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16/6/2014

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Feedback — Final Exam

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You submitted this quiz on Mon 16 Jun 2014 6:55 PM PDT. You got a score of 7.00 out of 20.00. You can attempt again, if you'd like.

You can have 2 attempts to do this final exam, but the solutions will be given at the due date of this exam.

Question 1 Check whether the statement is right or wrong: The directivity pattern of a dipole approaches to that of a monopole as frequency goes higher. Your Answer Right

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Question Explanation Directivity pattern of a dipole is independent with frequency.

Question 2 Check whether the statement is right or wrong: Sum of power reflection coefficient and power transmission coefficient can be less than 1. Your Answer

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Right Wrong

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Question Explanation Due to the absorption, sum of them can be less than 1.

Question 3 Check whether the statement is right or wrong: Wave propagation speed remains constant if the temperature changes with respect to time and space . Your Answer

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Question Explanation Wave propagation speed can be changed with respect to temperature.

Question 4 Check whether the statement is right or wrong: By the dispersion relation, waveform in time domain and space domain is related with each other. As the frequency of the wave increases, wavelength of the wave also increases linearly. Your Answer

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Question Explanation https://class.coursera.org/acoustics1-001/quiz/feedback?submission_id=7549

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Frequency is inversely related with its wavelength. (Wavelength=Speed of propagation/Frequency)

Question 5 Check whether the statement is right or wrong: If we add one more sound source, the SPL will increase twice in linear scale, therefore we will have 2dB increases in SPL. Your Answer Right

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Question Explanation It can be 2 dB when they are coherent, but not always. When they are incoherent, we will have 3dB increases in SPL.

Question 6 Check whether the statement is right or wrong: When the mass effect of a wall is dominant compared to the stiffness or internal damping, we can assume the wall as a limp wall. Your Answer Wrong

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Question 7 https://class.coursera.org/acoustics1-001/quiz/feedback?submission_id=7549

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Check whether the statement is right or wrong: If we have a duct with a finite length of L, and rigid boundaries at the both ends, then the phase difference between the pressure and velocity is 90 degrees and has zero mean intensity. Your Answer

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Question 8 Check whether the statement is right or wrong: There is a mean flow in a duct with the velocity u0. In this case, the governing wave equation can be given as the acoustic wave equation where the speed of propagation c is given as u0+cs. (cs: speed of propagation at rest) (acoustic wave eqation:

)

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Question Explanation acoustic wave equation is hold with cs, not c. After applying acoustic wave equation, we can then apply total mean flow. For a detail, check the derivation process of wave equation.

Question 9 Fill in the blanks. “To get a speed of propagation, Newton assumed that the process is ( A ). While he assumed the process is ( A ), which means the temperature is conserved during the https://class.coursera.org/acoustics1-001/quiz/feedback?submission_id=7549

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process, the speed of propagation is quite different with the real one. Laplace's modified equation considered the process as ( B ), which means the process occurs without the transfer of heat or matter between a system and its surroundings. When the process is ( B ) and ( C ), we can assume it as ( D ) process.” Your Answer

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(A) adiabatic (B) isothermal (C) reversible (D) isentropic (A) isothermal (B) isentropic (C) irreversible (D) adiabatic (A) isentropic (B) isothermal (C) reversible (D) adiabatic (A) isothermal (B) adiabatic (C) reversible (D) isentropic

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(A) adiabatic (B) isothermal (C) irreversible (D) isentropic (A) isentropic (B) adiabatic (C) irreversible (D) isothermal Total

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Question 10 In a manufacturing factory, we want to measure Sound Pressure Level (SPL, dB) of sound due to a machine at one point. First, 97dB was measured when a machine was in operation. Second, 90dB was measured when the machine was not operating. Please, estimate the SPL of the machinery noise itself. Your Answer

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7dB 92.5dB

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Question Explanation https://class.coursera.org/acoustics1-001/quiz/feedback?submission_id=7549

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If we assume that the machine noise and the factory background noise are incoherent, the measusred SPL in machine operating condition can be expressed as the sum of machine noise and background noise of factory itself as following: " 97 dB = 10*log10((|P_machine|^2+|P_factory|^2)/Pref^2) " where P_machine and P_factory are machine noise and factory background noise, respectively, and Pref is the reference sound pressure, 20e-6 Pa. Also when a machine is not operating, " 90 dB = 10*log10(|P_factory|^2/Pref^2) ". From above two equations, the noise contribution by a machine, |P_machine|^2, can be obtained easily. Thus, SPL of a machine noise only is equal to the 96 dB (" 10*log10(|P_machine|^2/Pref^2) = 93.3 dB ").

Question 11 When we excite a string with the boundary condition described as following figure, impedance at

the end is described as

. Determine the

condition of resonance.

.

Your Answer

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Question Explanation When the driving point impedance goes to 0, resonance occurs. Therefore, (mω^2-s)coskLρL*csωsinkL=0.

Question 12 If two fluids or materials make the flat surface of discontinuity as illustrated in the following graph, then there is a possibility to have a region of no transmitted wave. What will be the condition that makes this possible? (c0: speed of propagation at medium 0. c1: speed of propagation at medium 1) (Hint: See the problem in the wavenumber domain and calculate the critical angle.)

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Question Explanation For a detail, see the solution of advanced homework #3 (problem3).

Question 13 We will try to reduce the noise from a machine by installing a wall. The sound level of the machine is 80dB, which is not likely to be acceptable for an office. We would like to reduce it to 40dB. The available material that can be used for this case is a sheet of plywood of thickness 2mm. The mass per unit area is 10 kg/m2. Assume that the machine noise is mainly at 200Hz, and the resonant frequency of the wall is 5Hz. Assume that adding plywood will not change the resonant frequency of the wall, you are asked to design the partition. The density of air is 1.25kg/m3 and the speed of sound is 320m/sec. How many plywoods have to be used? (Consider the minimum required number) Your Answer

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7 Only 1 piece will be OK.

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Question Explanation For this condition, we can assume a plywood as a limp wall. Condition: transmission loss R_TL=20log(ωm/2ρc)>40dB -> ωm/2ρc > 100 when ωm/2ρc ≒ 15.7*(number of plywoods), we need at least 7 plywoods to reduce 40dB.

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