best of both
Describe Class D amplifiers
- very efficient use of batter current (best of all) - high saturation levels (best of all) - symmetrical peak clipping - small size - more expensive than all other amplifiers
How does internal feedback occur?
-seal b/w the receiver and the microphone inside the HA is broken so feedback will still occur even when you block sound for leaving the HA
Anderson refers to 4 typs of non-linear distortion, what are they?
1) Crossover distortion = produced from a discontinuity in the output of the waveform near the zero-crossings (where the wave cross the zero axis) 2)Overload distortion = occurs when an input signal is too large for the HA to process cleanly (exceeds saturation level) and the tops and/or bottos of the waveform is chopped (peak clipping) 3) Harmonic distortion 4) intermodulation distortion
Describe 2 disadvantages of directional microphone systems.
1) Internal noise: 2 mics = more technology = more noise (especially with a hearing aid that uses LF amplification, LF internal noise will be amplified more) 2) Wind noise: turbulent air that moves past the mics, and when directional they are more sensitive to sounds in the near field so wind noise will be received at a higher level in directional patterns than in omni patterns
What are the different polar plots? Be able to estimate the attenuation of sound originating from different angles.
1) Omni directional (circle) 2) Hypercartiod (little loop and bigger loop) 3) Bipolar (figure 8) 4) Cartiod (heart)
What are the 2 main hearing aid microphone categories?
1) omnidirectional 2) directional
Describe the 3 major limitations of receivers.
1) output is related to receiver size 2)susceptibility to wax 3) unwanted resonances (extra peaks in frequency response = BTE, lead to harsh quality)
List the 3 types of hearing aid transducers.
1)Microphones 2)Telecoil 3)Receiver
What are the different ways of characterizing directivity of directional microphone systems?
1)Polar Plots Single Number Characterization 2) Directivity Index 3) Front to back ratio
What are the different types of amplifiers? (worst to best?)
A, H, B, D
What is the order of amplifiers from most expensive to least expensive?
D, B, H, A
Explain and identify what an "O-T-M" switch does.
OFF-MIC-TELECOIL switch: allows you to switch between those different modes
What is the smallest and what is the largest amplifier?
Smallest = D Largest = B?
Define output
level (in dB SPL) of sound at the output of the HA; what comes out of the hearing aid after the sound has been amplified/processed
Define gain
output level - input level; the amount of "gain" added by the hearing aid; in dB
Define peak clipping
the effect on the signal when you drive the HA into saturation; peaks of the waveform are clipped producing distortion (harmonic and intermodulation distortion)
Define saturation
the level (in dB SPL) at which a further increase in input no longer produces a change in output - if you drive a amplifer into saturation it will lead to distortion - peaks are chopped/peak clipping and it will cause distortion of the waveform
Define maximum power output (MPO)
the max output of the HA