The purpose of this lab is to give you practice with concepts of a transfer function and to demonstrate the response characteristics of the external ear.The lab will also familiarize you with the structures of the external ear, review concepts related to impedance of the middle ear, and demonstrate the effects of sound frequency on middle ear impedance.

Two workstations with function generator, speaker, probe microphone, oscilloscope and digital multimeter. Otoscopes and alcohol pads. Virtual impedance bridge system and printer. Internet workstations.

III. Orientation Points

1. Demonstrate the techniques of otoscopy with emphasis on safety (of both ears and the otoscopes!)
2. Review the purpose of the external ear.
3. Review the concept of the transfer function.
4. Explain the transfer function measurements that will be made in lab.
5. Explain the operation of the equipment.

1. Review the concept of impedance and admittance.
2. Explain the logic behind tympanometry.
3. Review normative values.
4. Demonstrate the use of the Virtual system for obtaining canal volume, peak pressure, and static compliance.

1. Procedure
1. External Ear Anatomy and Otoscopy
1. Sketch the pinna of your partner. Label the following structures: tragus, concha, helix, ear canal, and lobe.
2. Compare your drawing of your partner’s pinna to your partner’s drawing of your pinna. Describe the differences between the two.
3. Using the otoscope, examine your partner’s ear canal and tympanic membrane. Sketch what you see and label the tympanic membrane, "cone of light", and the manubrium of the malleus, if visible. Describe what you see in the ear canal.

2. Transfer function of the external ear
1. Position the microphone near the subject’s ear.
2. Set the function generator to a 250 Hz sine wave.
3. Set the volume control knob so that the sound picked up by the microphone is dominated by the signal, not the noise, as seen on the oscilloscope. Be sure that the volume is comfortable for the subject.
4. Measure and record the voltage, in dB, using the digital multimeter on the rack.
5. Measure and record the voltages at 500, 1000, 2000, 4000, 6000, and 8000 Hz. Do not change the volume control knob.
6. Holding the subject’s head as still as possible, insert the microphone probe tube into the subject’s ear canal as demonstrated by the lab instructor.
7. Measure and record the voltages again at 250, 500, 1000, 2000, 4000, 6000, and 8000 Hz.
8. Remove the microphone probe tube.

3. On your graph paper make a chart for outside the ear dB, in the ear dB, and difference of the two in dB for each frequency tested. The difference is the effect of the external ear and is called the transfer function.
4. On your graph paper plot the external ear transfer function as dB vs. frequency for your subject. Make sure to label the axes.
5. Obtain and make a chart of the data obtained for subjects tested by other groups.
6. Plot the data for additional subjects on the same plot drawn in part

2. Questions
1. Are the functions plotted the same for each subject?
2. How are the response characteristics different?
3. Why do you think that they might be different?