Principles Of Sonar Flashcards
What is a Decibel (dB)?
1/10th of a bel (B)
these are two signals that have a power ratio of 10^(1/10)
Why use Decibels (dB) in acoustics?
Used due to the very wide dynamic range of many systems
Sound levels can range over scales from 1 to 10^16
Conversion to dB
Level in dB = 20log10(P_measured / P_reference)
Example for an underwater sound of 200μPa
= 20log10(200/1)
= 46 dB re 1μPa
Why do we use logs?
They are good for dealing with really big numbers or really small numbers
Its for large dynamic ranges
Sound propagation - Calculating Received level
Received level = Source Amplitude x Loss Factor
What is the Sound level?
Its a theoretical value that appears to radiate from an ‘acoustic centre’ of a source when it is observed at a distance from the source in the far field independent of the environment.
Reference to 1m
i.e. dB re 1μPa-m
Not the same as received level
Propagation equations in dB
RL = SL - TL
Received level = Source level - Transmission level
What is Transmission loss?
AKA propagation loss
It is modelled in most sound fields
It depends on the physics of the environment as the sound energy travels through it
How does sound radiate
Spherical spreading
Energy spreads over a larger and larger area
Square Wave law
Energy measured on the surface is lower the further away from the source you are
What is Spherical Spreading
Intensity (I) α 1 / 4πR^2
In dB 20log10(R)
R is range in m from the source
How can radiating sound be affected by boundaries?
Boundaries will be different medias
E.g. Surface or the seabed
–> Refraction - sound bends travelling through mediums
–> Reflection - sound strikes medium boundary in a way that it bounces back
–> Combination of both - some sound is refracted, some is reflected
What is the free field?
The area of water where sound propagates without encountering boundaries or obstacles.
E.g not near the surface or seabed
What is the surface and seabed like as a boundary?
Surface
–>It is very good at reflecting
–>Creates a large boundary as there is a large pressure differential between water and air.
Seabed
–> It might get some reflection and refraction
What is the law of refraction in underwater acoustics?
Snell’s law
n1 / n2 = sin(θ1) / sin(θ2) = ρ1 x c1 / ρ2 x c2
where ρ x c is acoustic impedance
ρ is density in kgm^-3
c is sound velocity in ms^-1
What is Cylindrical spreading?
When sound spreads from a source but is restricted between two boundaries. E.g. between the surface and seabed.
Similar to how fibre optics works
It has less geometrical losses than spherical spreading as energy is trapped between boundaries
How do you calculate Transmission loss (TL) only considering geometric spreading?
TL = Nlog10(R) in dB
For Spherical
N = 20
TL = 20log10(R) in dB
Intensity (I ) α 1 / 4πR^2
For Cylindrical
N = 10
TL = 10log10(R) in dB
Intensity (I) α 1/2πRD
What is Absorption / Attenuation (α) ?
Another loss effect
Absorption is different for given medias and different frequencies
Sound is absorbed by medium
It relates to a mediums molecules and elasticity
For absorption in salt water –> use the attenuation coefficients graphs to find the α value
How do you calculate Transmission loss (TL) considering geometric spreading and Attenuation?
TL =Nlog10(R) + α(R) in dB
N is dependant on type of spreading
α is attenuation coefficient
R is range from source
What are the limitations of geometric models?
Which N value do you use if there is interaction with both surface and bottom? The N value makes a huge difference
The range changes with the environment. E.g. Bathymetry –> Deeper, shallower, slopes, ground not flat
Is the signal broadband? Multiple frequencies involved? Which α value?
What is the critical angle and total internal reflection?
Critical angle
When the angle of incidence of sound strikes a boundary and undergoes total internal reflection. It is the minimum angle of incidence required for total internal reflection
Total internal reflection
Complete reflection of a wave at the boundary between two mediums
Prisms work like this
What is geometric dispersion?
Sound separates into components due to varying wave velocities or frequencies.
What are thermoclimbs and how do they affect sound?
Thermoclimbs are the distinct layers/ zones in the ocean. Created from temperature changes
They can be treated as different mediums so there are multiple boundaries.
Sound rays will therefore bend dues to sound velocity and density with depth.
Names results caused by refraction
Shadow zones
–> Waves refract toward the normal if objects are in the way, there are areas where its harder/ less waves reach that area
–> some areas the sound can’t reach at all which are shadow zones
Convergence zones
–> where multiple waves bend towards one point
–> from ‘constructive interference’ - most common paths taken
–> easier for energy to reach these areas
–> higher net energy in these areas
What is the Lloyds mirror effect
Sound waves reflect off the surface and submerged objects. They interfere creating distinct interference patterns and spatial variations in received sound field.
Monopole Vs Dipole sources
Monopole source
–> a sound source that radiates waves in all directions equally
–> similar to omnidirectional point source
–> spherical wavefronts propagate outward from source
–> E.g speaker
Dipole source
–> a sound source consisting of 2 sources even if one is virtual
–> the sources oscillate in opposite directions creating a dipole sound field
–> sound propagates primarily in perpendicular direction
–> E.g. fish
What is the low frequency cut off?
Mode stripping
–> when lower frequencies cannot propagate well dues to the channel being too shallow
Low frequency cut off
–> the point where signals will not propagate well
What is the Passive Sonar Equation?
SE = SL - TL - NL - DT
where
SE - Signal excess
amount of signal you have left. Ideally +ve, -ve means not enough signal, 0 means just about enough
SL - Source level
in dB re 1µPa-m (-m part makes it source level)
TL - Transmission loss
the one way propagation loss from the target in dB
NL - Noise level
in dB - e.g. ambient noise, ship noise
high noise, signal excess is lower
DT - Detection level
detection threshold in dB
Sometimes DI (Directivity Index) is included for directionality
- e.g. sound is lower when someone is talking in front of you compared to behind
Values are all in dB
What is the Wenz curves ?
Aka Wenz spectrum
A graphical representation of the power spectral density of underwater ambient noise as a function of frequency
Tells you about the distribution and intensity of noise at different frequencies underwater.
They came from experiments looking at how noisy the ocean is
What is the active sonar equation?
SE = SL + TS - 2TL - NL - DT
where
SE - Signal excess
SL - Source level in dB re 1µPa-m
TS - Target strength
2TL - 2x Transmission loss
accounts for the 2 journeys made, there and back
NL - Noise Level
ambient noise, ship noise, reverberation power summed
DT - Detection Threshold
All in dB
What is Target Strength?
TS = 10log(Iscat / Iinc)
where
Iscat
- scattered intensity referenced to a distance of 1m from target centre
Iinc
- intensity of the transmitted signal incident on the target
if 0dB then everything sent out comes back
can be greater than 0dB
e.g. with divers their air cavities just look like balloons from the lung capacity on an echo
For a large rigid sphere, of radius a metres:
TS = 10log(a^2 / 4)
where a 2m radius sphere has 0dB TS, larger spheres have positive target strengths
Generally:
TS = 10log(σs / 4π)
where σs is the effective scattering cross section
e.g might not see an individual fish but you will seen many fish densely packed together in a shoal which is an effective target
Near field Vs Far Field Vs Free Field
Near field
–> try to avoid
–> region close to source
–> field dominated by complex interactions
–> significant variations in pressure and particle velocity
–> individual parts can be detected
–> highly spatial and very wave dependant
Far field
–> source should only be calculated from here
–> sounds appear to come from same virtual infinitely small point
–> region father away from source
–> more stable
–> characterized by spherical wavefronts and decreasing intensity with distance following the inverse square law.
Free field
–> point where measured radiated energy only travels a ‘direct path’ from source to reciever
–> free of any multi paths
–> no reflections
–> free to radiate spherically
What is the acoustic centre?
The reference point where waves are considered to originate from
Its a virtual point / theoretical value observed from the far field
usually referenced to 1m
What is the Equal energy hypothesis?
all frequencies of sound have equal energy
loud sound –> short time
lower sound –> longer time
Both do same damage (Hearing damage)–> area under curve is the same
