Laboratory for Perception of Complex Auditory Stimuli
These laboratory exercises investigate various auditory phenomena. The experiences are quite different, but what links them is that they all involve relatively complex auditory phenomena.
Simultaneous Sounds
1. Auditory Beats
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:
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:
You know that not all combined sounds result in beats, right? If the frequencies of the two sounds are sufficiently different, then no beats will occur. Instead, you might hear one complex tone (i.e., 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?
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!
2. Noise
Noise is often unwanted, but some auditory research depends on noise (especially particular types of noise, such as white noise). For example, to create stimuli for a signal detection experiment, it may be essential to present tones embedded in (white) noise, so that they are not easily detected. First, listen to a 1000 Hz tone. Now, listen to the same 1000 Hz tone with some white noise added and with more white noise added. Obviously, were we to add white noise at an even higher amplitude, it could entirely mask the 1000 Hz tone. Note that it's also possible to mask sounds with other sounds that aren't presented simultaneously.
3. Complex Tones and The Missing Fundamental
In class, we've discussed the notion that complex sounds are comprised of a fundamental frequency as well as harmonics or overtones. (Note that the combination of these pure tones results in the perception of a single tone, without beats.) We've also experienced the phenomenon of the missing fundamental. Now, go to Dale Purves's site to play around with various combinations of a fundamental frequency and its harmonics. (Within the Sound and Music section, click on the lower icon.) Do you agree that with all the pure tones except the fundamental, you hear the same basic pitch as when the fundamental frequency is played by itself or with all the overtones? When you eliminate some of the overtones, does the pitch change?
Anomalies of Pitch Perception
4. Shepard Tones (discrete and continuous) & The Tritone Paradox
As is true for visual illusions, we can learn a lot about auditory perception by examining auditory illusions. Here are two illusions of pitch perception.
Shepard Tones (Shepard, 1964) create the illusion that sounds continually increase (or decrease) in pitch. These complex tones have component pure tones that create the illusion. Check out the following demonstration, with the display turned on (as seen in the window below). [Click on the Applet link, then click on the Start button.] You can play around with the number of component tones, but in comparing the graphical representation of Tones 1 and 11 with 6 components, you should get a sense of what creates the confusion. Articulate why you perceive the illusion of a constantly rising (or falling) pitch? What does this demonstration tell us about pitch perception?
Next, check out the continuous version of this demonstration at the Exploratorium site.
You should play around with the tritone paradox as well (Deutsch, 1987). You can learn more about these and other auditory phenomena by visiting Diana Deutsch's site (Research and Musical Illusions link), where you will find explanations of the tritone paradox, the octave illusion, and other demonstrations of auditory phenomena. You will find a link to the Philomel site, from which you can order CDs on which these demonstrations may be found in their entirety.
Outrageous Claims for Music?
5. Music and Mind/Brain
How does the brain respond to music? To see your brain on music, check out this video. The brain is affected by music and the brain certainly affects how we perceive music. That said, people have made a number of claims for the impact of music on the brain that may not be warranted. You've probably heard of the Mozart Effect. Explore the Mozart Effect Site, which makes lots of claims related to this phenomenon. Does the evidence appear to be compelling?
You should realize that a number of studies have called the Mozart Effect into question. Do you find any indication at the Mozart Effect site that their results may not be widely replicated? Think as critically as you can about the claims being made. You might be interested in a brief description of research at Eric Chudler's site (Neuroscience for Kids!!). You may also find a summary CNN article of interest.
Speech-Related Demonstrations
6. Vowel Sounds
Check out this Exploratorium demonstration of the production of vowel sounds.
7. Ladle Rat Rotten Hut
This Exploratorium demonstration is neat. If the link to the sound doesn't work on your computer, try this one. Do you think that you would hear the "proper" words (Little Red Riding Hood) if you were not already familiar with the story? What role does prosody (intonation, "melody") play in speech?
8. Speech Perception
David Lane has provided a number of useful web-based tools in the realm of statistics. He's also provided a useful page on speech perception. Read over the page, especially the portion titled "Is Speech Special?". In that section, go through the Categorical Discrimination demonstration. Then, to see the intersection between audition and vision, view the demonstration of the McGurk effect in the section on "How is Speech Perceived?". (You will likely have to wait a bit for the demonstration to download.)
9. Phonemic Restoration Effect
Click each trial, one at a time, and respond on the answer sheets provided in your worksheet.
For each trial you will hear a sentence spoken clearly. A cough will occur at some time during the sentence. Your task is to indicate where you think the cough occurred by placing a circle around the exact place you heard it in the typewritten sentence. Additionally, you need indicate, for each trial, whether the cough completely replaced the sound(s) that you circle or occurred in conjunction with it/them. You can download a pdf file that shows the soundwaves occurring in these trials, but please do so only after you've made your judgments.
Take your time and try to respond as accurately as possible to each stimulus. Listen to each trial only once.
More Outrageous Claims?
Is it possible that there are messages hidden in our speech, such that they can only be heard when our speech is played in reverse? A number of people claim that such embedded messages have occurred in music (e.g., Claiming that The Beatles were reverse-singing that "John is dead.") Do such messages exist? If they exist, are people sensitive to those messages? Could such "messages" have any impact on one's behavior? [For a useful introduction, see...Vokey J. R. & Reed, J. D. (1985). Subliminal messages: Between the Devil and the media. American Psychologist, 40, 1231-1239.]
First, can you hear any message in this segment of reverse music? Next check out this "hint," and then listen to the reversed music segment once again. Do you hear the phrase in the music now? Why?
Now listen to the following segments of reverse speech. Do you hear any "embedded messages?"
Now, check out some hints for phrases that you might hear in the reverse speech segments and listen to them one more time. Do you now hear those phrases?
Check out the ReverseSpeech site (Oates) and tell me if you find his claims persuasive?