perception in practice.docx
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8/13/2019 perception in practice.docx
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LAB 4 (CHAPTER 5): HEARING 1, SOUND & THE EAR
(Graded out of 50, worth 5% of final grade)
This lab consists of a single part, which will be done at home. If you wish to do the
lab in the lab room, you will need to bring headphones.
Equipment:
For this lab, you will need the following:
- A computer capable of playing sound. It is best if you use reasonably high
quality headphones or speakers, but just use the built in speakers if
necessary (though they are typically of poor quality)
- A program capable of playing “.wav” files (almost any computer will have
such software).
- A set of “.wav” files that each plays a given frequency of pure tone in a
descending sequence of amplitudes (we have provided you with this)
If you do not have access to the necessary hardware and/or software, you may use
the computers in the lab room, but in this case you must bring a set of headphones
or ear buds.
Handing In:
Hand in a physical copy of your lab to Alicia before Lab 5 begins.
Background: In class we discussed the idea that there are two fundamental
characteristics of sound: Frequency and Amplitude. Here we will examine how theabsolute threshold for sound amplitude varies as a function of sound frequency. As
we will see in the next chapter, this function is a fundamental characteristic of the
human auditory system. In preparation for this lab, you should read Chapter 5,
especially sections A2 and B2. You may also want to skim Chapter 6, section A1.
Procedure: You will be acting as researcher and participant for this lab. There is no
need for a partner.
What you will be doing in this lab is playing a series of sound files (e.g.,
HumBeh_p007_200Hz.wav). Each sound file plays a series of pure tones at a given
frequency. The tones drop in amplitude. You will be counting the number of tonesyou can hear for each file and deducing from that what your absolute amplitude
threshold is for each frequency.
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Here are the detailed instructions:
1) Adjust the volume on your computer for proper playback of the tones.
a) Use the best quality audio headphones or speakers available. If you use low
quality ones, you will find that some of the files, especially at the low
frequency end, produce no sound when you play them. This is because thesetones are outside the range of frequencies your speakers can produce. If this
occurs, simply mark "N/A" for the data for that frequency and plot the
function for the values that you can play. Even cheap speakers should be
able to play 200 Hz and above.
b) Do your measurements in the quietest room available. Background noises
will interfere with the sounds you are trying to hear, and may change your
threshold values. If you do the measurements in the lab room, be sure to use
headphones.
c) Play the 3500 Hz sequence (file name: HumBeh_p007_3500Hz.wav) and
adjust the volume so that you hear only the first 18 or 19 tones in the
sequence. There are 25 tones in the file, so this means that you have adjustedthe volume so that tones 20–25 are below your threshold of hearing.
d) Write down the number of tones you can hear in the 3500 Hz sequence, and
leave the volume setting at this level for all of the other files.
2) Listen to each file and count the number of tones you can hear.
a) Repeat at least five times for each file to assure an accurate count. You should
do this in some kind of random order (how you implement this is up to you).
b) Use the table in the results section to keep track of your counts.
c) Allow yourself about an hour for this step.
3) Convert your count to a threshold value for each frequency.
a) You will be plotting your results relative to your threshold for the 3500 Hztone sequence. We will assign 3500 Hz to 0 dB. Note that our measurements
here are necessarily relative to some standard, as we have not calibrated our
speakers to determine exactly what amplitudes are actually being played.
b) To calculate your threshold value for a given frequency, take the average
count for that frequency, subtract it from your count for 3500 Hz, and
multiply this number by 3 dB (because each tone in the sequence is
decremented in amplitude by 3 dB).
c) For example, say you have adjusted the volume so that you can hear the first
18 levels of the 3500 Hz tone sequence. Let's say you counted an average of 5
levels for the 100 Hz tone sequence. Your threshold for 100 Hz would then
be: (18 − 5) × 3 = 39 dB. Note that you can get negative values of dB.
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Results
First, record the number of tones you heard at 3500 Hz after adjusting the volume
(which should be 18 or 19): ________
Enter your results for each of the frequencies in the following table.
Frequency Count 1 Count 2 Count 3 Count 4 Count 5 Average Threshold (dB)
Graph
Produce a scatter plot showing the relationship between frequency and threshold.
This graph should be plotted on semi-log axes. This means that each tick along the x
axis represents a jump of 10x the previous value. To do this in MS Excel 2011:
- Enter your frequency data and threshold data in two columns
- Select your two columns of data (Frequencies and Thresholds)
- Select the "Charts" tab and choose "Scatter"
- A chart should appear. Double-click one of the numbers along the x axis.- In the formatting pane that appears, choose "Scale" in the upper left
- Click "Logarithmic Scale", ensuring that the base is set to 10.
Paste your graph in at the top of the next page.
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Discussion:
1. Describe your frequency-by-threshold function. Were there large differences in
threshold or not? If yes, where were you most sensitive? Where least?
2. We used a method of limits in this lab to find your thresholds, but we used only
descending sequences. What effect is it likely to have had on our measurements?
Can you think of a reason why this is a defensible research design option in this
case?
3. What is it about the anatomy of the human auditory system that makes us more
sensitive to certain ranges of frequencies than others?
4. Can you think of any evolutionary pressures that likely led to humans being more
sensitive to certain frequencies than others?
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