Underwater Acoustic Principles Flashcards

1
Q

Sound is what kind of wave?

A

A pressure wave

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2
Q

Why kind of particle motion occurs from pressure waves?

A

Longitudal Oscillation

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3
Q

What 2 things does speed of sound depend on?

A
  1. Distance between molecules

2. Strength of intermolecular interactions

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4
Q

Speed of sound increases with what 2 things?

A
  1. Temperature

2. Density

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5
Q

What are the 5 characteristics of sound waves?

A
  1. Amplitude
  2. Frequency
  3. Speed
  4. Wave Length
  5. Intensity
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6
Q

What is amplitude?

A

The distance from the resting state

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7
Q

What is intensity?

A

The amount of energy per second crossing a unit area

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8
Q

What is frequency?

A

The rate of oscillation

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9
Q

Waves with similar phase interact…….

A

Constructively

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10
Q

Waves with opposite phase interact……

A

Destructively

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11
Q

Give 6 things that affect SV

A
  1. Temperature
  2. The Coastal Environment
  3. Tidal Mixing
  4. Upwelling
  5. Salinity
  6. Depth
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12
Q

What has the greatest influence on SV?

A

Temperature then salinity then depth

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13
Q

What influence does temperature have on SV?

A

4.5m/s per 1 Deg Increase

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14
Q

What influence does salinity have on SV?

A

1.2m/s per one ppt Increase

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15
Q

What influence does depth have on SV?

A

1m/s per 60m of depth

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16
Q

What is the combination of absorption and scattering known as?

A

Attenuation

17
Q

What does spherical spreading cause?

A

A spreading of sound waves in all directions, uniformly, which decreases intensity and weakens the signal

18
Q

What is Target Strength?

A

The ability of an object to reflect sound

19
Q

What 3 things does Target Strength depend on?

A
  1. Reflectivity
  2. Size
  3. Aspect
20
Q

How does the Receiving Directivity Index influence an acoustic signal?

A

It affects the systems ability to detect an acoustic pulse, by filtering out noise in the water column, it facilities a stronger gain from the target

21
Q

Give 3 sources of noise with definitions

A
  1. Ambient (any sound not emitted by the source of interest)
  2. Self Noise (any sound emitted by the source of interest)
  3. Reverberation (persistence of sound in a particular space after the original sound is produced)
22
Q

What is acoustic impedance?

A

How well the medium facilities sound energy to pass through

23
Q

What would a reflection coefficient of 0 mean?

A

Weak reflecting surface

24
Q

What would a reflection coefficient of 1 mean?

A

Very strong reflecting surface

25
What would a reflection coefficient of 0.2 mean?
Sand
26
What would a reflection coefficient of 0.6 mean?
Clay
27
What would a reflection coefficient of 0.8 mean?
Rock
28
What would a reflection coefficient of 0.9 mean?
Steel
29
Give 4 points about Low Frequency Sonar
1. Effective transmission of the pulse 2. Some seabed penetration 3. Large footprint but reduced resolution 4. Long range capability
30
Give 4 points about High Frequency Sonar
1. Energy is easily dissipated in the water column 2. Very little penetration 3. Narrow beam and good resolution 4. Decreased range capability
31
What is the typical frequency of a Low Frequency Sonar?
30-35 kHz
32
What is the typical frequency of a High Frequency Sonar?
200-220 kHz
33
What is active sonar?
A system that transmits and receives
34
What is passive sonar?
A listening device
35
What does the sonar equation do?
It tells you whether target detection is possible using echo excess (EE = SL - TL + SG - DT) if EE > or = to 0 then it will be detected