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Basic Auditory Functions


Demonstrations (Direct Links)

Demonstration 10.1 The Missing Fundamental
Demonstration 10.2 Context and Loudness
Demonstration 10.3 Relationship Between Frequency and Loudness
Demonstration 10.4 Auditory Beats

Before You Start

• First, you may want to reflect on the visual system. How do the eyes lead us to the rich sense of lightness, color, visual space, etc.? As you've learned, the processes are far from simple. The transduction process in the ears is much different (transducing sound pressures instead of electromagnetic radiation), but in the end we perceive loudness, pitch, space, etc. How we do so is explored in this chapter. Do you expect the "basic" auditory processes to be simple?

• To what extent would you predict that these basic auditory experiences are solely due to the auditory stimulus? What role might you expect memory and cognitive processes to play?


Pitch Perception

Background: Early Theories of Pitch Perception

Developments in Place Theory

Demonstration 10.1 The Missing Fundamental Good demonstrations of the missing fundamental are available online and on CDs.  In all likelihood, however, you have already experienced the missing fundamental without thinking much about it.  To perform this demonstration you need a car and a source of music (the car's radio, a CD or an mp3 player).  All you have to do is drive and listen to music.  Go run an errand while you are at it.  Now the road and tire noise will be creating a cheap source of low frequency noise for you, which will wipe out most of the low frequencies in the music.  Keep the volume at a low to moderate level and notice you can still perceive low pitches (deep voices, bass guitars, etc.).  The fundamental frequencies of the complex musical and vocal sounds are being obscured by the road noise, yet you still hear the music as essentially the same as when the car is not moving.  Our ability to hear these frequencies is an example of the way in which the harmonics of complex sounds can give rise to a perception of their fundamental–even when it is missing!

One source for the missing fundamental is Wikipedia.

Developments in Temporal Theory

How Do We Perceive Pitch?

The Complex Relationship between Frequency and Pitch

Measuring Pitch

Link - You can experience a demonstration of Shepard tones from the Acoustical Society of America's CD. Why might you misperceive the tone height?


Loudness Perception

How Do We Perceive Loudness?

The Complex Relationship between Amplitude and Loudness

Demonstration 10.2 Context and Loudness Context continually affects our perceptual experience. Nowhere is that more apparent than in your perception of loudness. You've surely had the experience of starting your car only to find that the stereo sounds quite loud. When you'd last driven the car, the stereo didn't sound that loud, right? That's likely true for a number of reasons, including the fact that you were listening to the stereo in the context of road noise from driving the car. You'll also experience the impact of context on loudness when your alarm goes off in the morning. When you set the alarm before going to bed, you'd been listening to many sounds throughout your day, so it probably didn't sound too loud. However, after a night of sleep (presumably in a quiet environment), the alarm will likely sound much louder.

Here's a simple demonstration of the impact of context on loudness. If you can, find a clock or watch that ticks and place it where you can clearly hear it (the ticking should definitely be above threshold). Judge how loud the ticking sounds. [If you can't find a ticking watch or clock, substitute a sound source for which you can adjust the volume to a suitably low level--preferably just audible.] Then turn on a stereo or a radio so that it is loud, but not painfully loud. Leave it on for ten minutes. Now turn it off. Return to the location from which you originally heard the ticking watch [or other sound source]. Judge its loudness relative to your original judgment.

Demonstration 10.3 Relationship Between Frequency and Loudness The Fletcher-Munson curves provide a graphical illustration of the relationship between frequency and loudness. Here is an auditory demonstration of the relationship between frequency and loudness. In the demonstration there are nine different frequencies (10, 20, 50, 100, 500, 1000, 5000, 10000, and 20000 Hz) each played for about 2 seconds each. Each frequency is played at a constant amplitude, so the energy present is constant. Nonetheless, your experience of loudness should not be constant. Thus, some frequencies will sound louder than others, even though the energy present is always the same. (In fact, you may not even hear some frequencies.) Thus, your auditory system is more sensitive to some frequencies (between 1000 and 5000 Hz) than other frequencies that are much lower or much higher. Here is the demo.

Intensity Discrimination

Measuring Loudness


Auditory Localization

Sources of Information for Sound Localization

 

Physiological Basis of Auditory Localization

Measuring Localization Abilities

When Interaural Differences Aren't Enough: Localization Difficulties

Nonhuman Localizing Abilities

• Link - You can learn more about owls at the following site;

The Owl Pages

 

Link - You can learn a lot more about bats (videos, pictures, sounds) at the following sites:

Bat Conservation International (Merlin Tuttle)
Bat Bombers--an aborted attempt to use bats for military purposes
USGS Western Ecological Research Center
Wildlife Online
Australian Museum
University of Michigan Museum of Zoology
Batbox (mainly links)

Integrating Vision and Auditory Localization


Perception of Simultaneous Sounds

Perception of Pure Tone Combinations

Demonstration 10.4 Auditory Beats To best experience this demonstration, you should listen with headphones as well as speakers. The reason is that when listening through speakers, the sound waves combine outside your head (so physics can explain what you hear). However, when listening with headphones, the waves are kept separate in your two ears, so physics has little to say about your ultimate auditory experience.

When two tones are presented simultaneously, your experience will differ, depending in part on the frequencies you're hearing. For example, suppose that you hear two pure tones that are identical in frequency. Of course, you will simply hear that single frequency. Here's an example of a 100 Hz pure tone in stereo. Now, suppose that you hear a 100 Hz pure tone in one ear and a 200 Hz pure tone in the other ear (i.e., one octave higher). The effect is essentially hearing a 100 Hz tone with its first harmonic. The important point, however, is that you hear a single tone, even though each ear is actually receiving a different tone.

Auditory beats occur when two pure tones have similar frequencies. For instance, imagine a 100 Hz and a 103 Hz tone. Their wave forms may look like the two waves seen below:

separate waves

With the 100 Hz tone on the bottom and the 103 Hz tone on top, you can see that the two peaks will occasionally line up (e.g., as is roughly the case for the first wave from the left). That will produce a peak for the beat (Peaks Line up). At nearby points, the two troughs will line up, which will produce a trough for the beat (Troughs Line up). Finally, on other occasions, the peak of one wave will line up with the trough of the other wave (which will occur beyond the right end of the figure). That will produce a cancellation, or a zero point. The disparity between the two waves creates the beats that are heard when the two tones are played simultaneously. The beats actually occur with a predictable frequency that lies between the two contributing waves [ (f1 + f2)/2 ].

The beats would look something like this:

combined waves

As I illustrated earlier, not all combined sounds result in beats. If the frequencies of the two sounds are sufficiently different, then no beats will occur. Instead, you might hear one complex tone (e.g., a fundamental and its harmonic) or you might hear two distinct tones. Here are some combined pure tones for your listening pleasure. When do you hear beats? When do you no longer hear beats?

100 + 103 Hz Tones (Speakers or Headphones)

100 + 120 Hz Tones (Speakers or Headphones)

100 + 160 Hz Tones (Speakers or Headphones)

100 + 200 Hz Tones (Speakers or Headphones)

100 + 500 Hz Tones (Speakers or Headphones)

Auditory beats in the world are fairly easy to understand. That is, they have a physical explanation (as above). However, what happens if you present the 100 Hz tone to one ear and the 103 Hz tone to the other ear? Note that now the two waves are kept separate from one another, so there is no opportunity for the physical merging of the two waves. Will you still hear beats? Take a listen. Do you hear a "beat" that bounces back and forth between your ears?

100 Hz + 103 Hz Tones (Headphones On!)

Apparently, there is no accepted explanation for such binaural beats!

 

Masking

Noise

Link - OSHA noise guidelines


Test Yourself

1. Discuss the place and temporal theories of pitch perception, reviewing the evidence in support of each theory. Point out the problems with both theories, and illustrate ways in which they both contribute to pitch perception.


2. In the auditory tuning curves shown in Figure 10.2, notice that the curves are sharper for high-frequency tones, and that the curves are steeper to the right than they are to the left. Can you use what you know (especially Figure 10.1) to explain these phenomena?


3. Discuss why pitch is not perfectly correlated with frequency. Consider the factors that influence pitch perception. Repeat this same process with the relationship between loudness and intensity, including a discussion of the impact of frequency on loudness.


4. Using the equal loudness contours (Fletcher-Munson curves), answer the following questions:
      a. for a 1000-Hz tone at 40 dB, what sound pressure will be equally loud for a 300-Hz tone? A 50-Hz tone?
      b. for a 1000-Hz tone at 100 dB, what sound pressure will be equally loud for a 150-Hz tone? A 50-Hz tone?


5. Discuss the ways in which sound intensity and sound frequency might be measured, and contrast this with the ways in which pitch and loudness are measured.


6. Notice a sound in your present environment and list the cues that help you judge the direction of the sound source. What are the factors that could help you resolve potential ambiguities about the location of that sound? Using only intensity differences, what frequency range would best be localized by a mouse, a human, and an elephant?


7. Our perception of space is unitary—auditory and visual information provide us with consistent detail about the world. Compare the information we discussed in Chapter 6 on visual space perception with what you have learned in this chapter. How do you think the information might be integrated?


8. Consider the auditory systems of echolocating creatures such as bats and dolphins. How do they differ from humans? Try to describe what it would be like to perceive the world entirely through echolocation (cf. Nagel, 1974).


9. Discuss the difference between monaural and binaural beats. How might monaural beats be helpful to musicians tuning instruments?


10. Consider the combination of visual stimuli (e.g., additive color mixing) discussed in earlier chapters and the combination of auditory stimuli (e.g., combining pure tones, masking, noise) discussed in this chapter. How does the visual system differ from the auditory system in encoding these combined stimuli? How might those differences lead to perceptual differences?

 


Teaching Materials

Link - The Acoustical Society of America maintains a site with useful information, publications, and resources (e.g., listen to a whale call). The best resource they provide is a Auditory Demonstrations CD from the more basic (e.g., loudness scaling, interrelationship of pitch and intensity) to the more esoteric (e.g., missing fundamental, Shepard tones, Deutsch's music illusion).

Link - Albert Bregman's Lab page is a useful site, especially regarding Auditory Stream Analysis. Bregman and Ahad have produced demonstrations to accompany the book, available as a CD or for download in mp3 format (Demonstrations of Auditory Scene Analysis: The Perceptual Organization of Sound).

Link - The vOICe site illustrates one possible auditory assistive device for people who have visual impairments.

Link - Echolocation in humans...Ben Underwood video.

Link -


Recommended Readings

Bregman, A. (1990). Auditory Scene Analysis: The perceptual organization of sound. MIT Press.

Cook, P. R. (Ed.) (1999) Music, cognition, and computerized sound: An introduction to psychoacoustics. Cambridge, MA: MIT. (With CD of demos)

Middlebrooks, J. C. & Green, D. M. (1991). Sound localization by human listeners. Annual Review of Psychology, 42, 135-159.

Neuhoff, J. (2004). Ecological psychoacoustics. Academic Press.

Plack, C. J. (2005). The sense of hearing. Erlbaum.

Yost, W. A. (2006). Fundamentals of hearing, 5th Ed. Academic Press.