Physics102 Second Exam Nov 20, 2001

Instructions: This exam consists of 7 question pages and one NCS answer sheet. Before you start answering the questions, fill in your name in letters and in black circles on the NCS answer sheet, last name first. Then answer questions 1-33 on that sheet. Note that due to machine grading only one of the five answers to each question can be counted as correct. Therefore give the best answer of those offered, even if others also seem correct.

The exam will last 75 minutes, including the distribution of these pages, your reading these instructions, and your handing in the completed exam. Put these stapled pages, back page up, into the alphabetical trays, and put the NCS answer sheet into the single tray on the lecture table.

Grade scale: Each of the 33 multiple choice question is worth 2 points; each part of problems 34 and 35 is worth 3 points; one point for filling your name in properly and legibly; making for an exam total of 100 points.

1. A complex tone is synthesized from sine waves of frequencies corresponding to an overtone series. What is least important for the timbre (or sound quality) of the synthesized wave (as perceived by a typical listener)?

1. the amplitude of the fundamental
2. the amplitude of the higher harmonics compared to that of the fundamental
3. the the relative phases of the harmonics
4. the presence of a formant region in the spectrum
5. the presence or absence of even harmonics

2. Fourier's theorem states that
1. any wave that can be analyzed into single-frequency components can also be synthesized from these components
2. any periodic wave with some frequency f can be synthesized from sine waves of the frequency f and its harmonics
3. the amplitudes and phases of a complicated wave shape are related by a simple formula
4. any wave containing harmonics of some fundamental frequency f will repeat itself with a periodicity T = 1/f
5. none of the above

3. The main difference between the Fourier spectrum of a sine wave and a square wave of equal period and amplitude is that the sine wave has
1. a different amplitude of the fundamental
2. a different amplitude of the second harmonic
3. no higher (N > 1) harmonics whatever
4. a formant at harmonic N = 3
5. there is no difference between the spectra
4. Pink noise consists of all frequencies of the audible spectrum superimposed with an intensity that falls off at a rate of 3 decibels per octave. This means that the Fourier spectrum of pink noise consists of
1. discrete peaks in an overtone series with sizes decreasing by steps of 3 for each octave increase in frequency
2. discrete peaks in an overtone series with sizes decreasing by a factor of 3 for each octave increase in frequency
3. some amplitude at all frequencies, and with total power in each octave 1/3 of that in the previous octave
4. some amplitude at all frequencies, and with total power in each octave 1/2 of that in the previous octave
5. equal amplitude in every third octave

5. The air in an empty wine jug will resonate when you blow across the opening. Such a jug can be well treated, acoustically, as
1. a Helmholz resonator
2. an open tube
3. a closed tube
4. a sounding board
5. a receptacle for more wine

6. The distinguishing characteristic of the resonance curve of a Helmholz resonator is
1. a single, low, isolated resonant frequency
2. resonances at odd harmonics only
3. resonances even harmonics only
4. the absence of a fundamental
5. multiple closely spaced resonance frequencies

7. In one lecture demonstration we used a Fourier synthesizer to synthesize a 100 Hz square wave out of sine waves of different frequencies. The necessary frequencies were
1. 100 Hz, 300 Hz, and 500 Hz
2. 100 Hz, 200 Hz, and 300 Hz
3. 100 Hz, 200 Hz and 400 Hz
4. 100 Hz, 300 Hz, 500 Hz, and all other odd numbers × 100 Hz
5. 100 Hz only
8. In that Fourier synthesis the amplitudes had to be
1. all equal (independent of frequency)
2. decreasing with frequency
3. increasing with frequency
4. only a single amplitude was needed
5. none of the above
9. In that Fourier synthesis we could not get a perfect square wave because
1. phase errors made the wave lopsided
2. amplitude errors made the tops and bottoms rounded rather than flat
3. frequency errors gave an unsteady wave shape
4. the synthesizer could not put out all the necessary high frequencies, so the synthesized wave had small rapid wiggles at top and bottom
5. we DID get a perfect square wave

10. The tone of a police siren (a tone whose pitch repeatedly and smoothly rises and falls) can be achieved by what type of modulation:
1. AM
2. PM
3. FM
4. PWM
5. S&M

11. The sound of question 10 can be produced by inputting a low-frequency sine wave into a device and connecting the output of that device to a loudspeaker. The device is called
1. VCA
2. VCO
3. VCF
4. VCR
5. VAT
12. If a carrier wave of frequency F is double-sideband (balanced) modulated by a lower-frequency sine wave of frequency fm, the Fourier spectrum of the result contains
1. only F + fm
2. only F - fm
3. both F + fm and F - fm
4. F, F + fm and F - fm
5. all frequencies between F + fm and F - fm
13. The envelope generator in a synthesizer is typically used to
1. generate the audio signal
2. achieve a vibrato effect
3. as a substitute for the keyboard
4. control the attack and decay characteristics of the synthesizer output
5. generate envelopes in which to mail payments for the synthesizer

14. Suppose the oscillator of your synthesizer can produce only square waves and sawtooth waves. To obtain a wave as close as possible to a sine wave you start with a square wave and filter it. What kind of filter should you use?
1. a low-pass filter
2. a high-pass filter
3. a bandpass filter
4. a notch filter
5. a coffee filter

15. An AM radio is designed to tune in a narrow band of frequencies out of may frequencies that come into the antenna. What kind of filter is used for this tuning?
1. a low-pass filter
2. a high-pass filter
3. a bandpass filter
4. a notch filter
5. an oil filter
16. Why does the tuning filter in the AM radio pass a band of frequencies rather than the single station frequency?
1. because people prefer band music to single-instrument music
2. because an AM modulated signal contains a band of frequencies around the station's carrier frequency
3. because a single-frequency filter would cause unwanted phase shifts in its output
4. because only bandpass filters can be tuned to different carrier frequencies
5. because AM radios developed from crystal sets, which are resonant in a band of frequencies
17. The envelope of a flute tone and a guitar tone are different because
1. the flute has a reasonably long attack and the guitar has a very rapid attack
2. the flute has a steady-state sustain level, whereas the guitar has only attack and decay (or release)
3. the flute's decay time is much shorter than the guitar's
4. all of the above
5. none of the above
18. To produce the "choo-choo" sound of a steam engine (actually more like ch - ch, without the the "oo") we used what singal generator and which low-frequency modulation?
1. noise generator and AM modulation
2. noise generator and FM modulation
3. square wave generator and AM modulation
4. square wave generator and FM modulation
5. filtered sawtooth generator and AM modulation
19. Analog and digital synthesizers have in common that both
1. use modulation
2. have a keyboard
3. control attack and decay
4. have an analog output
5. all of the above

20. A constant-frequency sine wave signal is fed into a loudspeaker. The loudspeaker is (off-center) on a rotating turntable, so that both its distance and its speed vary, with respect to a stationary listener. (Such setups have actually been used in electronic organs.) Consequently the wave arriving at the listener is
1. amplitude modulated
2. frequency modulated
3. both amplitude and frequency modulated
4. pulse width modulated
5. all of the above
21. Our text states that "the ear responds to vibrations over the range of frequencies from about 20 Hz to 20 kHz, a factor of 1000 in frequency". What is that factor when expressed in dB (decibels)?
1. 1 dB
2. 3 dB
3. 10 dB
4. 30 dB
5. 100 dB
22. Recall that an octave denotes a frequency ratio of 2, and a ratio of 2 is described as 3 dB on the decibel scale. Therefore, from your answer to the previous problem, how many octaves are there in the range of human hearing?
1. 1/3
2. 1
3. 3 1/3
4. 10
5. 33
23. Which part of the ear performs the function of focusing the sound waves onto the eardrum?
1. the outer ear
2. the middle ear
3. the cochlea
4. the basilar membrane
5. the hair cells
24. The place theory of hearing holds that
1. the quality of a sound depends on the place where it is emitted
2. for each sound amplitude there is a different place in the brain where it is received
3. we place the source of a sound by observing the phase difference between the waves to our two ears
4. only one place in each wave shape triggers nerve signals
5. the frequency of a sound is correlated with a place where it is most strongly received
25. What is the relation between the critical band the and frequency JND?
2. The critical band is much smaller than the JND
3. The JND is much smaller than the critical band
4. The two refer to different frequency variables and cannot be compared
5. The JND has been long been criticised, but the critical band has only just been noticed
26. At what frequencies is the loudness level of the threshold of hearing equal to 0 phons?
1. at all audible frequencies
2. at frequencies below about 100 Hz
3. in the middle frequency range, 100-1000 Hz
4. in the range of greatest sensitivity, 1000-5000 Hz
5. above 5000 Hz.
27. Two tones, each with SIL of 50 dB, are sounded simultaneously. The SIL of the combination is
1. 52 dB
2. 53 dB
3. 56 dB
4. 100 dB
5. depends on the tones' frequencies
28. Now another two equally intense tones are added to those of problem 27 -- a total of 4 tones, 50 dB each.
The SIL of this combination is
1. 52 dB
2. 53 dB
3. 56 dB
4. 100 dB
5. 200 db
29. What is true about Ohm's law of hearing?
1. It states that the acoustical tension equals the product of acoustical resistance and acoustical current
2. it states that the sound quality of a complex tone depends on the amplitude and relative phases of its harmonics
3. it has limitations, as demonstrated by second-order or quality beats
4. it states that each tone quality corresponds to a unique wave shape
5. all of the above
30. The periodicity theory is supported by the observation of
2. ordinary (first-order) beats
3. critical bands
4. periodicity pitch
5. sharpening
31. Bernoulli's principle states that, in a steady-state fluid flow situation,
1. the flow velocity is greatest where the cross sectional area is least
2. the pressure increases when the fluid's cross sectional area is decreased
3. the pressure is lower where the flow velocity is greater
4. the pressure increases as the flow velocity increases
5. the pressure increases as the depth of fluid increases
32. A frequency region where the harmonics of the Fourier spectrum are emphasized relative to other harmonics is called
1. a formant region
2. a region of emphasis
3. a resonant region
4. a region of inharmonicities
5. a vowel sound
33. When a person who has inhaled a lungful of helium speaks or sings, his/her voice has the typical "Donal Duck" sound. This is because
1. the vocal folds vibrate more rapidly in helium
2. Bernoulli's principle is valid only in air, not in helium
3. not having air to breathe leads to an anxious, higher-than-normal speech
4. the sound waves from the person travel faster in helium than in air, hence arrive more quickly at the listener
5. the frequency of the vocal formants is increased because f = S/l, S increases, and l remains the same.
I pledge on my honor that I have not given or received any unauthorized assistance on this examination

34. This problem concerns the equal loudness curves shown on the right, from an internet reference (not our text). Assume that the Sound Pressure Level is the same as the SIL. The "dashed" curve is the threshold of hearing (0 phons). Give approximate answers, as well as can be read off the curves, but do not worry about exact interpolation between the curves or between the marked SIL or frequency values.
1. These curves were (fiendishly) drawn omitting the intensity scale (in watts/mē) on the right. Supply this scale at at least three levels, knowing that 0 dB corresponds to 10-12 W/mē
2. At what frequency is the ear most sensitive? That is, at what frequency can it hear the lowest intensity of sound?

3. We have a sinusoidal acoustic wave of SIL = 60 dB of variable frequency. The frequency starts out at 20 Hz and slowly increases up to 10,000 Hz.
1. at what frequency does this wave first become audible?

2. at what frequency is its loudness level 40 phons?

3. at what frequency does it appear loudest?

4. what it its loudness level at this loudest frequency?

4. The loudness level of a 100 Hz tone is 10 phons. What is its SIL?

35. Assume that a person with perfect hearing can hear all frequencies between 200Hz and 20,000Hz equally well. Recall that the Fourier spectum of a square wave contains only odd harmonics, whereas that of a sawtooth wave contains all harmonics. At sufficiently high frequencies, f > fd, two different wave shapes that sound different at lower frequencies no longer have different tone quality though they are still quite audible.
1. What is a likely reason for this?

2. What do you expect for the approximate value of  fd when comparing a square wave and a sine wave?

3. What do you expect for the approximate value of  fd when comparing a square wave and a sawtooth wave?