Two- tone unmasking and suppression in a forward-masking situation

Robert V. Shannon 1976

Spring 2009 HST.723 Theme 1: Psychophysics

Unmasking and GoalsUnmasking – addition of another frequency component to the masker signal resulting in decrease of threshold of signal detection.

Goals: Investigate masking and unmasking as a function of:Time : τ, tv (Forward vs. Simultaneous Masking)

Frequency: fp

Intensity: Lv – Lp, Lv & Lp

Phase

Author’s assumption:Unmasking is caused by suppression. Masking is caused by excitation.

Procedure:

Two- alternative forced-choice method

Masker:frequency: fp = 1 kHz, fv = variable duration: 500msintensity: Lp = 40 dB SPL and Lv = 60 dB SPL

Signal:frequency: fp = 1 kHz, fv = variable duration: 20ms intensity: Lp = 40 dB SPL

Total of 5 subjects: RS, PP, SK, MD and MC.

Temporal Parameters: Simultaneous vs. Forward masking

Note: The baseline (i.e. 0dB) is the amount of masking produced by the 1kHz masker alone (not the absolute threshold)

Simultaneous masking:• symmetric about 1000 Hz• most masking is at 1000 Hz

Forward masking:• most masking is at 950 Hz• at 1150 Hz amount of masking

decreased to -6 dB = unmasking

*Subject MC showed no unmasking

Temporal Parameters: Duration of the fv component

Stimulus:• duration of fp is 500 ms• duration of fv varied from 20ms

to 500 ms• fv always occurred at the end of

the masker

Minimum tv necessary for unmasking to occur?

Temporal Parameters: Duration of the fv component (cont.)

Variability across subject:• for RS unmasking at tv = 100 ms

• for MC unmasking at tv = 250 ms

Intensity Parameters: Intensity Difference Lv – Lp

Stimulus:• Lp = 40 dB SPL is fixed

• Lv = 30 - 70 dB SPL

(Effect of intensity level of variable frequency masker component onto the masking and unmasking pattern)

1. for Lv = 30 dB SPL no unmasking

2. for Lv = 40 – 70 dB SPL: - masking at 950Hz increased from 7 to 17 dB

- unmasking increased from 2 to 7 dB- maximum of unmasking is at fv=1150 Hz

Intensity Parameters: Intensity Difference Lv – Lp (cont.)

3. No unmasking for fv<fp

(unmasking for this condition might occur for Lv > 70 dB)

Intensity Parameters: Overall Masker level Lv and Lp

Stimulus:• Lv - Lp = 20 dB SPL is fixed• Lp = 30 -70 dB SPL• Lv = 50 -90 dB SPL

• for fv > fp amount of unmasking is almost the same for all Lp and Lv levels

• for fv <fp unmasking does not appear until Lv = 50 dB SPL, it then monotonically increases

Signal frequency and duration

Subject MCsimilar masking patterns for all frequencies

Subject MDshowed no unmasking for 1 and 2 kHz

Stimulus:• fp is 1, 2, 4 and 6 kHz

One-tone masker, Phase and Square Gating

One-tone maskerStimulus: fv alone

Results: 1. Amount of masking produced by two-tone masker is never less then amount of masking produced by the variable masker component

alone. 2. Unmasking is not observed in case where the baseline is an

absolute threshold

PhaseStimulus: initial phase between fp and fv in the masker were varied.

Result: phase had no impact on unmasking

Square gating Stimulus: signals with abrupt on and off transitions rather then smooth.Result: The existence of unmasking does not depend on whether or not the masker

and signal are turned on and off abruptly or smoothly.

SummaryUnmasking is not observed at absolute thresholds. It is only observed when the

masking at the signal frequency is raised by the presence of a masker signal.

When fv < fp unmasking is dependent on only, Lv, the intensity of the variable masker component.

When fv > fp unmasking is dependent on, Lv – Lp, the difference between the intensities of the two masker tones.

The variable masker component must be present 40 – 100 ms for unmasking too be observed.

Unmasking was interpreted as a result of the variable masker component suppressing the masker component at the signal frequency. If this interpretation is valid then the characteristics of the unmasking of the signal may be used to infer the characteristics of suppression in the masker.

Q&A