Underwater Acoustic Principles Flashcards
Sound is what kind of wave?
A pressure wave
Why kind of particle motion occurs from pressure waves?
Longitudal Oscillation
What 2 things does speed of sound depend on?
- Distance between molecules
2. Strength of intermolecular interactions
Speed of sound increases with what 2 things?
- Temperature
2. Density
What are the 5 characteristics of sound waves?
- Amplitude
- Frequency
- Speed
- Wave Length
- Intensity
What is amplitude?
The distance from the resting state
What is intensity?
The amount of energy per second crossing a unit area
What is frequency?
The rate of oscillation
Waves with similar phase interact…….
Constructively
Waves with opposite phase interact……
Destructively
Give 6 things that affect SV
- Temperature
- The Coastal Environment
- Tidal Mixing
- Upwelling
- Salinity
- Depth
What has the greatest influence on SV?
Temperature then salinity then depth
What influence does temperature have on SV?
4.5m/s per 1 Deg Increase
What influence does salinity have on SV?
1.2m/s per one ppt Increase
What influence does depth have on SV?
1m/s per 60m of depth
What is the combination of absorption and scattering known as?
Attenuation
What does spherical spreading cause?
A spreading of sound waves in all directions, uniformly, which decreases intensity and weakens the signal
What is Target Strength?
The ability of an object to reflect sound
What 3 things does Target Strength depend on?
- Reflectivity
- Size
- Aspect
How does the Receiving Directivity Index influence an acoustic signal?
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
Give 3 sources of noise with definitions
- Ambient (any sound not emitted by the source of interest)
- Self Noise (any sound emitted by the source of interest)
- Reverberation (persistence of sound in a particular space after the original sound is produced)
What is acoustic impedance?
How well the medium facilities sound energy to pass through
What would a reflection coefficient of 0 mean?
Weak reflecting surface
What would a reflection coefficient of 1 mean?
Very strong reflecting surface
What would a reflection coefficient of 0.2 mean?
Sand
What would a reflection coefficient of 0.6 mean?
Clay
What would a reflection coefficient of 0.8 mean?
Rock
What would a reflection coefficient of 0.9 mean?
Steel
Give 4 points about Low Frequency Sonar
- Effective transmission of the pulse
- Some seabed penetration
- Large footprint but reduced resolution
- Long range capability
Give 4 points about High Frequency Sonar
- Energy is easily dissipated in the water column
- Very little penetration
- Narrow beam and good resolution
- Decreased range capability
What is the typical frequency of a Low Frequency Sonar?
30-35 kHz
What is the typical frequency of a High Frequency Sonar?
200-220 kHz
What is active sonar?
A system that transmits and receives
What is passive sonar?
A listening device
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
