Lecture 9 Flashcards
What is masking?
Sound of interest becomes harder to hear when another sound is present. In the cochlea, the activity of the BM generated by a signal is disrupted or “covered up” by activity produced by masker.
What is the typical masking experiment?
Signal and masker presented at the same time, or one after the other. Properties of masker or signal are varied and compare level at which signal is barely audible in presence of masker to level at which signal is barely audible without masker.
Threshold of signal with masker and without = amount of masking.
Alternative expt: listeners presented with soft pure tone, proceeded at various intervals/decays by a masker. Level at which masker begins to mask signal (lowest masker level where signal can’t be heard) = MLT
What are psychophysical tuning curves and what do they measure?
Two-tone suppression measures this. It is better to present the masker at a different time than signal to avoid suppression. Tuning care sharpens when tones are not presented at the same time.
Measuring the auditory fibre frequency tuning and shape of tuning curve for auditory fibre.
In experiments, use low-level tone because you want/assume listener to listen to only one place on the cochlea.
Tone you are trying to hear is at threshold.
What happens when you increase the delay between the masker and signal?
The MLT also increases. You need higher thresholds of masker to mask the signal.
What is the point of masking?
Measure the masker level in threshold for maskers of different frequencies. Trying to figure out how BM responds at single specific place when you present different signals of different frequencies.
When does the largest amount of masking occur?
What is the consequence of this?
When frequency of masker is the same or very close to the signal.
If you are trying to listen to someone who is talking in a loud place - masking will be relatively large. In another room that has high frequency noise that is just as loud, it will be distracting but won’t mask because frequencies are slightly different compared to other room.
When does the psychophysical tuning curve become broader for masking?
When you increase the level of the masker. Sharp tuning curve when level of masking is low
What does masking tell us?
Properties measure behaviourally match the response that you observe from auditory nerve fibres because they are both due to properties of the BM and the mechanisms that happen for transduction to code the frequency information.
Our ability to hear sounds when noise is present due to properties of the BM.
What is the notched-noise technique?
Listeners asked to listen for a signal - a pure tone of a specific frequency. Masker presented at the same time or preceding signal. Masker has spectral notch/gap.
Frequency centered in notch, width is varied(remove energy within certain band) Preferably present masker just before signal.
Masker - Pure Tone (auditory filter) - Masker
What happens when you modify the notch in the notched-noise technique?
Narrow Width: more energy or masker is closer to frequency of signal); more masking needed to be able to hear the tone
Wide Width: threshold of signal is decreased, energy of masker decreased - need less energy of signal to be heard.
- signal threshold decreases
Threshold of signal determined by specific signal-to-noise ratio - need same amount of masking to mask signal
- signal threshold increases
How do you measure the signal threshold from a notched-noise technique?
Measure lowest level at which signal can be detected for each width of notch. Is lowest level at which listeners can detect signal.
What would be the resulting auditory filter shape as a result of the notched-noise technique?
Ex: of 800 Hz signal?
Shows like triangle - combines the threshold collected at different notch widths.
As frequency shifts left and right from the centre peak, the amount of attenuation is shown as (-) gain following filter function.
Ex. 800Hz signal would be attenuated at 20dB - movement of BM (at characteristic frequency), observe decrease in displacement at 20dB
How is bandwidth of the auditory filter described?
Equivalent rectangular bandwidth. Area is the same as that of the actual estimated auditory filter shape. Describes how bandwidth of filter changes as a function of centre frequency.
Measure of the attenuation that happens for different signal frequencies at one point on the BM.
Centre frequency = highest point in excitation pattern.
What is the result of a wider ERB?
When masker level not changed, more masker energy gets into filter - increases signal threshold. Have to make signal higher.
Higher frequencies - bandwidth of filter is wider, masker level doesn’t change - more energy contributes to masking of signal.
With broadband noise - as the pure tone frequency changes, the SNR and threshold increase as you increase the signal.
How do the bandwidth of auditory filters work on the basilar membrane?
Bandwidths increase as the centre frequency increases.
Represented by an array of overlapping band-pass filters.
The collection of auditory filters on the BM respond to signals according to the shape of filter.
Move frequency away, will attenuate energy.
