Underwater Acoustics Flashcards
Relate sound velocity / frequency / wavelength
c = fλ
c = is the sound velocity (ms-1)
f = is the frequency of the disturbance (Hz or s-1)
λ = wave length of the period of the disturbance (m)
What is the speed of sound in air?
330ms^-1
we live in a world of echos
speed in water is 5x faster than in air
What are acoustic disturbances?
how sound waves travel and spread through a medium
sound sources create vibrations that set off a series of compressions and rarefactions causing pressure changes
Compressions - areas of hight pressure
Rarefactions - areas of low pressure
What is ambient pressure?
Baseline pressure in a given environment when unaffected by external disturbances
What is simple harmonic pressure?
Regular back-and-forth motion around a central equilibrium position with a constant frequency and amplitude.
What are harmonic waves?
Waves that can be described by simple mathematical relationships, where the wave pattern repeats at regular intervals.
Typical Density values to know
Density of air: 1.225 kgm3 at at zero degrees Celsius
(but can change with temperature, pressure, humidity etc.)
Density of fresh water: 1000 kgm3 at 10 degrees Celsius
(been a standard for a long time)
Density of salt water ~1027 kgm3 at 10 degrees Celsius
(but both vary with temperature, pressure, depth, salinity, etc.)
Typical sound velocities to know
Sound velocity in air ~337 ms-1
(but can change with temperature, pressure, density, etc.)
Speed of sound in fresh water ~1482 ms-1
Speed of sound in salt water ~1490 ms-1
(but both vary with temperature, pressure, depth, salinity, etc.)
different between fresh and salt water doesn’t seem like much but think of mirages
mirages - reflections of the sky are seen in deserts - shimmers due to air currents from temperature
Pressure equation
Pressure = Force / Area
Sound velocity equations
c = sqrt(Ks / ρ)
where
c - sound velocity
Ks - coefficient of stiffness related to bulk modulus
ρ - density kgm^-3
c = sqrt( dp / dρ )
where
p - pressure Nm^-2
Humidity
how much water vapour is present
How does density affect divers?
They have issues with balancing buoyancy
pressure increases with depth - become more compressed
if there are perfectly balanced in freshwater then they are more buoyant in salt water
Pressure values to know
1 Newton exerted on 1 m^2 = 1 Pascal
underwater acoustics - use μPa micro Pascal
Other units of pressure:
atmospheres, bar, psi, mmHg (for mercury), dynes/cm^2
1 bar is 0.986923 standard atmospheres
(i.e. the average air pressure above our heads)
At the surface there is 1 atmosphere
At 10m underwater there is 2 atmosphere
Roughly every 10 m depth in water is another bar
1 bar = 100’000 Pascals or 10-5 micro-bar = 1 micro-Pascal (μPa)
i.e. a μPa = 10-11 atmospheres or bar
20μPa –> smallest sound humans can hear
Intensity definition
The acoustical power per unit area in the direction of
propagation with units Watts/m2
For humans in air 1 Watt/m2 is only
just audible where 10 Watts/m2 is often painful
Intensity ∝ |P|^2 / ρc
Intensity, power, energy all proportional to pressure squared
Acoustic impedance
Product of medium density (ρ) and sound velocity (c)
State some applications of sound
Sonar systems like radar (military, scientific, civilian, fisheries)
* Communication (long range: ATOC global communications links
short range: vehicles, divers, sensors)
* Military: (finding, detecting a threat)
* Monitoring (Missile launches CTBO, Tsunami warnings);
* Long term environmental monitoring (global warming, etc.)
* Environmental monitoring (marine mammals, fish, sediments)
* Conditioning monitoring (flood warnings, dam breakages,
underwater systems)
Passive VS Active SONAR
Passive - just listening - no active transmission
Active - sound transmitted and then observed when it comes back
What does SONAR stand for
SOund and NAvigation Ranging
Active SONAR systems
Sound is made
Measure time delay for echo
Divide by 2 –> for the 2 journeys there and back
Known sound of speed
Estimate range of object
Sound made/sent out has to be an pulse
Echolocation definition
A sensory ability used by certain animals to navigate and locate objects in their environment
Clicks (sound waves) emitted and echoes are listened for
animals like whales and dolphins use active SONAR
- transmit path vis melon ( fatty tissue on their heads)
- echo reception related to low jaw ( current theory)
Acoustic Thermometry of Ocean Climate (ATOC)
A scientific technique
Measuring ocean temperature using long range acoustic propagation
Low frequencies transmitted through the ocean to measure temperature changes over time
Low frequencies used as they can go a long way - can go all around the planet
Satellites only measure surface temperature
