Physics 1240 Notes

Physics 1240 Notes Tues 12/7/05

Final Exam: Saturday, December 11, 7:30-10pm

Review For Final Next Thurs

Last time: Formants

Singer's formant

Raised formants

throat singing

Today: Musical Illusions

Today's Outline

Synthesis of throat singing
Deutsch's high-low illusion
Ascending/Descending scale illusions (Shepard's scale)
Deutsch's Scale illusion
Other interesting effects

Demo: effect of transient on timbre

Demo synthesis of throat singing

Deutsch's High-Low Illusion

Last time we discussed the octave illusion, where octave intervals were played in left and right ears with the right ear hearing "high-low" and the left ear hearing "low-high". Typically, right-handers hear only the high note in the right ear. This illusion works best with headphones. If you put the headphones on backwards, you should still hear the high note in the right ear if you are a right-hander. Diana Deutsch was the scientist who discovered many of these types of illusions she is a psychologist at University of California, San Diego. She has recently done research showing that people who speak tonal languages (Mandarin and Vietnamese) have perfect (or absolute) pitch. That is the ability to determine exact pitch of a sound, not just relative differences in pitch between two sounds.

The high-low illusion is very similar to the octave illusion, except now the sound has a more interesting timbre. The octave interval is now voiced.

High-Low illusion, Track 6

The same illusion is observed as for the octave illusion, where the high note is localized. However, another effect occurs as well. The words themselves seem to transform into different words as the pattern continues. Also, different sounds are heard depending on the balance between the left and right channels, and the time delay between the left and right channel. So, as you move around in a room with loud speakers you will hear different things. People have reported hearing "blow pie", "high high", "pie pie", "buy loan", or "no no", "boat man" after listening for a while. Try listening to this track at home through loud speakers and move around, or change your listening location.

Ascending/Descending Scales

Let's now look at an illusion that is easier to explain then the Octave and high-low illusions of Diana Deutsch.

Listen to the two sounds and determine if the pitch goes up or down.

Track 48 IPO

Two sounds with partials: 800, 1000 Hz and 750, 1000 Hz.

Concept Test

The point is that the frequency goes down from 800 Hz to 750 Hz. However, we perceive the pitch going up. Why? Because of virtual pitch. The largest common denominator is 200 Hz for the first sound and 250 Hz for the second. So the virtual pitch goes up from 200 Hz to 250 Hz.

Shepard's Scale

This is a famous illusion that can be perceived as an ever ascending scale. It is in some ways similar to the illusion in Maurice Escher's famous drawing "Ascending and Descending."

IPO Track 52

The frequency of the partials are continually shifted upward with increasing then decreasing intensity. The lower pitch partials are faded in with low amplitude and the high pitch tones fade out with low amplitude. In the Shepard's scale the frequency of the partials are increased in half-steps. In the Descending scale of Jean-Claude Risset the partials are decreased continuously.

Tape: Jean-Claude Risset, "Sud, Section III: "Afternoon, Evening" (1987). Tape 137 in the music library.

There are other some good examples of electronic music using interesting illusional effects in the music library on Disc 3338, "New Directions in Music: significant contemporary works for the computer."

Deutch's Scale Illusion

Scale Illusion: Track 7

This illusion is constructed from an ascending and descending major scales with notes switching from channel, as shown in the Figure B below. The actual patterns played to the left and right channels are shown in Figure A.

Listen to this pattern with headphones and your sensory system takes a sound that is musically quite random (A) and makes sense out of it (C). Your ear and brain are making musical order out of musical chaos! Reverse the headphones. Do you hear the same pattern? Figure (C) shows what people typically hear, though other patterns are perceived as well. It also interesting to listen to Duetsch's Scale Illusion with loudspeakers and adjusting the balance. This clearly shows how two complex patterns are merged into much two much more simple patterns by your brain. Like with the octave illusion, right-handers tend to hear the higher pattern in their right ear, whereas for left-handers there are differing results regarding which pattern is heard in which ear.

This effect of making musical order out of chaos is more general than this one example and can be important musically. Suppose two instruments are playing parts that seem to jump around in an incoherent way, yet sense could be made out of their combined sound. This of course assumes a regular musical pattern exists within the notes of the two combined parts. The ear can, in many situations, find the ordered musical patterns. Like the way that C is perceived from A above.

Glissando Illusion

Track 8

This illusion is made up of an oboe sound that has a fixed pitch combined with a sine wave whose pitch glides up and down. Both sounds are alternated between the left and right channel. Listen to this track with headphones. What do you hear? Many people hear the oboe sound alternating from left to right hear, whereas the gliding tone seems to be well connected or balanced between the two ears. Listen to only one channel, using either the balance control on your stereo, or by just listening through one earphone. Notice that the sensation is quite different when listening to one channel.

Many of these effects are especially important in electronic music where there is a lot of control over the sounds that go to the left and right channel.

We will continue our discussion of the illusion and perception of sound, including binaural effects, categorical perception and effect of transients on timbre on Monday.

Deutsch's High-Low Illusion Track 6, like the octave illusion, right-handers tend to hear the high note in the right ear.

Ascending/Descending Scales, "Shepard's Scale"

Deutch's Scale Illusion Track 7, the ear makes musical order out of musical chaos.

Categorical Perception

Perceptual tendency to categorize sounds into either/or categories: "b" or "p", inanimate or living. Natural or electronic.

Jean-Claude Risset's Sud

Glissando Illusion

Track 8

Tone duration

It is interesting to ask how fast can one play notes? Eventually, we will no longer perceive distinct notes. This time limit for identifying pitch is around two periods, or the time for two cycles.

Demo: Countdown, "Giant Steps," John Coltrane

Effect of sound duration on loudness

Another important perceptual effect is that loudness depends on the duration of the sound. If we take a given steady sound, keep the intensity fixed, but vary the duration we perceive the loudness to increase with increasing duration up to about 200 milliseconds. The ear tends to average the sound energy over 200 ms intervals.

Importance of Transients on Timbre

The ADSR envelope has quite a big impact on our perception of timbre, or how an instrument sounds.

IPO Track 54-56

Reverberation

We often don't really perceive the reverberation because we expect it to be there, but it's existence becomes very clear when we play sounds backwards.

IPO Track 70