Physics 4830 Notes                                                                            Tues 12/4/01

Last time:  Masking

Today:  Musical Illusions

Today's Outline

Forward/Backward Masking

Last time we discussed forward/backward masking and I wanted to play you some examples.  The effect is subtle.

Forward Masking: IPO Track 25.

You will hear a 10 ms 2000 Hz burst following a 250 ms burst of white noise.  The dB level will drop 4dB each time.  The interval between the tone and the noise will be decreased from 100 ms to 20 ms, to 0 ms.

Forward Masking: IPO Track 24.

Now, the same thing except the tone burst will precede the noise burst.

A clear example of masking is the following example (IPO Track 43).  This track plays a familiar tune with a pure tone alternating with virtual tone.  Low-pass and high-pass noise are used to mask the  tune. 

Ascending/Descending Scales

Analytic versus synthetic pitch: 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. 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", Figure 14-7.

IPO Track 52

The physical/perceptual explanation of this effect is shown in Fig. 14-6. The frequencies 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."

 Octave Illusion

We are now beginning Chapter 14 and are going to discuss interesting auditory paradoxes and illusion, some that can be explained, some that cannot. Many of these illusions are related to how our brain processes musical sounds. Some of these illusions are useful for creating interesting music. Let's look at a simple example.

Track 5: Octave illusion

An octave interval is played high-low in one ear, and low-high in the other ear. It is important to listen with headphones so that the signal to each ear is well separated. Where do you hear the high pitch? Right-handers typically hear the high pitch in the right ear and the low pitch in the left ear. For left-handers the results are mixed. For some, the illusion does not work well. So, now switch the headphones. Where do you hear the high pitch now? Most people hear it in the same ear they did before. So, how did the high pitch switch channels? Well, it did not. This is the illusion. The illusion is that high-low is played to each ear in opposite order, however, one ear hears "high tone-silence-high tone-silence" and the other ear hears "silence-low tone-silence-low tone". This illusion is not understood, but Diana Deutsch was inquisitive enough to discover the effect in the 1980's.

It seems this illusion is correlated with what is known as brain lateralization and that certain functions are localized to one hemisphere or the other. Brain lateralization is that fact that the two hemispheres of the brain behave differently. It has also been discovered, first by Paul Broca (a French Neurosurgeon) in the 1800's that right-handed people have language functions in the left brain and spatial functions in the right brain. Whereas left-handed people can have the same lateralization as right-handers or sometimes the or opposite. More recent research with PET scans indicates that brain lateralization is not so simple and that the brain is a highly interconnected network with many regions active or "firing" when complex tasks are performed.

Deutsch's High-Low Illusion

Like 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. Make sure to try this illusion with headphones, it is Track 5. 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.

 

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 Deutsch'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.