Part II
- - 5. For answers 1-5, use the code
- phase shift
- frequency modulation
- amplitude modulation
- balanced modulation
- damping
to identify the wave property shown in the pictures:
- The mathematical representation of the equivalence between a complex waveform and a mixture of pure sine waves is
- Huygens principle
- Bernoulli's principle
- principle of superposition
- Fourier analysis and synthesis
- equivalence principle
- A unit of frequency is
- hertz
- cps (cycles per second)
- rpm (repetitions per minute)
- sec-1
- all of the above
- A difference between a mode and a node is
- the same as that between Mote and a note
- a system may oscillate in one mode, but there is no oscillation at a node
- different nodes of oscillation are distinguished by the number of modes they have
- an oscillating system may have a number of nodes, but it must always oscillate in just one mode
- there is no difference
-
The change in propagation direction when a wave crosses a boundary between two different media is called
- reflection
- refraction
- reproduction
- resonance
- release
- In order that the superposition of two waves produce a steady pattern of interference, the waves must have the same
- amplitude
- phase
- frequency
- damping
- 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.)
- - 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.
- - 20. Repeat the previous question for a triangle wave of same amplitude, offset, period, and starting amplitude.
- 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?
- How many numbers, possible with the number of binary bits you chose, are not used in representing the amplitudes 1 to 5?
- 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
- How many components (harmonics) are there in the Fourier spectrum of a pure sine wave?
- 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?
- What number (base 10) is represented by the binary number 101?
- By what factor does the intensity of sound from a (point-like, isotropic) source increase when I halve my distance to that source?
- By what factor does the amplitude of the sound wave increase when I halve my distance to the source?
- A wave of amplitude 3 and one of amplitude 2 are 180° out of phase. What is the amplitude when they interfere?
- 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?
- If the SIL of a tone increases by 5 dB, by what factor does the intensity increase?
- 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?
- Take the speed of sound to be 320 m/s. How many seconds does it take sound to go 1600 m (about a mile)?
- A string with fixed ends is vibrating in the third harmonic mode. How many nodes are there (not counting the ones at the ends)?
- What is the harmonic number of the second possible standing wave in a closed tube?
- By what factor does the frequency of a sound increase when the pitch increases by one octave?
- How many strong, low, isolated resonant frequencies does a Helmholz radiator have?
- 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?
- What voltage is being delivered to the speaker of the previous question?
- 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)?
- 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
- decreases
- increases
- remains the same
- no beats are heard
- 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?
- less power is delivered
- some of the source signal is reflected
- the signal does not pass efficiently from source to device
- the impedance matching is inadequate
- all of the above
- The difference between a dynamic and an electrostatic microphone is that
- 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
- only the second puts out an electrical signal
- only the first reproduces the dynamics (loud vs. soft passages) of music well
- the first uses the law of changing magnetic fields (induction), the second uses the law of electrostatics relating charge and voltage on a capacitor
- "dynamic" and "electrostatic" are brand names of competing manufacturers for identical devices
- In a lecture demonstration, an amplifier was driven by a sine wave beyond its linear range. What happened?
- the output had Fourier components at frequencies other than that of the input sine wave
- the output had Fourier components at fewer frequencies than the input sine wave
- the output was a sine wave, but shifted in phase compared to the input
- the output had a different fundamental frequency than the input
- as the input amplitude was increased beyond the amplifier's linear range, the output decreased in amplitude
- Which type of tube can never have a node at its center?
- a tube closed at both ends
- a tube open at both ends
- a tube open at one end and closed at the other
- the second harmonic of all types of tube has a node at the tube's center
- Which aspect of a synthesized tone is normally not controlled by the envelope generator?
- period
- attack
- release
- sustain
- decay
- A loudspeaker usually sounds better when mounted on a baffle or in an enclosure because
- the baffle prevents nonlinearities
- the baffle prevents unwanted reflections
- the baffle improves the speaker's efficiency at high frequencies
- the baffle prevents sound from the back of the speaker from interfering with the sound from the front
- all of the above
- A frequency region in which harmonics present in some tone are emphasized relative to the other harmonics is called a
- resonance
- focus
- filter
- formant
- fipple
- The perception of pitch is (mainly) correlated with the physical variable
- fundamental frequency
- frequency of harmonics
- harmonic number
- phase
- tension
- How can you increase all the formant frequencies present in your voice?
- speak after having inhaled a gas, such as Helium, in which the speed of sound is greater than in air
- speak in a falsetto voice by increasing the tension in the muscles of the vocal folds
- speak through a long, resonant tube
- run a tape recording of your voice at a decreased tape speed
- make all your vowels sound like French nazalized vowels.
- 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?
- 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
- 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
- the tones consisted of a rapid frequency modulation over one octave
- Each note started at a higher pitch than the previous one, but actually decreased in pitch while it was being sustained
- What is the difference between white noise and pink noise?
- Pink noise results from mixing white noise with a low-frequency sine wave
- Pink noise has only Fourier components that are multiples of some low frequency, wheras white noise has all frequencies present with equal intensity
- The frequencies present in pink noise cover only one octave, those in white noise cover all octaves
- pink noise has less power at high frequencies than white noise
- pink noise has more power at high frequencioes than white noise
- An important support for the place theory of hearing is that
- there is a critical band of frequencies within which two tones affect each other, and make the combination sound "coarse"
- we perceive pitch on a logarithmic scale
- we perceive loudness on a logarithmic scale
- we can hear combination tones
- we can hear the periodicity pitch when there is a missing fundamental
- The timbre of a sustained sound is most closely correlated with which physical characteristic of a sound:
- frequency
- amplitude
- Fourier spectrum
- phase of harmonics
- loudness
- Of the following, the sound with the most rapid attack is that of
- the roar of the crowd at a football game
- a car speeding past
- a flute
- the "wow" sound programmed on an analog synthesizer
- a jet "breaking" the sound barrier
For the following true-or-false questions, mark 1 (A) for true, 2 (B) for false.
- Period and frequency measure the same characteristic of simple harmonic motion.
- When a body moves in simple harmonic motion (SHM), its velocity as a function of time also has the SHM shape.
- Longitudinal waves always travel more slowly than transverse waves.
- The speed of sound is constant and independent of the medium in which the sound propagates.
- Huygens's principle applies to both longitudinal and transverse waves.
- The inverse square law is never valid for transverse waves.
- Quincke's tube shows the interference of sound waves.
- Diffraction of sound makes it possible to hear sound around corners or behind barriers.
- Superposition of two waves always yields a wave with larger amplitude than that of either of the two waves.
- When two waves are superposed and beats can be heard, the beat frequency equals the difference between the frequencies of the two superposed waves.
- The Doppler effect always makes the frequency of a moving source appear higher than it really is.
- Standing waves occur only for transverse waves, longitudinal waves are always travelling waves.
- The frequencies of standing waves on a stretched rope with fixed ends are integral multiples of a fundamental frequency.
- 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.
- Reflections of sound waves occur both at a closed end and at an open end of a tube.
- Although the air in a tube can vibrate in many different modes, only one of these (the fundamental) is a resonance.
- The control voltage of a VCO determines the wave shape that the VCO produces.
- The low-frequency control oscillator in a synthesizer can be used to produce a vibrato effect in the synthesizer output.
- 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.
- Resonances of the vocal tract produce the formants present in spoken vowels.
- VI, IČR, and VČ/R are formulas for electrical power.
- In a loudspeaker, the voice coil is rigidly mounted on the speaker's magnet.
- A magnetic tape can record either analog or digital signals.
- It was a BIG mistake that we did not get to Chapter 8 in the text (Room and Auditorium Acoustics).
End of Exam
Have a Happy Holiday!
Look for preliminary final grades starting tomorrow (Wednesday)