Amplification 2 > Directivity > Flashcards

Directivity Flashcards

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1
Q

Why would we care about directivity?

A

We care b/c we need a way to help patients in those challenging listening situations. Ex. speech in noise, speech intelligibility, and wind noise

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2
Q

Why would a chatter cab driver need a reverse cardiod or figure-8 pattern polar plot

A

it is a special situation where speech is coming from behind the cab driver

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3
Q

How could you improve intelligibility?

A

moving the mic closer to the source or by using a directional microphone

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4
Q

WHat is a directional microphone?

A

it is a mic or group of mics with more than 1 port of entry. Either 2 ports of entry w/ 1 mic or 1 port of entry w/ 2 mics

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5
Q

What are acoutic dampers?

A

It is for rear port entry noise. it holds sounds coming in the rear for .1 ms until it is convenient to release the sound. When it is released it cancels out the sound the came from the front port of entry. Speech is unaffected.

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6
Q

What is the progression within a directional hearing aid sounds/speech travel?

A

speech comes in from front port to the diaphragm, then to the amplifer, then tranfers sounds to receiver.

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7
Q

Directional mic is also know as

A

microphone array, beamforming array, or beamformer

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8
Q

What does the directional mic do?

A

suppress noise coming from some directions while retaining good sensitivty to sounds arriving from 1 direction. Focuses in on 1 directions while suppressing the other directions.

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9
Q

What is port spacing? Why would you choose to give a patient large port spacing?

A

it is the distance b/w the 2 mics (the 2 port of entry). Large PS maximizes mic senstivity and minimize internal noise (ideally 5-12 mm)

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10
Q

External Delay

A

calculated by dividing port space by speed of sound

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11
Q

Internal Delay

A

integral to the acoustic filter/electronic delay

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12
Q

Delay Ratio

A

ratio of internal delay to external delay

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13
Q

what does the delay ratio detemines?

A

determines shape of senstivity pattern (aka polar directivity pattern)

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14
Q

As delay ratio decreases from 1 to 0, what do the polar patterns change from? What happens to the senstivity to sound?

A

moves from a cardioid pattern to a super-cardioid, to hyper-cardioid, to a figure-8 pattern. Sensitivty to sounds from the back grows, but sensitivty to sounds from the sides dimish.

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15
Q

Using the delay ratio decreaing from 1 to 0, would you want your ratio closer to 1 or 0 for a patient who needs more sound sensitivity to the sounds coming from his/her sides.

A

closer to 1 b/c as the ratio decreases the sounds to the back grow and the sides dimish

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16
Q

where is zero degrees azimth?

A

infront of the patient

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17
Q

where is 90 degree azimuth?

A

to the right of the patient

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18
Q

where is 180 degree azimth?

A

behind the patient

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19
Q

What is the front to back ratio?

A

includes frontal sensivity (zero degree azimuth) to rearward sensitivity (180 degree azimuth). Not effective b/c sounds usually hits HA from different directions

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20
Q

Unidirectional Index (UI)

A

sensitvity averaged across all frontal directions (clockwise 270 to 90 degrees azimuth) relatvie to sensitivty averaged across all rearward direction (clockwise 90 to 270 degree azimuth).

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21
Q

Is unidirectional Index more or less useful than directivity index?

A

Not as effective b/c speech and noise would be averaged together.

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22
Q

Directivity Index (DI)

A

Ratio of sensitivity for frontal sounds (0 degree) relative to sensitivity averaged across all other directions. Measure of benefit provided by directionality. (2D or 3D)

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23
Q

List the types of directional microphones

A

First-order subtractive, second-order subtractive, and addictive directional arrays

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24
Q

First-order subtractive

A

widely available in most HAs, it a single subtraction of 2 signals, either mechanical or electronic subtraction. Linear with no distortion

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25
How does first-order subtractive directional microphones work?
Speech coming from the front, noise from the back. Same as earlier that noise gets hold and then nullified. Speech is not affected. Simplest and easiest one to follow. Mechanicals subtract of noise; electronical things needed from audiologist.
26
Second-order subtractive
subtract 2 signals each of which is the o/p of a first-order subtractice directional mic resulting in a super-directional pattern
27
How does second-order subtractive directional microphones work?
focus only on front, everything else has been eliminated. This is a more advanced beam focused on sounds and speech infront. behind will be suppressed.
28
Additive directional arrays
instead of reducing sensitivity for sounds from all directions other than the front, it produces max sensitivty for frontal sounds as compared to the other directions (best suited for hand held microphones)
29
Broadside mic placement
multiple mics on chest, on headband, on the front of the spectacle frame microphone arranged from forehead to feet
30
broadside
Frontal target signal (speech) reaches all mics in-phase - added up; while other ambient signals (noise) reach the mics sequentially – phase-cancellation External and internal noise sounds when reaching the 2nd mic; it is added up to be double as loud. When at 3rd mic it will be triple as loud.
31
down side to broadside
Any noise not coming from directly infront is not taken into account. External sound and the internal sound when get to second mic it is added up and be double as loud. When both get to third mic it will be triple as loud.
32
End-fire, delay-and-add
multiple mics from anterior to posterior e.g. on the temple of a spectacle. Linearly microphone arrangement from frontal to back out reached hanf from fingers to shoulder. Sound from front hits the first mic the input gets to second mic.
33
Broadside or End-fire are all mics perpendicular to the msot sensitivte direction
broadside
34
Broadside or End-fire are all mics in a line pointing to the msot sensitivte direction
end-fire
35
Electronical delay T equals
time taken for sound to travel acoustically from one mic to next
36
What is the advantage of additive mics over subtractive mics
reduce in internal noise
37
What is the disadvantage of additive mics over subtractive mics
effective only for HF sounds (>4 kHz); cosmetically less appealing
38
Complex direcitonal arrays
change thier directivity pattern to minimize pick-up of the dominant noise present and/or to maximinze pick-up of the signal coming from a target direction at any time
39
Adaptive Delay
similar to the first-order subtractive arrays, but with variable internal time-delay to minimize pick up of sounds from the rear
40
Adaptive weight
vary gain and phase of each mic by different amounts at different frequencies; then add the filtered signals. Can increase SNR by up to 30 dB
41
what factors affect directivity
directivity decreases as frequency decreases, distance of the talker and noise sources.
42
How would directivity be affect when increasing the distance will affect the speech to noise ratio?
It is not necessarly the SNR that is affect, it does not directly affect the intelligibility. bc distance does not affect speech intelligibility. If the speaker is near noise, there is a chance the microphone is going to pick their voice up and assume it is apart of noise; suppressing both
43
How do HAs know when to be directional?
HA follows 4 basic knows, when all four are met, then the HA knows to go into directional focus
44
1st basic know
the overall sound level is high enough to indicate that voice levels are raised So speech noise raise = noise around them and person talking louder to over talk the noise
45
2nd basic know
background sound level is greater than 60 dB SPL; suggesting that there is noise in the environment
46
3rd basic know
the o/p of the directional mic has deeper envelope(peak) fluctuations (4-20 Hz) than the o/p of the omni-directional mic; suggestive that there is a talker in the front
47
4th basic know
the signal present is not characteristic of wind noise
48
Multi-channel HAs
can choose b/w directional and omni-directional modes independently at different channels. AKA split-band/split-channel directivity
49
what is the disadvantage to multi-channel HAs
no increase in SNR for low frequencies sounds due to internal noise
50
wht is the disadvantage of traditional directivity
assumption that noise is to the rear or side, increased internal noise, wind noise .. can be disruptive or distorted if HA keeps switiching back and forther
51
What is the clincial dB SNR improvment level?
5-10 dB
52
What is the real-life clinical application level?
1/3rd of listening situations
53
Highest 3D DI (equal 6.0 dB)
hyper cardioid
54
Highest 3D UI (equal 11.6 dB)
super cardioid
55
Highest 2D DI (equal 4.8 dB)
super cardioid
56
which polar pattern in UI averages equal 0.0 dB
firgure-8
57
Smaller the port distance
mic itself becomes to noisy and decreases the micophones sensitivty, but internal noise generated by mic stays the same so becames increasinly apparent by comparison with the signal More evident to HA wearers bc directional mic will be noisier in quiter situations if the 2 mics are equalized to have the same gain-freq response for frontal sounds
58
What type of HA would a directional mic be found in
larger HA with enough port spacing; BTE and ITE.... never likely CIC b/c no room and pinna diffraction creates complex sounds field near the faceplate of HA
59
Wavelengths lengthen as
freqs decrease
60
Mismatching in gian freq response causes what
a progressive deteriation in their matching, hence directivty. ex sensitivty mismatch as small as 1 dB, or a phase mismatch as small as 5 degrees can decrease the direcitonal index by 1/3 at 100 Hz, with progressively much worse effects as freq decreases below 1000 Hz
61
Directivity pattern on the head can approximately be predicted by
adding (in dB) the omni-directional pattern of the head position to the free-air pattern of the directional mic
Amplification 2
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