Sarahs Masking In Pta

Question 1

What noise is used in masking and why?

Answer 1

Narrowband involving test frequency but also adjacent frequencies

Because:
Beats (fluctuations in amplitude) can occour when tones of similar frequencies presented together

Harmonics and combination tones

Narrow band as slim envelope critical band of CONSTANT SPECTRAL DENSITY

Question 2

What are the three types of masking presentation?

Answer 2

Simultaneous
Forward
Backwards

Question 3

What mechanisms do we have for encoding?

Answer 3

TEMPORAL FIRING RATE (e.g. Low frequencies for encoding at 200Hz 200AP action potentials per second required from single nerve cell.

TEMPORAL VOLLEY PRINCIPLE (e.g Other nerve cells recruited to produce more AP. Used for Mid frequencies

PLACE PRINCIPLE (e.g the tonotopical organisation of basilar membrane. Mainly for higher frequencies

Question 4

What are tuning curves?

Answer 4

A graph of auditory threshold intensity at various frequencies for a single neuron.

Individual neurons wired to particular frequency places along the membrane adjacent neurons can be recruited dependent on type and intensity of sound

More energy is needed to recruit adjacent frequencies

Question 5

What are Psycophysical tuning curves?

Answer 5

Generates a threshold curve for a single auditory fibre. Used to estimate shape and bandwidth of auditory filters at different frequencies

Shape of auditory filter can be found by rotating tuning curve by 180 degrees

Question 6

How would the shape of a psychophysical tuning curve be altered with abnormal ears?

Answer 6

Poor hearing= flattened

Normal ears=sharper

Question 7

What is the upwards spread of masking?

Answer 7

Low frequencies are capable of masking a wider range of frequencies than high frequencies are able to.

( a reason background noise interferes with speech detection)

Less likely to happen with higher frequencies

Question 8

What is an auditory filter?

Answer 8

An internal filter on the basilar membrane
It acts as a BAND PASS FILTER (only certain frequencies allowed in)

Can tell one sound from another

Question 9

What is a critical band?

Answer 9

Bandwidth of an auditory filter at a
particular frequency
Group of hair cells act as a filter for a particular frequency

Question 10

What is ERB (critical band)
Equivalent
Rectangular
Band

Answer 10

Equivalent rectangular band 
covers the width of the critical band
Equivalent shape of critical band
Bandwidth can change due to frequency but shape stays the same
Usually shallower on low frequency side
Usually steeper on high frequency side

Question 11

Effectiveness of equivalent rectangular band?

If the EEB is filled with masking noise is this more effective

Answer 11

Yes. The more the ERB is filled with masking noise the more effective the masking is. Ideally whole width filled

Width of ERB determined by frequency
Shape stays the same

Question 12

What are notched noise filters?

Answer 12

The narrower the notch the better the masking effect

Can be time consuming

Question 13

How are bandpass filters arranged?

Answer 13

OVERLAPPING bandpass filters

Question 14

What is lateral inhibition

Answer 14

Occours in CNS central nervous system
Lateral inhibition prevents excitation of neighbouring neurons
E.g. If single neuron stimulated will respond strongly. If neighbouring neurons become involved response will be weaker from each one

Question 15

What is nerve fibre adaption?

Answer 15

Can lead to reduction in detection of any signals which follow the masking
(May be useful when trying to obtain temporary tinnitus relief)

Question 16

What is on and off frequency listening?

Answer 16

On frequency- the auditory filter which is centred around the target signal (centre frequency is used)

Off frequency-listener chooses an alternative auditory filter to help reduce the effects of noise and improve SNR