ISR Fdn 25-2 > Acoustics > Flashcards

Acoustics Flashcards

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1
Q

How does temperature affect the speed of sound in water?

A

For every 1 deg Fahrenheit increase in temperature, the acoustic velocity will increase by 6ft/s.

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2
Q

How does pressure affect the speed of sound in water?

A

As depth increases the pressure grows. For every 100ft increase in depth, the velocity will increase by 1.6ft/s.

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3
Q

How does salinity affect the speed of sound in water?

A

For every increase of 1 part per thousand (salt to water) the speed will increase by 4.6ft/s.

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4
Q

What is the HALT law?

A

High Away, Low Towards

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5
Q

What is the mixed layer of the 3 layer ocean model?

A

Influenced by atmospheric conditions. Water warmed by the sun is mixed with wind and wave actions. Generally isothermal.

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6
Q

What is the permanent thermocline of the 3 layer ocean model?

A

Usually contains a pronounced negative temperature gradient

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7
Q

What is the deep water layer of the 3 layer ocean model?

A

Generally isothermal, cold

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8
Q

Label this diagram of the three layer ocean model.

A
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9
Q

What is the relationship between ambient noise and frequency?

A

Noise is higher in the lower part of the frequency spectrum, especially below 500GHz. Noise decreases as frequency increases.

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10
Q

What is ambient noise?

A

Any noise within the ocean that is not target related.

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11
Q

What are the main groups of background noise in the ocean?

A

Natural Noise
Man Made Noise

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12
Q

What are the sub-divisions of natural noise?

A

Biological
Ice
Weather
Seismic

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13
Q

What is the relationship between noise level and volme?

A

An increase of 3dB in the noise level result in an increase of twice the volume.

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14
Q

What are the sub-divisions of man-made noise?

A

Shipping noise
Seismic surveying
Drilling
Active transmissions

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15
Q

What are choke points?

A

Geographical features, above or below the surface of the sea, that restrict the freedom of movement of a submarine.

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16
Q

What are the 3 main types of bottom composition?

A

Sand
Rock
Mud and Slit

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17
Q

What are the 3 main provinces of the ocean bottom?

A

Continental Shelf
Continental Slope
Continental Rise

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18
Q

What are the topographical features that that characterise the deep ocean basin?

A

Abyssal Plain
Deep Trenches
Ridges
Seamounts

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19
Q

What is the name for a jagged seamount?

A

pinnacles

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20
Q

What is the named for a flat topped seamount?

A

guyots

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21
Q

What are the 3 main requirements of a successful underwater detection system?

A

Good penetration (active) or sensitivity (passive)
High resolution
High Data Rate

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22
Q

What is an active underwater detection system?

A

Emits an acoustic pulse into the water and listen for corresponding returns (echo)

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23
Q

What is an passive underwater detection system?

A

Listens silently for noises from the submarine without drawing attention to the presence of a sonar buoy.

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24
Q

What are the elements/stages of an airborne acoustic detection system?

A

1st - Detection
2nd - Processing
3rd - Display

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25
Complete this diagram of an airborne acoustic detection system
26
What are the dimensions of a Lofargram?
Time, frequency, intensity, direction
27
What is doppler?
The apparent change in frequency due to relative motion between a source and a receiver
28
What are the pieces of information that can be gained from DSCPA?
F0, Speed, Range, Bearing, Track, Depth
29
In this diagram, what is the phenomenon when, as the target passes point Z, the apparent frequency shifts from Higher than natural frequency, to lower than natural frequency?
Doppler Shift Closest Point of Approach (DSCPA)
30
What is the formula to calculate speed from DSCPA?
Spd = (Fmax-Fmin) x C / F0 Spd = Speed Fmax = maximum frequency seen Fmin = minimum frequency seen F0 = centre (true) frequency C = Constant 1500
31
What is the spd of a boat with an Fmax of 301 and Fmin of 299?
Spd = (Fmax-Fmin) x C / F0 Spd = (301-299) x 1500 / 300 Spd = 2 x 1500 / 300 Spd = 2 x 1500 / 300 Spd = 3000 / 300 Spd = 10 kts
32
What is the Lloyds mirror effect?
When CPAs exhibit a constructive and destructive interference pattern derived from sound arriving at the hydrophone from two different paths.
33
What 3 factors does doppler shift CPA depend on?
Target velocity Frequency Range
34
Define Shaft Rate
The number of times a propeller shaft rotates in one second
35
Define Turns Per Knot
The number of times a shaft has to turn to make the submarine travel at a speed of one knot
36
Define reinforcement
When lines overlay each other, creating a line of greater intensity than either of them separately
37
Define blade rate (BR)
The number of times a blade passes a given point in one second BR = SR x no. of blades
38
Define the Crankshaft Rate (CSR)
The number of times the crankshaft rotates in one second CSR = ERPM / 60s
39
Define Engine Firing Rate (EFR)
The number of times the engine fires in one second EFR = CFR x no. of cylinders
40
Define cylinder fire rate (CFR)
The number of times an individual cylinder fires in one second CFR = EFR / no. of cylinders
41
Define Engine RPM (ERPM)
The number of times the crankshaft rotates in one minute ERPM = CSR x 60s
42
What are the primary sonar buoys?
AN/SSQ-36B Bathy (Expendable Bathythermograph) AN/SSQ-53G DIFAR (Directional Frequency Analysis & Recording) AN/SSQ-62E DICASS (Command Activated Sonar Buoy System)
43
What is the operating depth of the AN/SSQ-53G DIFAR?
90, 200, 400, 1,000ft
44
What is the operating life of the AN/SSQ-53G DIFAR?
0.5, 1, 2, 4 or 8 hrs
45
What are the selectable channels of the AN/SSQ-53G DIFAR?
1-99 selectable (aside from 57, 58, 93)
46
What are the two depth settings for the AN/SSQ-62E DICASS?
D1 (Shallow), D2 (Deep)
47
What is the operating depth of the AN/SSQ-62E DICASS?
D1: 50, 150 or 300ft D2: 90, 400 or 1,500ft
48
What is the operating life of the AN/SSQ-62E DICASS?
60min; 50 ping seconds
49
What are the selectable channels of the AN/SSQ-62E DICASS?
1-99 selectable (aside from 57, 58, 93)
50
What is the operating life of the AN/SSQ-36B Bathy?
12mins
51
What is the operating depth of the AN/SSQ-36B Bathy?
2625ft
52
What are the selectable channels of the AN/SSQ-36B Bathy?
1-99 selectable
53
Complete the sonar buoy deployment pattern diagram shown below
54
What are the 5 main types of propulsion used by submarines?
DE BE TR TE AIP
55
Which propulsion method is most common among submarines?
DE
56
Define frequency.
Frequency is the number of wave cycles passing a fixed point in 1 second.
57
Define the reflective properties of sand
Fairly good reflector
58
State 2 categories of ambient noise and list the components of each.
Natural - Biological - Seismic - Weather - Ice noise Man made - Shipping - Drilling - Mil & Civ active transmissions - Seismic surveying
59
List two of the main contributors to biological noise in the ocean
Whales & Shrimps
60
Define the sonic layer depth
The fastest acoustic velocity in the first 1000ft of water
61
State the blade and shaft fit for military vessels
Multi-shafted and fitted with 4-7 blades
62
Explain what is meant by the term fundamental frequency
The base or origin frequency measured in cycles per second and expressed in Hz
63
Label this diagram of the ocean floor
64
Name four types of propeller blade
Azipod Contra rotating High Speed Skewed
65
Name two types of propeller
Fixed pitch Variable pitch
66
List all the countries that currently operate nuclear powered submarines.
China, France, India, Russia, UK and USA
67
What are the two main types of submarine propulsion?
Nuclear and Diesel
68
What does SSC stand?
Submersible Ship, Coastal (conventionally powered)
69
What does SSK stand?
Submersible Ship, Killer (conventionally powered)
70
What does SSP stand for?
Submersible Ship, AIP (conventionally powered)
71
What does SSAN stand for?
Submersible Ship, Auxiliary, Nuclear
72
What does SSN stand for?
Submersible Ship, Nuclear
73
What are the advantages of a nuclear powered boat?
Range, Speed, Endurance
74
What are the 3 factors that affect the speed of sound in water?
Temperature, Pressure, Salinity
75
What does SSBN stand for?
Submersible Ship, Ballistic Missile, Nuclear
76
What does SSGN stand for?
Submersible Ship, Guided Missile, Nuclear
77
What is the advantage of a diesel powered boat?
Very quiet to operate on batteries
78
What is snorting?
Surfacing a submarine to run the diesel engines in order to re-charge batteries
79
What does AIP stand for?
Air Independent Propulsion
80
What are the disadvantages of a nuclear powered boat?
Constant noise sources
81
What are the disadvantages of a diesel powered boat?
Must snort to recharge batteries
82
What is DD and how does it work?
Diesel Direct - diesel engine connected directly to the propeller shaft
83
What is DR and how does it work?
Diesel Reduction - diesel engine drives the propeller through a gearbox
84
What is DE and how does it work?
Diesel Electric - Diesel engine charges batteries which in turn power the propeller through an electric motor
85
What is BE and how does it work?
Battery Drive - The battery powers the propeller through an electric motor, used by diesel submarine when divedr
86
What is AIP and how does it work?
Air Independent Propulsion - allows battery charging without running diesel engines via oxygen/argon
87
What is TR and how does it work?
Turbine Reduction - turbine turns the propeller through a gearbox
88
What is TE and how does it work?
Turbine Electric - turbine constantly charges batteries which power the propeller through an electric motor
89
Label this diagram of a basic snort system
90
Label this diagram of a diesel electric drive system
91
Define sound
A periodic variation of pressure, particle displacement or particle velocity in an elastic medium
92
Label this diagram of a turbine reduction system
93
What is sound pressure measured in?
Decibels (dB)
94
What is the frequency at which sound is being transmitted measured in?
Hertz (Hz)
95
How does an increase of 2x in volume effect noise level?
increase of 3dB
96
How does an increase of 5x in volume effect noise level?
increase of 7dB
97
How does an increase of 100x in volume effect noise level?
increase 20dB
98
What measurements are used on deep water charts?
meters and fathoms
99
What is the speed of sound in water?
5,000ft/s
100
What is broad band?
When received energy is spread over a wide spectrum and individual frequencies are not distinguishable. Think large lorry
101
What is broad band effect?
Caused by multiple paths of BB energy reflecting off the sea bed
102
What is narrow band?
When received energy is concentrated in one discrete spectrum (Think Ferrari)
103
What are transients?
Noise that occurs over a short or long term over irregular intervals (Think subs using hydraulics to power fins)
104
What is the typical blade and shaft fit for merchant vessels?
Single shafted. 3-4 blades for small or medium vessels, 4-7 for large.
105
Explain what is meant by harmonic frequency
An echo or numerous echoes of the fundamental frequency throughout the spectrum
106
Define CPA
Closest Point of Approach - The point at which the vessel passes from high to low frequency and the track of the vessel is at 90 to the buoy
107
How do you calculate range?
Range = speed x time x 17
108
What are the sizes of sonar buoys and which fall within category?
A ○ DIFAR, DICASS & Bathy G ○ DIFAR F ○ BATHY
109
What is the maximum cable length of the Bathy?
2625 ft
110
What are the Operating Depths of the DIFAR?
90, 200, 400 and 1,000ft
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