Underwater Transducers Flashcards
(37 cards)
What is a hydrophone?
Underwater acoustic transducer
– acting as a receiver
– “underwater microphone”
Usually makes use of a piezoelectric element
Converts acoustic pressure in the sound wave to
electrical voltage
Sensitivity: volts per pascal,
… or more usually…………….dB re. 1 V / µPa
transmitter –> µPa/V
What sort of Frequency Response do we want
ideally ‘flat’ with frequency
with broadband - want to be ideally flat
What sort of Sensitivity do we want
ideally high sensitivity
What size do we want
small compared to sound wave – so approximates to a “point receiver”
Large transducer - could get diffraction affecting nearby transducers
What Beam pattern to we want
omnidirectional – sensitivity doesn’t vary with direction of sound wave
For dolphins - ~15 degrees wide
- narrow beam for transmitting and receiving
- easier to get information on specific things it’s point at
-simply turn and scan to get more information
- good for signal to noise ratio
- narrow beam cuts down reverberation noise
What Noise performance do we want?
ideally low inherent noise
What sort of Linearity do we want?
output varies linearly with input pressure
why would you want non-linear?
- maybe for better dynamic range
- for really loud and really quiet signals
What sort of Stability do we want?
response independent of environment (pressure and temperature) and time
Name some types of hydrophones
- Measuring hydrophones
- Towed arrays
- Planar arrays
- Miniature ultrasonic hydrophones
What is a Parasitic noise?
unwanted, unintended, or extraneous noise that interferes with the desired signal or communication
e.g. wind howling - it doesn’t really exist
microphones are more sensitive than human ears
Examples of Active elements in hydrophones
Piezoelectric crystal (old designs)
– e.g. quartz, Rochelle salt, tourmaline
Piezoelectric ceramic
– e.g. lead zirconate–lead titanate (PZT), barium titanate
Piezoelectric polymer
– PVDF (Polyvinylidene fluoride)
Fibre optic hydrophone element
What is a Spherical piezoelectric hydrophone
Spherical so omnidirectional
Most common hydrophone element
Two hemispheres glued together
Radially poled – internal and external electrodes
Simple design – usually air-filled
Fairly easy to predict response
Spherical piezoelectric hydrophone formulae
For the low frequency sensitivity for a thin walled shell:
M ≈ -b g31
where
b is external radius
g31 is appropriate piezoelectric constant
For the low frequency capacitance:
CLf = 4πεb ((b/t) -1)
where
ε is dielectric constant
t is wall thickness
Resonance frequency
fs = sqrt [ (1/(2πb)^2).(2.E11/ ((1-σ)ρ) )]
where
ρ = material density
σ = Poisson’s ratio
E11 = elastic modulus
Cylindrical piezoelectric hydrophones
Common design with easily predicted
response
In most basic form it is a PZT cylinder,
often with an end-cap
Need to take care with aspect ratio (thickness to diameter ratio)
Clever designs to improve damping –
i.e. reduce resonance behaviour
- slotted cylinders, division into rings, internal filling material to provide damping
Cylindrical hydrophone formulae
Low frequency sensitivity of an end capped, thin walled, cylinder:
M ≈ (3/2)bg31
where
b is external radius
g31 is appropriate piezoelectric constant
Low frequency capacitance:
CLf = 2πεLa/t
where
ε = dielectric constant
t = wall thickness
L = length
Radial & Length mode resonance frequencies
f ≈ c/2πr
fL ≈ c/2L
where c is the longitudinal wave speed in the ceramic
Adv of PVDF (Polyvinylidene fluoride) hydrophones
– high inherent piezoelectricity
– lower mechanical Q
– better impedance matched to water
Dis of PVDF (Polyvinylidene fluoride) hydrophones
– low electrical permittivity
– only available in sheets
– connections difficult
Successes of PVDF (Polyvinylidene fluoride) hydrophones
– planar receive arrays
– very high frequency ultrasonic hydrophones
Fibre-optic hydrophones
Converts pressure into phase modulation of light
– changing pressure alters length and refractive index of fibre
– signal recovered using interferometric techniques
Mainly used for LF towed arrays
How to get around piezoelectrics not being waterproof?
Make them from a material that impedance matches water so energy will go through
Advs of Fibre-optic hydrophones
– inherent multiplexing
– lightweight
– immunity to EM
– no wet end electronics
What needs to be considered with hydrophone performance?
Require knowledge of the sensitivity – calibrated
Require flat frequency response
– Crucial for faithful reproduction of acoustic signal
– Choose resonance to be well above frequency of interest
– Phase response also important
Need good signal to noise ratio for accurate measurement
– High gain/sensitivity no good if also very noisy
Require knowledge of the directivity
– Sensitivity can also vary with angle
What is Hydrophone sensitivity?
The voltage produced when expose to a given acoustic pressure (Volts per Pascal or μPa)
Often states as a negative number (e.g. –211 dB re 1V/ μPa)
Sensitivity = 0 dB –> 1V generated when exposed to a sound wave with pressure of 1μPa
Negative
- bigger number has worse sensitivity
Our –211 dB re 1V/ μPa hydrophone would generate a 28 pV signal
when exposed to a sound wave with a pressure of 1 μPa
* Also known as 28 μV per Pa
Which is worse? a Hydrophone sensitivity of -205 dB re 1V/ μPa or -211 dB re 1V/ μPa
-211 dB re 1V/ μPa is less sensitive than -205 dB re 1V/ μPa
It is 6 dB worse
Our –211 dB re 1V/ μPa hydrophone would generate a 28 pV signal
when exposed to a sound wave with a pressure of 1μPa
AKA 28 μV per Pa
