Shepard tone

The Shepard - scale or Shepard - scale, presented in 1964 by psychologist Roger Shepard, is the illusion of an infinite ascending or descending scale, which never exceeds the limit of one's hearing.

This effect is achieved by a number of different sine waves (usually there are more than eight ), which slowly increase in frequency and decrease and are cyclically interchanged by a slow time -delayed increase and decrease the volume. The frequencies of the individual sine waves are both apart by an octave and moved slowly in parallel over a limited frequency range. Tones that are approaching the limit of the frequency range are hidden; for each tone that falls at one end of the frequency range, a new appear at the other end. The spectrum of the resulting sound gets so practical a steady bell-shaped envelope, which is traversed by the sequence of partial tones.

Occurs, depending on the direction of the frequency shift while the listener the impression of a constantly rising in pitch or constantly falling tone sequence.

Find the change in frequency in discrete sound steps in the form of short sounds instead that follow with a time interval to each other, it is called the Shepard scale. If the partials permanently maintained and changed their frequencies continuously referred to the resulting effect after the composer Jean- Claude Risset as Shepard - Risset glissando.

The human ear can not distinguish by the current time-delayed rise and fall of the individual sine waves, which note the fundamental frequency and which is an overtone: The auditory impression here changes imperceptibly between fundamental and harmonic wave. This acoustic deception is comparable to the optical illusion at a barber pole illusion, seemingly forever moves depending on the direction up or down.

The perception of the Shepard scale is based on the gestalt law of proximity. Looking at the whole basis of the scale, for example, a jump from C to C sharp is perceived as a rising semitone, a jump from C to H, however, as falling semitone. With increasing size of the intervals loses this law of proximity in intensity and culminates at the tritone in the tritone paradox.

Sample

A minute long Shepard - Risset glissando with seemingly constantly deeper werdendem sound? / I