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INDIANA UNIVERSITY, DEPT. OF PHYSICS
P105, Basic Physics of Sound, Fall 2008
Midterm Exam #2
Thursday, 13 November 2008, 7:30 – 9:30 p.m.
Closed book. You are allowed a calculator.
There is a Formula Sheet and Equal Loudness Curves at the back of the exam
Fill in the answers on the exam sheets themselves. If you run out of room, use the back of
the sheets. In some cases I give partial credit – try not to leave blank answers. Show your
work for fill in the blank and multiple-choice questions also for partial credit.
1.
Organs galore:
(a) (3 pts) An organ pipe (closed at one end, open at the other) is carefully tuned to middle
A, i.e., 440 Hz at 20°C. Assuming that the length of the pipe remains fixed, what is the
fundamental frequency of this pipe at 35°C?
Hz
(b) (3 pts) Only the longest organ pipes found in cathedrals can reach down into the
infrasonic (and cause chills to run down your spine...). To produce a fundamental frequency
of 16 Hz at 20°C (i.e., vsound = 343 m/s) in a stopped pipe (one end open, other end closed),
the required length of the pipe would be L =
m.
(c) (2 pts) What would be the frequency of the fifth harmonic, f5, in such a long
pipe?
Hz.
Midterm Exam #2, P105, Basic Physics of Sound – 13 November 2008
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(d) (4 pts) Sketch below what the standing wave pattern would look like at this fifth
harmonic, clearly labeling nodes and anti-nodes with "N" and "A". Give the resulting
wavelength of the sound. λ =
m.
(e) (2 pts) The organist really cranks it up and is producing a sound intensity level of SIL =
80 dB at a distance of 10 m. What is the sound intensity at this distance?
W/m2
(f) (2 pts) What is the sound power being put out by the pipe assuming that sound is
spreading spherically?
W.
(g) (3 pts) How far away would you have to stand so that the new sound level intensity
would be SIL = 65 dB?
(h) (4 pts) Using the attached plot of equal loudness curves at the end of the exam, if the
organ is playing a note at a frequency of 4000 Hz at a SIL = 65 dB, what is the perceived
loudness?
phons.
What would the sound intensity level have to be for the same perceived loudness, but at a
frequency of 125 Hz?
dB.
Midterm Exam #2, P105, Basic Physics of Sound – 13 November 2008
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(i) (4 pts) The organ continues to play at a sound intensity level of 65 dB and then the choir
kicks in at a sound intensity level of 67 dB. What is the sound intensity level of the two
sounds combined if they add incoherently (i.e., without interference)?
dB
2. (3 pts) What is white noise? Describe the difference between white noise and pink noise.
3. (4 pts) Fill in the two missing blanks in the block diagram below of a typical analog
synthesizer, and describe briefly what each of the missing components does.
4. (3 pts) (i) Two pure tones are close enough in frequency to overlap in their response on
the basilar membrane are said to lie within the same
band.
(ii) Speech sounds originate in the
with vibrations of the vocal
Midterm Exam #2, P105, Basic Physics of Sound – 13 November 2008
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5. (4 pts) For the given frequency spectrum input to a filter resulting in the output frequency
spectrum shown, draw the filter function and name the type of filter.
6. (3 pts) (i) A complex waveform repeats itself every 4.5 ms. The frequencies of the first
three Fourier sinusoidal frequencies are
Hz,
Hz, and
Hz.
(ii) If this complex wave is actually a sawtooth (ramp) wave and the amplitude of the
fundamental frequency f1 is 6.0 W/m2, then the amplitude of the third harmonic f3 is
W/m2
7. (a) (6 pts) According to the Place Theory of Hearing, describe how the human inner ear
detects each of the following aspects of sound:
(i) Pitch/Frequency:
(ii) Amplitude/Intensity:
Midterm Exam #2, P105, Basic Physics of Sound – 13 November 2008
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(iii) Timbre:
(b) (2 pts) Give two roles of the middle ear in the hearing process.
8. (i) (4 pts) The figure (a) below shows the waveform just outside of the glottis or vocal
cords of the famous opera singer Pavarotti. The time T between each of the waveforms
shown is 0.005 sec. If the vocal tract of the person has formants at F1 = 600 Hz and F2 = 1800
Hz, sketch the approximate frequency spectrum of the resulting sound in figure (b), labeling
the fundamental f1 and the formant frequencies F1 and F2.
Midterm Exam #2, P105, Basic Physics of Sound – 13 November 2008
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(ii) (3 pts) If this vocal tract can be modeled by a simple tube open at one end (the mouth)
and closed at the other, estimate the length of the vocal tract.
9. (3 pts) Tremolo of a frequency of 4 Hz is applied to a pure tone of frequency 440 Hz. This
is an example of
modulation. If one examined the frequency spectrum
of this sound, three peaks would be observed: one central peak at a frequency of 440 Hz,
and two sideband peaks at frequencies of
and
Hz.
10. (2 pts) If three sinusoidal sounds, each with a frequency of 600, 840, and 1080 Hz are
played together, the pitch of the sound heard would be
Hz.
Remember that the lecture on Friday (tomorrow) is cancelled. Also your next CALM homework
will be due next Thursday, not Tuesday. Have a good weekend!
Midterm Exam #2, P105, Basic Physics of Sound – 13 November 2008
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Equal loudness curves:
Midterm Exam #2, P105, Basic Physics of Sound – 13 November 2008
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