10g: Other Interesting Auditory Phenomena

Most musical instruments produce a combination of frequencies that change in amplitude as the note is played. These transitory pitches are called attack frequencies and they sometimes enable us to tell the difference between different instruments. When music is played backwards, particularly on stringed instruments it often sounds very strange because the attack frequencies occur in the wrong place. Our perception of timbre is determined not only by harmonics, intensity and duration as we have discussed previously, but also by the attack frequencies. Here are YouTubes of a piano played backwards; one, two.

The following is a list of other examples where our ears are fooled into perceiving something that isn't actually present. Definitions and examples of several of them can be found at Wikipedia or Richard Waren's site. NOTE: Many of these effects can only be heard using high quality headphones.

Shepard's Illusion. The illusion of a continuously rising (or falling) scale of notes. YouTube example. The continuous version of Shepard's Illusion is called the Risset rhythm.
The Octave Illusion. Two notes an octave apart are played alternating back and forth between each ear (eg. 400 Hz to left ear, 800 Hz to right ear, then switch). Most listeners perceive a the low tone in one ear and a high frequency tone in the other (instead of back and forth). Which ear depends on handedness (right handed people hear the high note in the right ear). Wikipedia (with sound sample).
Deutsch's Scale Illusion. If a two different patterns of notes which alternate up and down in pitch are played simultaneously to each ear the perception of a rising scale is heard.
Glissando illusion. An instrument or voice can be made to appear to change from one side of a stereo output to the other by simultaneously playing a sine wave that is shifting in pitch.
Continuity of Tones. If you interrupt a continuous rising tone with a brief burst of noise your perception will be that the tone did not stop during the break.
McGurk effect. Your vision affects what you think you hear. For example if you see a set of lips forming the sound "ga" while you are hearing the sound "ba" you will perceive the sound "da". There are many other examples where visual clues affect what you think you hear; YouTube example.
Context effects. The sound immediately preceding a sound may affect the how the second sound is perceived. For example "al" plus "da" sounds like "al ga" but "ar" plus "da" sounds like "ar da". Similar to this is prior information affects what you hear: YouTube example
Chromatic Auditory Illusion. Alternating pitches may be perceived as two scales going in opposite directions.
Melody illusion. Keeping the same notes but changing their octave can make a melody unrecognizable; YouTube example.

Video/Audio examples:

Many auditory illusions are demonstrated in YouTube videos.
The Auditory Neuroscience Website is a wonderful source of sound samples and videos of various auditory illusions, demonstrations of acoustic phenomena and indepth discussions of hearing loss.
List of auditory examples demonstrated in class.
A list of auditory CDs along with a description of each. You may be able to find others in a search on Amazon.
Sonification is the process of making data audible in order to detect patterns in a large data set. Technical link: Sonificaiton Handbook.
A test of the masking effects of frequency developed by Dr. Dan Boye and Dr. Greta Munger at Davidson College is available. Note: Internal computer speakers may not be able to reproduce all of the frequencies played in this test, you may wish to use external speakers or headphones. Take the test and record your answers so that you can compare your response with others. What did you find out about your ability for distinguishing two tones played at the same time?

10h: Animal Hearing

(NOTE: Much of the information about animal perception in this chapter and in Chapters 14 and 16 comes from the excellent book Engineering Animals: How Life Works by Mark Denny and Alan McFadzean.)

Many animals, including dogs, can hear frequencies in the ultrasonic range (above 20,000 Hz). Dog whistles used to train dogs have frequencies between about 23,000 Hz and 54,000 Hz so dogs (and many other animals) can hear them but humans cannot. As mentioned in Chapter 8, humans are about the best existing species in being able to hear frequencies in the range of 1000 Hz to 5000 Hz. In this frequency range, humans and elephants can hear softer sounds than dogs and cats, who can hear better than rats, who can hear better than horses and cows. Birds and fish generally hear a much smaller range of frequencies than mammals and also do not hear softer sounds as well. Some insects hear very specific bands of frequencies, for example crickets have a hearing range for signals generated by other crickets for communication purposes but they have a different, unconnected range of hearing to detect the echolocation signals coming from bats, which are a predator.

ranges

As a rule of thumb, small animals tend to make and hear higher frequencies and larger animals are more likely to make and hear lower frequencies, although there are many exceptions as can be seen in the chart. In general the shape of the outer ear is also related to which frequency range and the direction an animal is listening to. For example rabbits have tall ear lobes which makes them more sensitive to sounds coming from a horizontal direction because their predators are mostly terrestrial. Mice, on the other hand, have rounder ears which makes them more sensitive to sounds coming from above, since birds of prey are predators for them. Elephants have very broad, flat ears which are more receptive to lower frequencies with which they communicate. Owls and some other animals can change the relative orientation of their outer ears or will move their entire heads to achieve better accuracy in determining the direction from which sounds are coming. Many animals, such as deer, have muscles that can point the pinna in different directions, enabling better predator detection (humans have atrophied versions of these same muscles but they no longer produce much motion of the ear). Further discussion of animal hearing and its use for orientation and navigation will be postponed to Chapter 16: Acoustics.

Video/audio examples:
- Article by Peter L. Tyack in Physics Today about Human-generated sound and marine mammals.

Summary

The ear is composed of three main parts. The shape of the outer ear gives us information about the direction sound is coming from and funnels sound into the middle ear. The middle ear overcomes the impedance mismatch between air and liquid and transmits sound to the cochlea via the ossicles. The cochlea gives us our sense of balance and turns mechanical vibrations into nerve impulses. There are two theories of hearing, the place theory and the temporal theory. Neither can account for the richness of our perception of sound such as the missing fundamental, critical bands or other interesting aural illusions. Both theories may operate but in different frequency ranges. Age related hearing loss, presbycusis, generally lowers the loudness of high frequencies as we grow older. Loud sounds can also cause either conductive or sensorineural hearing loss.

End of chapter exercises: Perception.