Laboratory for Sound Stimuli and Anatomy of the Auditory System

1. The Auditory Stimulus

Shape of the Waveform

Although a great deal of research is conducted using pure sine waves, such waves are atypical. (You'll see examples of pure sine waves shortly.) Most sound waves are complex, as seen below:

Click on the wave forms to hear what they represent. The final wave is called White Noise, a random compilation of frequencies that is analogous to white light, in that it contains a full range of frequencies.

We will consider three basic characteristics of sound waves: intensity, frequency, and phase angle. In the pure sine wave depicted below, you can see a couple of complete cycles (beginning at a phase angle of 0° and ending at 360°). The amplitude (intensity) of the wave is illustrated by the height of the wave. The frequency of the wave is the number of complete cycles that occur in a second (cycles per second = Hertz, Hz).

Intensity or Amplitude

As illustrated in the four figures below, the frequency is constant (440 Hz). However, the height of the wave gets smaller with each successive wave, indicating less energy in the wave. Click on each figure to hear the differences in amplitude. Generally speaking, the physical dimension of intensity gives rise to the psychological experience of loudness. The relationship, however, is not a simple one, as you'll learn when studying loudness perception.

Sound pressure is typically measured in decibels (dB SPL). In this case, dB SPL = 20 log (Sound Pressure / Standard Sound Pressure), where the Standard Sound Pressure is .0002 dynes/cm^2. Note that a dB SPL is a logarithmic unit, so an increase of 20dB SPL is huge! To give you a sense of the relationship between dB SPL and sounds you may have experienced, look at the table below.

Frequency

The three waves below illustrate different frequencies of pure sine waves. Generally speaking, the physical dimension of frequency gives rise to the psychological experience of pitch. As is true for the intensity/loudness relationship, the frequency/pitch relationship is not a simple one.

As noted earlier, frequency is measured in cycles per second or Hertz (Hz). Each of the three frequencies below is an octave (doubling) apart, and would be the note A. Musicians typically tune to 440 Hz.

220 Hz

440 Hz

880 Hz

2. Auditory Anatomy

There are a number of potentially useful sites to provide you with information about auditory anatomy:

Virtual Tour of the Ear

Promenade 'round the Cochlea

a. On your lab worksheet, identify each of the major parts of the auditory system and their functions. Try to include information that goes beyond that found in your textbook.

Do you wonder why your ears will "pop" as you change altitudes? Here's an explanation of the role of the eustachian tube.

b. On your lab worksheet identify each of the structures of the inner ear as well as their functions. Again, you'll find Promenade 'round the Cochlea to be useful.

To get a sense of how the Organ of Corti moves and then produces neural impulses, check out this site at the University of Wisconsin. Click on both the Organ of Corti animations.

c. The afferent fibers of the auditory nerve carry information from the cochlea to the auditory cortex. The intermediate structures are indicated below. On your lab worksheet provide detail on the function of each structure indicated.

3. Auditory Tests, Disorders, and Treatments

Lots can go wrong with your auditory system (scary, huh?).

a. an audiogram illustrates at which frequencies (if any) there are hearing deficits. Check out the Simon Fraser University site to learn about audiograms.

b. high-frequency hearing loss

To get a sense of how a particular type of hearing loss would affect one's auditory perception, check out the demonstrations at the Phonak site. Note that the mild and moderate hearing loss being illustrated affect the higher frequencies. Such high-frequency hearing loss is most likely due to environmental noise (check out H.E.A.R. if you go to loud concerts)

c. tinnitus

Can you now understand how annoying this disorder would be? Note that the actual noise heard by people varies greatly. However, the noise is incessant! Learn more about tinnitus on the web. You can use any search engine you want to conduct the search. However, here are a few useful sites: (American Academy of Otolaryngology; Oregon Health Sciences University). Note that your search will likely lead you to a number of sites making strong claims about treatment for tinnitus. Suppose that you or a friend was suffering from tinnitus and you wanted to learn more about treatment on the web. On the basis of your search, what form of treatment do you think would be most useful? Why?

d. Otitis Media (middle ear infections) occur with some frequency among children. Learn what you can about such infections and the recommended treatment by searching the web. (Here's a site to start you off on your search.) Why are such infections more prevalent in children than in adults? Search the WebMD site for Otitis media. Again, presume that your child has developed otitis media. What form of treatment would you pursue, given what you learn on the web. Note the typical qualifications now found about performing a myringotomy/tympanostomy (insertion of tubes).

e. Cochlear implants are relatively expensive, but effective (especially when provided to younger children). Cochlear implants are actually quite controversial. The National Association of the Deaf also has a position paper on cochlear implants that may interest you. What is your sense of the efficacy of cochlear implants? For what sorts of hearing loss is such treatment useful? If you had substantial hearing loss, would you undergo such an operation?