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# Acoustics Final P1

### Turning answer bank into flashcards

Question | Answer |
---|---|

SONAR is an acronym for [_____] | SOund Navigation And Ranging |

Which is NOT an acoustic system? [Active SONAR, Seismic, Passive, Metric, Underwater Comms & Telemetry] | Metric System |

An underwater projector converts [_____] energy into [_____] energy | Electric, sound |

[T/F]: Energy propagating through acoustic waves has two forms; kinetic and potential energy | True |

For a plane wave of sound, which law relates acoustic pressure to particle velocity? [Ohm's, Snell's, Newton's, Energy Conservation] | Ohm's Law |

[____] is defined as power over unit area, and is often in terms of watts/m^2 | Intensity |

Which is used to define max allowable TL for Passive SONAR or max 2-way loss for Active when TS = 0? [NL, Fig. of Merit, Performance Figure, DI_T] | Figure of Merit |

[T/F]: The Performance Figure is defined as the difference between the SL and NL measured at the TRANSDUCER | True; Performance Figure = SL - (NL - DI) |

The following equation applies to a SONAR system that is [____]: SL - 2TL + TS = NL - DI + DT | Active |

What are the differences between the Active & Passive SONAR equations? | 1) TL is one-way for passive, but two-way for active. 2) TS is only seen in active, not in passive. |

[T/F]: A Passive SONAR system relies on the target to produce a signal so it can be detected | True |

The reference pressure of [____] is used in underwater acoustics. | One micropascal |

An underwater acoustic hydrophone converts [____] energy into [____] energy. | Sound, electric |

[____] and [____] are 2 fundamental laws used to derive the wave equation for sound wave propagation. | 1) Ideal gas law, conservation of mass 2) Continuity equation, Euler equation |

Name 2 common sources of ambient noise underwater | Shipping noise, tides, surface waves, ocean turbulence, thermal noise. NOT machinery noise/self-noise |

Volume reverberation is mainly caused by [sea surface, sea bottom, marine life, shallow water] | Marine life |

The following term is used IN the computation of the detection threshold for a SONAR system. What does the d stand for? What does that represent? d = (mu_s+n - mu_n)^2/(0.5(sigma_s+n^2 + sigma_n^2)) | Detection Index; processed signal-to-noise ratio (ARRAY GAIN) |

Which are true about explosive pulses? a) They're mobile b) Signal perfectly repeated each time explosive used c) Pressure generated by shock wave is function of weight & type of explosive d) Makes series of (+) pressure pulses caused by gas globe | a, c, & d; NOT b |

[T/F]: Noise is anisotropic in the real ocean | True |

For a plane wave, the [_____] pressure is related to particle velocity by: p = rho*c*u | Acoustic pressure |

In the given equation, rho*c is often referred to as [____]: p = rho*c*u | Specific acoustic resistance |

Which is NOT a SONAR equation? [Active, Passive, Active reverberation, Passive reverberation] | Passive reverberation is NOT real |

A company needs to evaluate where they should drill an oil exploration well in deepwater. Which system is most applicable? [Side scan SONAR, sub-bottom profiler, Seismic, CHIRP] | Seismic |

When studying sea sound propogation, the given equation is decribed by which law? cos(theta_2)/cos(theta_1) = c_2/c_1 [Ideal gas, Snell's, Euler's, Wave equation] | Snell's law |

What are the 4 layers of a typical velocity profile? In order from free surface to seafloor. | Surface Layer, Seasonal Thermocline, Main Thermocline, Deep Isothermal Layer |

Seawater absorption is a result of which of the following? [Viscosity of pure water, magnesium sulfate effects, density of seawater, boric acid effects] | 1) Viscosity of pure water 2) Molecular relaxation of magnesium sulfate > 100 kHz 3) Molecular relaxation of boric acid > 100 kHz |

What is the name of the widely used relation for evaluating absorption in the ocean, as a sum of the effects of boric aid, magnesium sulfate, and pure water? [D'Alembert, Francois-Garrison, Bernoulli, Euler-LaGrange] | Francois-Garrison |

If a sound wave 500x that of a plane wave re 1 muPa, then the N level is what? | N = 10 log(actual level/ref. level) = 10log(500/1) = 27 dB re 1 muPa |

[T/F]: It is important to keep all units consistent when solving for intensity and pressure | True, the fuck |

If a system is active with the parameters SL = 180 dB, TS = 50 dB, NL = 20 dB, DI = 10 dB, & DT = 25 dB, what is the allowable TL? [48.75 dB, 97.5 dB, 195 dB, 87.5 dB] | SL - 2TL + TS = NL - DI + DT -> TL = 0.5(180 + 50 - 20 + 10 - 25) = 97.5 dB re 1 muPa @ 1 yd. |

What is the reverberation active sonar eq? | SL - 2TL + TS = RL + DT |

How do you change the reference pressure? | N_3 = N_2 + 20log(P_2/P_3) |

Which is NOT an equipment related parameter in the SONAR equations? [Projector SL, Ambient NL, DI_T, DT] | Ambient NL is never related to equipment (at least, not related to the equipment involved in the sound wave exchange) |

Which is NOT a medium related parameter? [RL, Ambient NL, TL, TS] | TS is not medium related |

[T/F]: When a signal is a unidirectional plane wave (perfectly coherent) & when noise is isotropic, the AG reduces to DI. Since noise is actually anisotropic, AG should be used instead if N & S characteristics are known. | True |

For a perfectly coherent signal & partially coherent noise, AG is expressed as the following: AG = 10log(n/(1+(n-1)*rho)) What is n? [Euler's density factor, # of elements in array, NL, frequency coefficient] | # of elements in array |

Sound waves are [___] waves since the molecules transmitting the wave move back & forth in the direction of [___] of the wave, producing alternate regions of compression & rarefaction. | Longitudinal, propogation |

An offshore installation contractor is required to place a subsea structure on the seabed in deep water. Which system should they use that's independent of depth? [USBL, LBL, Hydrophone array, Side scan SONAR] | LBL: Long BaseLine |

[T/F]: When a ray is incident from the faster medium, there is no critical angle. Refraction occurs for all angles of incidence, and the angle between the refracted ray and the horizontal will never be less than theta_c. | True I guess? What's the truth, there's no theta_c or there is theta_c? Are we talking about different sound rays here? Badly worded question smdh. |

Given that there are an infinite number of ray paths illustrated below, what is the most critical item (actually measured) in underwater acoustics? [time, path length, horizontal range, depth] [Pic: Deepwater sound ray paths] | Time |

Common sources of ambient noise in deep water include all of the following except what? [Seismic disturbances, ship traffic, machinery noise, thermal noise] | Machinery noise is not ambient. |

Sources of radiated noise include which of the following? [Machinery, propeller, hydrodynamic, thermal] | Machinery, propeller, and hydrodynamic noise. |

Johny and Sarah want to search for an old ship wreck, so they should outfit their boat with which system? [USBL, LBL, Hydrophone array, Side scan SONAR] | Side scan SONAR is used to search the seafloor |

A [___] is defined as the solid angle of a sphere subtended by a portion of the surface whose area is equal to the square of the sphere's radius. | Steradian |

[T/F]: Sound velocity INCREASES as temperature, salinity, and depth INCREASE. | True |

Offshore dev. is trying to determine anchor placements and needs to verify supporting spoil data required by checking variability of soils to depth of 50m. Which system? [USBL, LBL, Sub-bottom, side scan] | Sub-bottom profilers penetrate up to about 100 m below seabed. |

[T/F]: A shadow zone occurs at a depth where the sound velocity is maximum. | True |