Probe ,
you will hear the difference between the modulated (presented
first) and unmodulated probe tone (presented second). It is this
difference in modulation of the probe (3800-Hz carrier) that you
are to detect in the three test examples. Modulation Detection Interference (MDI) is a phenomenon in which the ability to detect amplitude modulation of one tonal carrier (the probe tone) is disrupted when a tone with a very different carrier frequency is similarly amplitude modulated and added to the probe tone. Performance is typically measured by comparing the ability of listeners to detect whether the probe tone is modulated in each of three conditions. The three conditions are shown in the Figure. In the probe alone (PA) condition, the probe tone (in the demonstration: a 3800-Hz carrier is amplitude modulated at a rate of 10 Hz) is either sinusoidally amplitude modulated (in the signal interval) or not modulated (in the non-signal interval). The depth of modulation is varied to find that depth for which the listener can just discriminate between the modulated and the unmodulated probe (a typical threshold is about -25 dB when the depth of modulation is expressed in decibels, i.e., 20 log m = -25 dB, which is about a 5% change in the depth of modulation, i.e., m is about 0.05). In the unmodulated masker (UM) condition, a masking tone (a 1050-Hz carrier) is added simultaneously to the probe tone in both the signal and non-signal intervals. The masker is unmodulated. The listener's task is still to determine if the probe tone is being amplitude modulated. Typical results indicate that listeners can do almost as well at detecting probe modulation when the unmodulated masker is added (UM condition) as when the masker was absent (in the PA condition). This is not surprising, since the masker and probe carriers are very far apart in frequency. As a result, they should each be easily resolved by the auditory system. In the modulated masker (MM) condition, the masker is modulated at the same rate as the probe (10 Hz) and at 100% depth of modulation. Again, the listener's task is to determine if the probe tone is amplitude modulated. In the MM condition, it is difficult to detect probe amplitude modulation even though each carrier should be well resolved by the auditory system. A typical threshold for detecting modulation of the probe in the MM condition is about -10 dB. The difference in threshold between probe modulation detection in the UM and MM conditions, expressed in decibels, is the usual measure of MDI. For many conditions, MDI is about 15 dB (-25 dB threshold in the UM condition minus -10 dB threshold for the MM condition).
By clicking on Probe ,
you will hear the difference between the modulated (presented
first) and unmodulated probe tone (presented second). It is this
difference in modulation of the probe (3800-Hz carrier) that you
are to detect in the three test examples.
In each of the three test examples, count the number of times (out of ten) that you can detect a difference between the two stimuli that are paired together. The signal stimulus contains the modulated probe and the non-signal stimulus contains the unmodulated probe. The depth of modulation of the probe is decreased (by 3 dB) on each successive presentation of the stimulus pair.
PA: Click on Probe Alone (PA:1 to PA:10) conditions to hear 10 presentations of the difference between the modulated and unmodulated probe presented alone with the depth of modulation for the probe decreasing by 3 dB on each successive presentation.
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| PA: 1 | PA: 2 | PA: 3 | PA: 4 | PA: 5 | PA: 6 | PA: 7 | PA: 8 | PA: 9 | PA: 10 |
UM: Click on Unmodulated Masker (UM:1 to UM:10) conditions to hear 10 presentations of the difference between the modulated probe plus unmodulated masker and the unmodulated probe plus the unmodulated masker with the depth of modulation for the probe decreasing by 3 dB on each successive presentation.
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| UM: 1 | UM: 2 | UM: 3 | UM: 4 | UM: 5 | UM: 6 | UM: 7 | UM: 8 | UM: 9 | UM: 10 |
MM: Click on Modulated Masker (MM:1 to MM:10) conditions to hear 10 presentations of the difference between the modulated probe plus the modulated masker and the unmodulated probe plus the modulated masker with the depth of modulation for the probe decreasing by 3 dB on each successive presentation.
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| MM: 1 | MM: 2 | MM: 3 | MM: 4 | MM: 5 | MM: 6 | MM: 7 | MM: 8 | MM: 9 | MM: 10 |
The typical listener hears a difference between the two stimuli in the PA and UM conditions for about the first 6-8 presentations. However, they hear a difference between the two stimuli for only the first few presentations in the MM condition. That is, it is more difficult to detect modulation of the probe in MM condition than it is in the UM condition. A rough estimate of MDI in decibels can be determined by computing the difference in the number of times you can tell a difference in the MM condition from that for the UM condition, and multiply the difference by 3 (3 dB). For example, if you detected a difference for the first seven presentations of the UM condition and only the first two for the MM condition, MDI is 15 dB; 3x(7-2)=15 dB.
Sample References:
Yost, William A. and Stanley Sheft, Across-critical-band processing of amplitude-modulated tones, Journal of the Acoustical Society of America (85), 848-857, 1989
Yost, William A., Auditory Perception and Sound Source Determination, Current Directions in Psychological Sciences, Vol. 1(6), 1992
Yost, William A. and Sheft, Stanley. Modulation Detection Interference: Across-Spectral Processing and Sound Source Determination, Hearing Research, 79 (1-2), 48-59, 1994