Part II
1. - 5. For answers 1-5, use the code
1. phase shift
2. frequency modulation
3. amplitude modulation
4. balanced modulation
5. damping
to identify the wave property shown in the pictures: 1. The mathematical representation of the equivalence between a complex waveform and a mixture of pure sine waves is
1. Huygens principle
2. Bernoulli's principle
3. principle of superposition
4. Fourier analysis and synthesis
5. equivalence principle
2. A unit of frequency is
1. hertz
2. cps (cycles per second)
3. rpm (repetitions per minute)
4. sec-1
5. all of the above
3. A difference between a mode and a node is
1. the same as that between Mote and a note
2. a system may oscillate in one mode, but there is no oscillation at a node
3. different nodes of oscillation are distinguished by the number of modes they have
4. an oscillating system may have a number of nodes, but it must always oscillate in just one mode
5. there is no difference
4. The change in propagation direction when a wave crosses a boundary between two different media is called
1. reflection
2. refraction
3. reproduction
4. resonance
5. release
5. In order that the superposition of two waves produce a steady pattern of interference, the waves must have the same
1. amplitude
2. phase
3. frequency
4. damping
5. intensity

The following question have numerical answers in integers between 1 and 5. Mark the corresponding integers on the answer sheet. (That is, marking the first box will mean the answer is the number 1, and so on.)

1. - 15. Consider a sawtooth wave of amplitude 2. Add to it an offset 3, so the wave varies between 1 and 5. Sample it 5 times per period, and mark five successive sampled amplitudes in answers 11-15, starting at amplitude 1.
1. - 20.  Repeat the previous question for a triangle wave of same amplitude, offset, period, and starting amplitude.
1. How many binary bits do you need (at least) to represent the waves of the previous questions, whose amplitude varies (in integral steps) between 1 and 5?
2. How many numbers, possible with the number of binary bits you chose, are not used in representing the amplitudes 1 to 5?
3. How many of the following list of properties of an acoustical wave are unimportant in determining how the wave sounds to a human listener:
amplitude, frequency, exact wave shape, phase, Fourier spectrum
4. How many components (harmonics) are there in the Fourier spectrum of a pure sine wave?
5. I send a 1000 Hz square wave through a low-pass filter with cut-off at fF = 6000 Hz. How many harmonics are present in the output from the filter?
6. What number (base 10) is represented by the binary number 101?
7. By what factor does the intensity of sound from a (point-like, isotropic) source increase when I halve my distance to that source?
8. By what factor does the amplitude of the sound wave increase when I halve my distance to the source?
9. A wave of amplitude 3 and one of amplitude 2 are 180° out of phase. What is the amplitude when they interfere?
10. A sine wave of amplitude 4 is amplitude modulated by a (lower frequency) sine wave of amplitude 2. What must be the least offset level of the lower frequency wave to avoid overmodulation?
11. If the SIL of a tone increases by 5 dB, by what factor does the intensity increase?
12. The SIL of one source of some tone is 20 dB. By how much (in dB) does the SIL increase if two such identical sources are played simultaneously?
13. Take the speed of sound to be 320 m/s. How many seconds does it take sound to go 1600 m (about a mile)?
14. A string with fixed ends is vibrating in the third harmonic mode. How many nodes are there (not counting the ones at the ends)?
15. What is the harmonic number of the second possible standing wave in a closed tube?
16. By what factor does the frequency of a sound increase when the pitch increases by one octave?
17. How many strong, low, isolated resonant frequencies does a Helmholz radiator have?
18. An amplifier is putting a power of 36 watts into a speaker with impedance 9 W. How much current (in amps) is flowing into the speaker?
19. What voltage is being delivered to the speaker of the previous question?
20. The amplifier gets its power from a 12 V car battery. How much current must it draw to supply the 36 W (disregard the fact that actual amplifiers need considerably more input power than what they put out)?

21. Two frequencies are close to each other but not equal. As one of them changes to become closer to the other one, the frequency of the beats heard
1. decreases
2. increases
3. remains the same
4. no beats are heard
22. What happens when the output impedance of a source of electrical power does not equal the input impedance of the device into which the source should deliver its power?
1. less power is delivered
2. some of the source signal is reflected
3. the signal does not pass efficiently from source to device
4. the impedance matching is inadequate
5. all of the above
23. The difference between a dynamic and an electrostatic microphone is that
1. the first will work when carried around (when attached to a performer, for example), the second works only if it is fixed in one place
2. only the second puts out an electrical signal
3. only the first reproduces the dynamics (loud vs. soft passages) of music well
4. the first uses the law of changing magnetic fields (induction), the second uses the law of electrostatics relating charge and voltage on a capacitor
5. "dynamic" and "electrostatic" are brand names of competing manufacturers for identical devices
24. In a lecture demonstration, an amplifier was driven by a sine wave beyond its linear range. What happened?
1. the output had Fourier components at frequencies other than that of the input sine wave
2. the output had Fourier components at fewer frequencies than the input sine wave
3. the output was a sine wave, but shifted in phase compared to the input
4. the output had a different fundamental frequency than the input
5. as the input amplitude was increased beyond the amplifier's linear range, the output decreased in amplitude
25. Which type of tube can never have a node at its center?
1. a tube closed at both ends
2. a tube open at both ends
3. a tube open at one end and closed at the other
4. the second harmonic of all types of tube has a node at the tube's center
26. Which aspect of a synthesized tone is normally not controlled by the envelope generator?
1. period
2. attack
3. release
4. sustain
5. decay
27. A loudspeaker usually sounds better when mounted on a baffle or in an enclosure because
1. the baffle prevents nonlinearities
2. the baffle prevents unwanted reflections
3. the baffle improves the speaker's efficiency at high frequencies
4. the baffle prevents sound from the back of the speaker from interfering with the sound from the front
5. all of the above

28. A frequency region in which harmonics present in some tone are emphasized relative to the other harmonics is called a
1. resonance
2. focus
3. filter
4. formant
5. fipple
29. The perception of pitch is (mainly) correlated with the physical variable
1. fundamental frequency
2. frequency of harmonics
3. harmonic number
4. phase
5. tension
30. How can you increase all the formant frequencies present in your voice?
1. speak after having inhaled a gas, such as Helium, in which the speed of sound is greater than in air
2. speak in a falsetto voice by increasing the tension in the muscles of the vocal folds
3. speak through a long, resonant tube
4. run a tape recording of your voice at a decreased tape speed
5. make all your vowels sound like French nazalized vowels.
31. In lecture and on the internet we showed a sequence of tones that appeared to be rising in pitch, but which returned to the exact initial tone after apparently increasing by an octave ("Shepard tones"). How was this done?
1. the input to the sound system of the lecture hall had truely increasing pitch, but there was a strong resonance in the sound system, which emphasized a particular frequency whenever it was present
2. the frequencies in the tone's spectrum did increase, but lower frequency components were slowly added, and higher frequency components eliminated, keeping the spectrum under a constant envelope
3. the tones consisted of a rapid frequency modulation over one octave
4. Each note started at a higher pitch than the previous one, but actually decreased in pitch while it was being sustained
32. What is the difference between white noise and pink noise?
1. Pink noise results from mixing white noise with a low-frequency sine wave
2. Pink noise has only Fourier components that are multiples of some low frequency, wheras white noise has all frequencies present with equal intensity
3. The frequencies present in pink noise cover only one octave, those in white noise cover all octaves
4. pink noise has less power at high frequencies than white noise
5. pink noise has more power at high frequencioes than white noise
33. An important support for the place theory of hearing is that
1. there is a critical band of frequencies within which two tones affect each other, and make the combination sound "coarse"
2. we perceive pitch on a logarithmic scale
3. we perceive loudness on a logarithmic scale
4. we can hear combination tones
5. we can hear the periodicity pitch when there is a missing fundamental
34. The timbre of a sustained sound is most closely correlated with which physical characteristic of a sound:
1. frequency
2. amplitude
3. Fourier spectrum
4. phase of harmonics
5. loudness
35. Of the following, the sound with the most rapid attack is that of
1. the roar of the crowd at a football game
2. a car speeding past
3. a flute
4. the "wow" sound programmed on an analog synthesizer
5. a jet "breaking" the sound barrier
For the following true-or-false questions, mark 1 (A) for true, 2 (B) for false.
1. Period and frequency measure the same characteristic of simple harmonic motion.
2. When a body moves in simple harmonic motion (SHM), its velocity as a function of time also has the SHM shape.
3. Longitudinal waves always travel more slowly than transverse waves.
4. The speed of sound is constant and independent of the medium in which the sound propagates.
5. Huygens's principle applies to both longitudinal and transverse waves.
6. The inverse square law is never valid for transverse waves.
7. Quincke's tube shows the interference of sound waves.
8. Diffraction of sound makes it possible to hear sound around corners or behind barriers.
9. Superposition of two waves always yields a wave with larger amplitude than that of either of the two waves.
10. When two waves are superposed and beats can be heard, the beat frequency equals the difference between the frequencies of the two superposed waves.
11. The Doppler effect always makes the frequency of a moving source appear higher than it really is.
12. Standing waves occur only for transverse waves, longitudinal waves are always travelling waves.
13. The frequencies of standing waves on a stretched rope with fixed ends are integral multiples of a fundamental frequency.
14. Mersenne's laws imply that if the tension T and the linear mass W of a string are both multiplied by the same factor, the string's fundamental frequency does not change.
15. Reflections of sound waves occur both at a closed end and at an open end of a tube.
16. Although the air in a tube can vibrate in many different modes, only one of these (the fundamental) is a resonance.
17. The control voltage of a VCO determines the wave shape that the VCO produces.
18. The low-frequency control oscillator in a synthesizer can be used to produce a vibrato effect in the synthesizer output.
19. Ohm's law of hearing states that the sound quality of a complex tone can be produced by a superposition of a suitable number of simple sine waves.
20. Resonances of the vocal tract produce the formants present in spoken vowels.
21. VI, I˛R, and V˛/R are formulas for electrical power.
22. In a loudspeaker, the voice coil is rigidly mounted on the speaker's magnet.
23. A magnetic tape can record either analog or digital signals.
24. It was a BIG mistake that we did not get to Chapter 8 in the text (Room and Auditorium Acoustics).

End of Exam

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