This module explores ocean acoustics, focusing on the propagation and detection of sound in the marine environment. Sound propagation strongly depends on refraction caused by variations in sound speed, and reflection off the seafloor and sea surface. Detection of sound subsequently depends on its propagation path, background noise from human and natural sources, and the sensors used for detection.
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After completing the module, the learner should be able to do the following:
Physics of sound propagation
- Explain why sound is a pressure wave
- List and describe the important characteristics of sound waves
- Describe the relationship between sound pressure and sound intensity
Sound Speed
- List the factors that affect underwater sound speed (temperature, salinity, pressure)
- Describe how sound speed changes with changes in temperature, salinity, and pressure
- Describe typical sound speed profiles in the world's oceans and major marginal seas
Refraction and Reflection
- Given a sound speed profile, describe how sound will refract
- Describe the relationship between refraction and frequency
- Describe the factors that affect sound reflection off of the seafloor or sea surface
Propagation Paths
- Describe the surface duct, mixed layer, and sonic layer depth and their interrelationship
- Describe sound propagation in the surface duct
- Describe the conditions that lead to sound channels
- Describe the deep and secondary sound channels
- Describe the convergence zone
- Describe factors that lead to bottom bounce
Ocean Structures
- Describe how sound propagation is affected by oceanic fronts and eddies
Sources of Change in Ocean Acoustic Features
- Describe seasonal and diurnal effects on ocean sound speed profiles
- Describe the effect of large storms on ocean sound speed profiles
- Describe the effect of internal waves and tides on sound propagation
Scattering and Absorption
- Describe the factors that affect scattering and absorption of sound at the seafloor
- Describe how bathymetry affects scattering and sound propagation
- Describe how seawater absorbs sound energy
- Describe how air bubbles affect sound propagation
Factors Affecting Detection of Sound
- List the factors that affect the detection of sounds under water
- List sources of ambient noise in the ocean
- Describe the difference between spherical and cylindrical spreading of sound waves
- Describe how sound propagation paths affect the detection of sound sources
Sensors
- Explain the difference between active and passive sonar systems
- Describe the strengths and weaknesses of active and passive sonar systems
- Describe the factors that determine target strength
SONAR Equation (Advanced Topic)
- List the variables in the sonar equation
- Recall the difference in the sonar equation for active and passive systems
- Describe the Figure of Merit (FOM)
Sound, propagation, refraction, reflection, scattering, reverberation, absorption, sound speed profile, surface duct, mixed layer, sonic layer depth, sound channel, convergence zone, bottom bounce, spherical spreading, cylindrical spreading, sonar
