Psychoacoustic Audio Illusions

Psychoacoustic Audio Illusions by Nigel Parsons

1. Diana Deutsch – Scale Illusion

The Scale Illusion was discovered by Deutsch in 1973, first reported at a meeting of the Acoustical Society of America (1974)1 and first published in the Journal of the Acoustical Society of America (1975)2. The pattern that produces the Scale Illusion is shown in Figure 1A. This consists of a major scale with successive tones alternating from ear to ear. The scale is played simultaneously in both ascending and descending form; however when a tone from the ascending scale is in the right ear a tone from the descending scale is in the left ear, and vice versa. (Figure 1B shows these ascending and descending scales separately, and you can see that the pattern shown in Figure A is produced by the superposition of the patterns shown in Figure 1B). The tones are equal-amplitude sine waves, and the sequence is played repeatedly without pause at a rate of four tones per second.

Figure 1. The pattern that produces Deutsch’s Scale Illusion (a) and a way it is often perceived when listening through headphones (c). The notation in (b) shows how the pattern is composed of ascending and descending scales.

The Scale Illusion is best heard through stereo headphones. Make sure that the loudspeakers on your sound system are turned off, and that the left and right channels are balanced for loudness. Then read just the settings on your amplifier so that the sounds, as heard through the headphones, are somewhat on the soft side.

Now listen to this pattern. If you hear higher tones in one ear and lower tones in the other ear, decide which ear is hearing the higher tones. Then reverse the earphone positions and listen to the pattern again. Decide again which ear is hearing the higher tones.

Play Deutsch’s Scale Illusion

An illusion that many people experience when listening to the Scale Illusion through headphones is shown in Figure 1C. A melody corresponding to the higher tones appears to be coming from one earphone, and a melody corresponding to the lower tones from the other one. When the earphone positions are reversed, the ear that had heard the higher tones often continues to hear the higher tones, and the ear that had heard the lower tones often continues to hear the lower tones.

Right-handers and left-handers tend to differ in how they hear this pattern.  Right-handers tend to hear the higher melody on the right and the lower melody on the left, whereas left-handers vary considerably as to where the higher and lower tones appear to be coming from.


2. Shepard Tones

A Shepard tone, named after Roger Shepard, is a sound consisting of a superposition of sine waves separated by octaves. When played with the base pitch of the tone moving upward or downward, it is referred to as the Shepard scale. This creates the auditory illusion of a tone that continually ascends or descends in pitch, yet which ultimately seems to get no higher or lower. It has been described as a “sonic barber’s pole”.


3. Falling Bells

Falling bells (MP3) – This is a recording of a paradox where bells sound as if they are falling through space. As they fall their pitch seems to be getting lower, but in fact the pitch gets higher. If you loop this sample you will clearly see the pitch jump back down when the sample repeats. This reveals that the start pitch is obviously much lower than the finishing pitch.


4. McGurck Effect

The McGurk effect shows how hearing and vision are used for speech
perception. Named after the man who found it, Harry McGurk, it says that people do not hear speech with only their ears, they use their other senses too. The McGurk effect may be experienced when watching a video of a person saying /ga/ with a sound-recording saying /ba/. When this is done, a third sound is heard: /da/.

The McGurk effect is robust: that is, it still works even if a person knows about it. This is different from certain optical illusions, which do not work anymore once a person can see it.