Unit 2: ACOUSTIC WAVE TRANSMISSION - Practice Quiz

1 What type of wave is sound traveling through the atmosphere?

2 Which atmospheric property most directly affects the speed of sound in air?

3 What happens to sound intensity as a spherical wave moves farther from its source?

4 What is atmospheric refraction of sound?

5 Which effect causes the observed frequency to change when a sound source and observer move relative to each other?

6 For a stationary observer, what happens to the observed frequency as a sound source approaches?

7 What is the Mach number of a moving sound source?

8 What is a Green's function commonly used to represent in acoustics?

9 Generalized Green's formula is mainly used to express an acoustic field using which information?

10 Which mathematical operation commonly appears over the boundary in Green's formula?

11 The Lighthill equation is primarily used to describe which phenomenon?

12 Which flow feature is a major source of sound in Lighthill's acoustic analogy?

13 In Lighthill's analogy, the governing flow equations are rearranged into what familiar form?

14 How is a standing wave commonly formed?

15 What is a node in a standing wave?

16 What is an antinode in a standing wave?

17 What is the distance between two adjacent nodes in a standing wave?

18 What does the beam width of an acoustic source describe?

19 A highly directional acoustic source generally has what kind of beam width?

20 What does a larger directivity index indicate?

21 During a nighttime temperature inversion, atmospheric temperature and sound speed increase with altitude. How do sound rays emitted near the ground generally bend?

22 A sound wave travels downwind through a uniform wind of speed . If the sound speed relative to the air is , what is its ground-relative propagation speed?

23 Two tones of equal initial intensity, at and , propagate the same long distance through air. Which outcome is normally expected?

24 A source emitting moves toward a stationary observer at . Taking , what frequency does the observer receive?

25 An aircraft moves at Mach through still air. What is the approximate Mach-cone half-angle ?

26 For a uniform mean flow in the -direction, which operator replaces the ordinary time derivative in the linear acoustic wave equation?

27 In a source-free acoustic region, generalized Green's formula expresses the pressure at an interior point primarily in terms of what information?

28 Which three-dimensional free-space Green function represents an outgoing wave for the Helmholtz equation under the convention?

29 If the acoustic boundary condition prescribes the normal pressure gradient everywhere on a surface, which type of Green function can simplify the boundary integral?

30 Which equation correctly gives the principal form of Lighthill's acoustic analogy in a stationary uniform reference medium?

31 In unbounded turbulent flow with no solid surfaces, the dominant source represented by Lighthill's stress tensor has which multipole character?

32 According to Lighthill's low-Mach-number scaling, jet acoustic power is approximately proportional to . If jet speed doubles while other parameters remain fixed, by what factor does the acoustic power increase?

33 In a standing-wave tube, adjacent pressure maxima are separated by . If the frequency is , what is the sound speed?

34 A tube is closed at one end and open at the other. If its length is and , what is its fundamental resonance frequency?

35 A standing-wave apparatus measures a pressure standing-wave ratio of . What is the magnitude of the pressure reflection coefficient?

36 Which pressure conditions occur at the ideal closed and open ends of an open-closed standing-wave tube?

37 A circular piston has diameter and radiates sound of wavelength . Using , what is the approximate first-null angle?

38 An acoustic source has a directivity factor . What is its directivity index?

39 At a fixed observation distance, the half-power points of an acoustic beam occur where the pressure amplitude has fallen to what fraction of its on-axis value?

40 The frequency radiated by a fixed-size circular piston is increased while the sound speed remains constant. What generally happens to its main beam width and directivity index?

41 In a horizontally stratified atmosphere, the effective sound speed along the propagation direction is . If is measured from the vertical, which ray invariant follows from horizontal translational symmetry?

42 A spherical acoustic wave travels from to in an atmosphere with pressure-amplitude absorption coefficient . What is the total decrease in sound-pressure level?

43 For a stationary harmonic source in a uniform downstream wind , compare the ground-fixed downstream and upstream wavelengths. Which ratio is correct?

44 At a height where and , the temperature gradient is . Using , what downwind shear makes ?

45 A source moves toward an observer at , while the observer moves in the same direction away from the source at . The medium is stationary. What is ?

46 A compact source moves steadily at Mach number through a quiescent medium. What is the half-angle of its Mach cone?

47 For a uniform mean flow , a plane acoustic disturbance has the form . Which positive-intrinsic-frequency dispersion relation is correct?

48 In the retarded-time field of a moving compact monopole, which factor produces strong amplification as the source approaches the radiation direction at sonic speed?

49 Let in volume , and let . With the normal directed outward from , which representation is correct for ?

50 For a non-self-adjoint convected acoustic operator , which reciprocity statement generally replaces the symmetry ?

51 The boundary term in a Helmholtz representation is . If is prescribed on the entire boundary, which condition on eliminates the unknown boundary pressure?

52 Which exact free-space acoustic analogy follows from the compressible Navier–Stokes equations when ?

53 For geometrically similar, compact, low-Mach-number turbulent flows, Lighthill's eighth-power law gives when other parameters are fixed. If doubles, by what factor does acoustic power change?

54 A compact quadrupole has only the component nonzero. In the far field, its pressure amplitude is proportional to . What is the amplitude at from the axis relative to that on the axis?

55 In a standing-wave tube with negligible transmission through the termination, the measured pressure-amplitude maximum and minimum are and . What is the normal-incidence absorption coefficient?

56 Successive resonances of a closed–open tube occur at and . If and the physical tube length is , what open-end correction is implied?

57 An ideal one-dimensional standing wave has a pressure antinode at . What is the magnitude of the local specific acoustic impedance at a position where ?

58 A uniformly vibrating baffled circular piston has diameter and operates at in air with . Approximately where is its first directivity null?

59 A source has normalized intensity over the forward hemisphere and zero intensity over the rear hemisphere. What is its directivity index?

60 A uniform linear array has element spacing and is electronically scanned to , where angles are measured from broadside. Which statement about visible grating lobes is correct?