Does the amplitude of vibrato modulation affect tone segregation?
by Katarina Bulovic
This illusion is an extension of the basic streaming sequential grouping illusion, which presents different versions of a galloping sequence of tones of two different frequencies (ABA-ABA-ABA-...). When the tones corresponding to A and B are far enough apart in frequency, or the sequence is played quickly enough, the tones appear to segregate and become two distinct streams instead of one galloping stream. Additionally, in the 2019 illusion gallery, one of the experimentors found that making one of the two tones vibrato caused the tones to segregate. This showed that the timbre of the tones also mattered for sequential grouping of tones. However, the experiment did not test what would happen if both the A and B tones were vibrato.
This study aims to answer that question. Although it is clear from the 2019 experiment that if the tones have different timbre, they are less likely to be sequentially grouped, I wanted to experiment with what aspects of timbre would cause segregation even if the A and B tones had the same timbre. To test this, I created a galloping tone modeled off of the one from the 2019 experiment, but in this case both the higher and the lower tone have vibrato. I then experimented with changing the amplitude of the frequency modulation. In each of the sounds below, the tones are at 1000 and 1500 Hz, they are both vibrato with a modulation frequency of 25 Hz, and the durations of A, B, and the rest are all 100 ms. The first of the three tones has no frequency modulation (amplitude 0 Hz), the second has a frequency modulation with an amplitude of 15 Hz, and the third has a frequency modulation with an amplitude of 30 Hz.
No Frequency Modulation
Frequency Modulation with Amplitude: 15Hz
Frequency Modulation with Amplitude: 30Hz
van Noorden, L.P.A.S. (1975). Temporal coherence in the perception of tone sequences.
Kennedy, Joachim (2019). Can vibrato make it easier for tones to segregate?.
Comments
Katarina Bulovic
c) These reports do seem to answer the original question: increasing the amplitude of frequency modulation is correlated with faster segregation of the tones.
d) Follow-up question: Since it does seem as if the amplitude of frequency modulation affects tone segregation, it would be interesting to do another experiment to test other qualities of frequency modulation. For example, does the frequency of the vibrato also affect tone segregation?
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Maddie
My perception is similar to Nicholas. In the no-fm case, I heard galloping all the way through. In the 15 Hz fm, I heard segregation after ~4 repetitions . In the 30 Hz fm, I heard segregation after two repetitions. If I listen to them all again, the thresholds do vary but I think the overall ordering is the same.
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Nicholas Guiliano
a) In the no-fm case, I was able to consistently hear the "gallop" (though if I focused hard enough, I could hear separate streams). In the 15Hz fm, I perceived a segregation of tones after about half a second. In the 30Hz fm, the tones segregated even faster but I couldn't quantify it.
b) The introduction of two vibrato tones appears to make the segregation more apparent with increasing frequency modulation. Therefore, this confirms the hypothesis. The likely explanation is due to the more drastic frequency differences at different points in the vibrato tones.
