Week 3 Flashcards
(28 cards)
How to microphones work?
All HI have 2 microphone ports and a database of different shapes of amplification patterns based on the environment. These shapes are called polar plots and tell microphones how to behave
What dictates the directions the microphones amplify in?
PORT SPACING: accuracy increases with distance between ports
INTERNAL DELAY: low pass filter analyzes noise to amplify away from it
What are the 4 main polar plots?
OMNIDIRECTIONAL: 360 around
CARDIOID: only amplify in front
FIGURE 8: front and behind, no sides
HYPER CARDIOID: mostly front focus with some amplification behind
What are the two kinds of directional microphones?
Fixed directional microphones and adaptive directional microphones.
Under what conditions are adaptive directional microphones better than fixed?
- Dominant nearby noise source
- Dominant noise is stable
- Dominant noise source in non amplified area
How do SNR and distance affect the effectiveness of directional microphones?
SNR; less benefit with higher ratio (little noise or a strong signal)
Distance: the great the distance between sound and listener the less benefit
What are the drawbacks of directional microphones
No side or rear amplification
Decease in low frequency listening
Reduced localization of HI don’t coordinate
Wind noise greater for directional mics
What is noise reduction?
The goal is to provide less amplification of noise while keeping the level of speech sounds the same. Aim to improve the SNR.
How does digital noise reduction work?
Listens for the modulation of a signal (fluctuation found in speech) and doesn’t reduce that sound
What are the 2 most common noise reduction algorithms?
Weiner filter
Spectral subtraction
What is feedback?
Feedback is the whistling sound when the output of a microphone gets back to the microphone input
List some causes of feedback
- Someone/sth too close to mic
- wax
- HI poorly inserted
- Poorly fit/sealed mild
- Crack in shell or tubing
List the three types of feedback control
- gain frequency response control
- Phase control
- Feedback path cancellation
Describe gain frequency response control
Want to reduce the volume at a channel-specific group of frequencies to do to the output provided to the user
Describe phase control
Detect the frequency causing feedback and use inverted phase response to turn it down. This is inefficient if more than 1 frequency involved
Describe feedback path cancellation
Use internal feedback path created by HI. The HI monitors the external feedback path and if it makes the HI whistle then the internal path kicks in
What is frequency lowering
Process where a HI takes HF information and moves it to a lower and more audible range
What are the different types of frequency lowering?
- Frequency compression
- Frequency transposition
- Frequency translation
Describe frequency compression
HF information compressed into LF range
Frequency transposition
Shifted HF sounds overlap with LF information, so we get a mix of sounds at those channels
Describe Frequency translation
Manipulate HF information to be at wowed frequency to make it more audible. The distorted sound is overlayed onto the other sound
Who are good candidates for frequency lowering
HFHL (>2kHz) that exceeds 80 dB HL
Infants and young children
Those with feedback issues
What are some advantages of bilateral hearing aids?
- Speech intelligibility
1a- head diffraction effects
1b- binaural squelch
1c-binaural redundancy - Localization
- Sound quality
- Avoid late onset auditory deprivation
What are head diffraction effects?
In unilateral HI wearers, if the speech is on the bad side and the noise is on the good side you can lose your good SNR
