Analog Box 2 : Audio Research : Temporal Coherence : Part 1

Temporal Coherence in the Perception of Tone Sequences

Part 1:
Temporal Coherence, Fission and Fusion

This circuit plays an alternating tone sequence of two tones, tone A and tone B. With it we will demonstrate temporal coherence, fission and fusion. You can adjust various parameters of the sequence by turning knobs and sliding sliders. For this example however, we are only going to adjust the frequencies and the tempo.

Adjust the frequency of tone A by turning the tone A frequency knob. Likewise, adjust tone B with the tone B frequency knob. The difference between the two frequencies can be measured in semi-tones. This is displayed on the semi-tones between A and B readout.

The tempo of the sequence is adjusted using the T slider. The two readouts connected to it display the repetition time in seconds, and the equivalent tones per second in Hertz.

When you adjust the knobs and sliders, do not be overly concerned with getting the exact values stated in the text. Two or three significant digits is close enough to experience these phenomena.

We start with temporal coherence: If the frequency interval between A and B is small, 1000Hz and 1059Hz, 1 semitone apart for example, and the repetition rate is fairly fast, 10 tones per second, we hear the alternation of the two tones ABAB... This is what you normally might expect, nothing special. What is special is that we perceive the two notes as a single item. Our mind has constructed a coherent whole out of the two notes, thus the term temporal coherence.

But if we now raise the frequency of tone B to 3240Hz, 20 semitones apart, we do not alternation ABAB... but instead hear two simultaneous lines of tones: A A A ... and B B B ... You can alternately direct your attention to the high line and to the low line of tones. This phenomenon we call fission, the notes have split into two separate streams.

Especially at slower tempos, there is an intermediate range of tone intervals where either temporal coherence or fission can be heard at will. Try to hear either phenomena in a sequence with a tone frequency interval of 7 semitones and a tempo of T = 0.2 sec.

Apart from the phenomena of temporal coherence and fission, there is the phenomenon of fusion. Fast tone sequences with small tone intervals sound like continuous tones with frequency modulation. Listen for example, to a sequences with A at 1000Hz, B at 1040Hz and T at 0.06 sec.

Finally, in sequences with tone repetition times of more than 2 seconds, we can hardly speak of temporal coherence any more. The tones seem to become isolated events in time.