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Amplification II > 6 - Noise Reduction > Flashcards

6 - Noise Reduction Flashcards

1
Q

What is noise?

A

Noise can be described undesirable or unwanted sound, or a stimulus that is deemed to be detrimental to the intelligibility of speech

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2
Q

What 4 features make speech speech-like?

A

Hearing aids can understand speech characteristics very well
1) Patterning
2) Modulation and rate
3) Envelope characteristics
4) Amplitude

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3
Q

What 2 ways helped with noise reduction before today’s digital technology?

A

1) Fixed directional microphones
2) Low frequency gain reduction strategies

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4
Q

What are 3 low frequency gain reduction strategies that were used to reduce noise before today’s technology?

A

1) Noise suppression switch (operated by user)
2) Multi-memory devices with low-frequency gain cut in one program
3) Automatic low-frequency gain cut with increases in input level

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5
Q

How do low frequency gain reduction strategies work? What is the problem with this?

A
  • Assumes noise has mostly low-frequency energy (true for some noise, e.g., traffic, machinery)
  • Main problem? Gain reduction also reduced low-frequency speech cues
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6
Q

What 4 ways do HAs analyze incoming signals to determine whether noise is present?

A

1) Analyzing the intensity level in a given channel
2) Analyzing the spectrum, modulation, and rate
3) Analyzing the redundancy of the input
4) Classification of noise aims to improve speech primarily

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7
Q

What things can HAs classify?

A

Is the input quiet? speech in quiet? Speech in noise? Speech in loud noise? Music? Noise only? Is the noise steady-state? Is the noise transient? Is the noise wind?

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8
Q

If speech and noise are both present, what happens?

A
  • HAs will use the information of the SNR to determine if they must turn on noise reduction or not
  • You will typically see these classifiers in premium technology
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9
Q

What is the HA input analysis?

A
  • These is telling us which classifiers patients are in
  • This can be helpful to tell us which environments a patient is in
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10
Q

How well does a HA do at separating speech from noise?

A

If you put the speech in noise, it can be difficult for the HA to separate the two

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11
Q

What are the 5 different noise reduction approaches?

A

1) Modulation approach
2) Spectral approach
3) Digital noise reduction
4) Gain-reduction digital noise reduction (G-DNR)
5) Traditional-digital noise reduction (T-DNR)

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12
Q

How does the modulation approach work?

A

A modulation detector is used to determine if the incoming signal is speech or noise

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13
Q

What is the modulation detector?

A
  • Looks at temporal and amplitude patterns (modulations) of incoming signal and determines what it is analyzing.
  • If “speech” is around 10-15 S/N or better, no gain reduction (HA interprets main signal as speech)
  • Speech has greater amplitude modulation depth
  • Works best if noise is steady-state and narrowband. Noise tends to have less amplitude fluctuations, and can be easily identifiable
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14
Q

What is narrowband noise?

A

When the noise is in a specific frequency band and is staying steady

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15
Q

How does the spectral approach work?

A

1) The HA tries to determine the speech spectrum, and then estimates the noise spectrum from the total signal spectrum.
2) Spectral subtraction - estimated spectrum of the noise is subtracted from the total signal
3) Speech and noise spectra must be uncorrelated
4) Works best for steady-state noise

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16
Q

What is digital noise reduction?

A

1) Hearing aids continuously analyses the incoming signal to determine if it is speech or noise, and this analysis is taking place every few milliseconds
2) If noise is detected in one or more channels, the gain can be reduced in the channel(s) where noise is present

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17
Q

What are the 3 challenges to digital noise reduction?

A

1) Speech cues in that channel are also reduced
2) Noise is often unwanted/undesired speech, but may be an important environmental cue
3) Noise is often perceptual to the listener

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18
Q

When is G-DNR used?

A

Used when noise is the DOMINANT SIGNAL

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19
Q

How does G-DNR work?

A
  • Reduce noiseby reducing the gain in the channel where noise is present
  • This takes TIME… Time to determine that noise is dominant, time to start reducing noise, time to settle into the maximum amount of reduction
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20
Q

When is T-DNR used?

A

Used when noise is not the dominant sign, but speech is present as well

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21
Q

How does T-DNR work?

A
  • Reduce noise by reducing the gain in the channel where noise is present and separate this out from speech
  • How? Noise reduction occurring between segments of speech (in between words)
  • Often used in combination with G-DNR
  • This takes TIME… Time to determine that noise is dominant, time to start reducing noise, time to settle into the maximum amount of reduction
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22
Q

Can HAs classify noise accurately all the time?

A
  • Challenges exist
  • Very difficult, especially when different acoustic signals are all occurring at the same time and may be further complicated by reverberation or room acoustics
  • Hearing aids use probabilities
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23
Q

HA might identify the main signal as speech in noise based on the following probabilities:

A
  • 55% chance that the signal is speech in noise
  • 30% chance that the signal is music
  • 15% chance that the signal is noise only
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24
Q

How helpful is noise reduction for patients?

A
  • Noise reduction does not improve speech intelligibility on it’s own
  • Adults prefer HA with noise reduction (they want to hear everything around them)
    • More comfortable and relaxing
    • Decrease in listening effort
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25
Q

What is the acceptable noise level and noise reduction?

A
  • ANL = annoyance by noise
  • High number = very annoyed by noise
  • Low number = less annoyed by noise
  • When adding a little bit of noise reduction, they perform better for an ANL test
  • Significantly lower ANL scores with any noise reduction strategies than no NR.
  • People with poor ANL at baseline showed greater benefits of noise reduction.
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26
Q

How does noise reduction vary?

A

1) how quickly the NR system reacts when noise is detected and how quickly the gain recovers
2) threshold of activation
3) amount of gain reduction and in what frequency range

27
Q

The “aggressiveness or strength” of the Noise Reduction system is usually ____

A

Programmable

28
Q

____ in hearing aids alone, generally does not improve S/N substantially

A

Digital noise reduction (DNR)

29
Q

DNR helps with what 3 things?

A

1) Better listening comfort
2) Less annoyance of noise
3) Less listening effort

30
Q

What frequencies are important for speech understanding?

A

1000, 2000, 3000, 4000 Hz

31
Q

How do HAs optimize speech input?

A
  • Some hearing aid models slightly increase gain in channels where speech is determined to be the main input
  • If noise is detected in low-frequency channels, there may be a slight increase in high-frequency gain in addition to the reduction in low- frequency gain
32
Q

How fast does noise reduction engage?

A
  • Depends on the hearing aid model itself
  • Hearing aids may gather data for 10-15 seconds before deciding whether or not to engage the noise reduction system.
  • Activation time may be varied
33
Q

How much gain reduction can we expect?

A
  • Great deal of variation
  • Gain reduction ~2-3 dB to ~15-20 dB depending on HA models and settings of NR. This is often something that can be adjusted in the hearing aid software
  • Amount of gain reduction is often based on input level: more gain reduction with louder noise.
  • There may be less gain reduction in the mid-high frequency range (2-3 kHz) than in low frequencies.
  • There may be gain boost in speech frequencies if noise is detected in low frequencies (speech enhancement)
34
Q

Is more gain reduction better?

A
  • Is it better for the noise reduction systems to reduce gain by~20 dB instead of ~5 dB for example
  • The short answer is “NO”.
  • Remember, speech cues are also subjected to the gain reduction.
  • Everything may be too quiet with aggressive noise reduction, and speech may be inaudible
35
Q

What are the 4 choices of noise stimulus on the verefit 2?

A

Choice of Stimulus on Verefit 2:
- Air Conditioner
- Vacuum
- Multi-talker babble
- Pink Noise
Signal level dBSPL

36
Q

What is the verefit 2 verifying for noise reduction?

A
  • In real time, we can measure how long it takes for the curve “to separate”.
  • Air conditioner at 60 dB (NR of ~3dB; after 23s of playing the air conditioner)
  • Vacuum at 80 dB (after 14s, gain reduction kicked in ~4dB NR)
37
Q

What are the 3 ways of verifying noise reduction systems?

A

I) Compare the following two REAR with a 65-dB noise input (S-REM or REM):
II) Compare the following two 2-cc coupler or REM curves, with NR activated:
III) Use speech (or speech-like) input with a bias tone (Fonix)

38
Q

Explain comparing the following two REAR with a 65-dB noise input (S-REM or REM).

A

I. Compare the following two REAR with a 65-dB noise input (S-REM or REM):
a) REAR with NR de-activated
b) REAR with NR activated
- to verify audibility of aided response with noise reduction “on” (i.e. are we still meeting targets)
- to view and measure how much gain cut and at what frequencies

39
Q

Explain comparing the following two 2-cc coupler or REM curves, with NR activated.

A

II. Compare the following two 2-cc coupler or REM curves, with NR activated:
a) Carrot passage
b) Noise input
or :
a) Modulated-ICRA input
b) Unmodulated-ICRA input
- Can test at different input levels to see how NR performs for different levels of noise.

40
Q

Explain using speech (or speech-like) input with a bias tone (Fonix).

A

III. Use speech (or speech-like) input with a bias tone (Fonix)
- Tone simulates frequency-specific noise.
- Results show the HA’s gain reduction in channel where tone (noise) is present.
- Bias tone = tone that’s like speech, but has a tone overlayed at different frequencies (shows if the NR is actually doing anything)

41
Q

What are sudden impulse noises or transient signals?

A
  • Comfort feature
  • Hearing aids can reduce annoyance andincrease comfort of sudden and loud signals through gain reduction without affecting speech greatly if this is fast acting (~1ms)
42
Q

What is a very big challenge for noise reduction?

A

Wind noise

43
Q

What is so challenging about wind noise?

A
  • Aggravating and interferes with speech perception
  • Wind turbulence creates air pressure fluctuations at the hearing aid mics- this is a physical disturbance
  • Think about where microphones are placed dependant on hearing aid model (an over the ear model can be greatly impacted by wind noise)
44
Q

Wind noise is a greater problem for devices with ____

A

Directional microphone configuration

45
Q

Why is wind noise a greater problem for devices with DM configuration?

A
  • Spectrum of wind noise depends on wind speed
  • Slow wind = lower frequency; fast wind = higher frequency
  • There can be rapid and massive changes to amplitude on different microphones (simultaneously)
  • In DM configurations wind noise is uncorrelated at each mic (turbulence is different at each mic)
46
Q

What 3 ways is wind noise handled?

A

If wind noise is detected, the hearing aids may:
- automatically switch from directional to omni mic mode
- reduce the gain for frequencies below ~1000 Hz.
- newer technologies use adaptive filters to remove the wind noise from the signal in the frequencies that it is detected (rapid gain reduction)

47
Q

What are the 3 main take home messages regarding noise reduction?

A

1) Hearing aids alone cannot remove the noise entirely without affecting the speech signal
2) Hearing aids can reduce the overall loudness of noise, making the wearing of devices more acceptable to the user
3) Improves comfort in noise but not speech intelligibility directly

48
Q

What is the purpose of DMs? Why?

A
  • The overriding function and purpose of directional microphone technology is to improve sound from one direction and attenuate sound from another
  • WHY? To provide better speech understanding in noise by improving the signal to noise ratios (SNR) and create a comfortable listening environment for users
49
Q

Who is a candidate for DMs? How much can DMs improve SNR?

A
  • Patients who have challenges in noise (i.e. show impairment in performance of speech in noise testing) benefit from this technology
  • DMs can improve the signal-to-noise ratio by approximately 3dB when noise is to the rear of the listener
50
Q

Directionality is most effective when the patient is within ____feet of sound source

A

10

51
Q

What are the 5 polar plots?

A

1) Omnidirectional
2) Bi-directional
3) Cardioid
4) Super/Hyper Cardioid
5) Anti Cardioid

52
Q

How do you chose a microphone mode?

A
  • Level detector: automatic switch to DM mode when signal is louder than a certain level (65-70 dB SPL)
  • Modulation detector (similar to that used for noise reduction algorithms): switch to DM when noise is present
  • Analysis of differences in levels at the front and back mic to estimate the location of the dominant signal
53
Q

Why would a patient want user control for DMs?

A

Patients can override what the patient is switching for them

54
Q

What are automatic and adaptive directional mics?

A
  • Automatic switch from omni to directional
  • Adaptive polar plots depending on noise source (null steered where dominant noise is located).
  • Signal processing in a single channel or hearing aids may have adaptive mic
    • Different polar plots in different bands so that multiple noise sources may be attenuated at the same time.
55
Q

What are 8 potential challenges with DM?

A

1) Directional hearing not desirable in all situations (DM may steer to a signal that is not of interest)
2) Adaptive DM don’t consider omni preference in certain noisy situations.
3) DM effects are reduced or lost in reverberant rooms
4) DM benefits are reduced with increasing vent size
5) No benefits if speaker and noise are from the same direction
6) User’s voice may impact on activation
7) Wind noise- turbulence on mic is a challenge
8) Dirty ports / mismatched mics

56
Q

What is asymmetric directionality?

A
  • A Slightly Different Approach …
  • One hearing aid is in a directional mode, the other in an omni setting
  • The idea behind this is that the hearing aids are not “cutting off” the listener to the immediate sound environment
57
Q

Who would do best with asymmetric directionality? Who would do poorly?

A

Best:
- Good working memory
- Good cognitive health
- Good auditory processing skills
Poorly:
- APD may not do very well with this strategy

58
Q

What 6 ways do you maximize speech intelligibility in noise?

A

1) Provide audibility of speech cues over a wide frequency range
2) Bilateral hearing aids for better speech audibility and S/N improvement
3) Directional microphones for better S/N
4) Noise reduction systems for more comfort in noise
5) Remote microphone technologies for better S/N
6) Modifications to the environment when possible

59
Q

Digital noise reduction improves ____ in noise

A

comfort

60
Q

Directional mics improve ____ in noise

A

speech perception

61
Q

Digital noise reduction vs directional microphones

A

The best combination of these things translate to better comfort, more wear time, and more acceptance of HAs for our patients

62
Q

Do we have to counsel patients on technology?

A

Yes (balancing features, user control, usability)

63
Q
A

Amplification II (13 decks)

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