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Diana Deutsch refers to this ambiguity as the Tritone Paradox.  She and her collaborators set up various experiments dealing with the Tritone Paradox.  In one experiment, subjects were presented with pairs of Shepard tritones and asked to judge whether each was an ascending or descending sequence.  As Deutsch points out, with intervals involving a smaller distance between tones, proximity helped in making such a determination. With tritones, however, the aspect of proximity could not be used as such an aid.  As such, if there are patterns of consistency in judgement for a given subject (which indeed there were), there would have to be a basis for those consistencies other than proximity.

In one experiment, Deutsch played a sequence of pairs of tritones for different listeners, asking them to note whether each was up or down.  Specifically, the listener was instructed to note whether the second of the two tones in a given pair was higher or lower than the first.  The experiment was conducted such that each of the 12 pitch-classes had been represented many times as the first tone in a pair.

To get some sense of how this worked, we provide a very brief replication of this experiment.  In this brief experiment, you will hear a set of seven tritone pairs (a more complete replication is forthcoming within the next couple of pages).  Each pair will consist of two tones played close together in time.  Each pair will be followed by a pause of 4 seconds.  During this pause jot down whether you heard the second tone of the pair as higher or lower than the first tone. Make sure your computer speakers are on.

When you are ready, click "Start."

Bear in mind that there is no correct answer -- this is because, the spectral components of the paired tones suggest literally an ambiguity as regards to height.