| CHAPTER 1 HEARING: Page 1 |
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 WHAT DO YOU THINK?
A certain stereo system is advertised as producing "a smooth, flat response."
- What do you think the frequency response of an audio system is?
Record your ideas about this question in your Active Physics log. Be prepared to discuss your responses with your small group and the class. |
 FOR YOU TO DO
- Your teacher will provide an oscillator or a keyboard synthesizer for the sound source. This source will be connected to an amplifier with an equalizer and a loudspeaker. There will be one sound system for the whole class. Each group will have a sound level meter. Your teacher will sweep the sound through a range of frequencies. You will use the sound level meter to measure the sound level.
 a) As your teacher sweeps the sound through a range of frequencies, record in your log the lowest and highest frequencies you can hear.
- 2. If your teacher has an oscillator, decide as a class which frequencies you will use. Select at least 15 different frequencies in the range of human hearing. If you use a keyboard synthesizer, select at least 15 notes to measure. You might try every other white key.
- a) In your log, record the frequencies you decide to use. Indicate why you decided on these frequencies.
- 3. Make sure that your sound level meter is set on C weighting, so that the response of the meter is "flat" and does not favor any frequencies.
- 4. Your teacher will apply a "pure tone" (sine wave) at different frequencies, one at a time, to the input of the amplifier. Each team will measure the sound level at that frequency. Decide in advance whether to aim the sound level meter at the speaker or at the ceiling. Then keep it aimed the same way throughout the experiment. The tone controls or equalizer controls on the amplifier should be set in the "flat" position.
- a) In your log prepare a table similar to the following one, but with more rows.
- b) Record your team's data in one column.
- c) When the measurements are completed, record the data for other teams as well.
- d) Next to your table, describe how far from the speaker the sound level meter was placed. Also, tell where it was aimed.
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- 5. Now you can draw a graph of your data. You will use two sets of axes. One is linear, which is the kind you have used many times before. The other is called semi-logarithmic and is shown in the diagram on the next page. Take a look at the logarithmic paper.
- a) What is unusual about this graph paper?
- b) Look at the spacing between 10 and 20. Is it the same as the spacing between 80 and 100? Does that seem unusual? Why?
- c) Is the spacing between 10 and 20 the same as the spacing between 100 and 200? Is it the same as the spacing between 1000 and 2000?
- d) Look at the spacing between 10 and 100. Is it the same as the spacing between 100 and 1000? How about between 1000 and 10,000?
- e) Draw graphs on the linear and the logarithmic graph paper.
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- 6. Use your graph to answer the following questions.
- a) Describe the frequency response of the system you tested.
- b) Share your results with the other groups. How much variation in response is there in different parts of the room?
- c) Which kind of graph paper do you prefer? Why?
- 7. Change the setting of one of the equalizer levels to reduce the midrange frequencies.
- a) Take the data again and plot it on the same graph.
- 8. What is it like to have a hearing loss? You can use the equalizer to find out. Connect a radio to the input of the amplifier. With the equalizer set for a "flat" frequency response, listen to a station that has a lot of talk. Set the volume so that the sound level is comfortable but not loud. Then adjust the equalizer to remove frequencies of 2000 Hz and above, corresponding to a typical hearing loss. Also try listening to music.
- a) Describe how well you can understand the speech you are hearing.
- b) Describe how the music sounded.
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