Underwater Transducers Flashcards
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
Why do we have Hydrophone preamplifiers
Amplifiers for underwater microphones
Hydrophone elements can have quite high electrical impedance
Add a pre-amp to:
- act as impedance buffer
- increase signal level
some hydrophones have them integrated
What should you consider about Preamplifiers?
- Input impedance (high)
- Noise (low)
- Gain (depends on signal level)
What does the Directivity index tell us?
It tells us the directional sensitivity measure of the hydrophones
It quantifies how well the hydrophones can discriminate between sounds arriving from different directions
High directivity index –> greater ability to detect sounds from specific directions
Usually in vertical directivity
–> poor sensitivity where the cable would be
What factors affect a hydrophones performance?
– Cable extensions
– Instrument loading - what instrument you plug into (standard is 50Ω)
– Mounting
– Wetting
– Depth
– Temperature
A hydrophone being dragged up and down will change the hydrostatic pressures
- e.g. waves hitting the boat the transducer is attached to
What should you consider when deploying a hydrophone?
– Surface motion can generate large LF signals on the hydrophone output
– Cable strumming can also generate spurious LF signals
How do extension cables and loading affect hydrophones?
Hydrophone electrically loaded by extension cables= results in loss of sensitivity
At frequencies well below resonance a simple correction is possible, at higher frequencies the correction is more complex
Corrections can also be applied for instrument loading
Best to use a pre-amplifier to drive long cables
Using a preamplifier will also usually avoid any problems with instrument loading
What is wetting?
- Hydrophone surface can repel water
- Bubbles or a film of air adheres to surface - gas bubbles building up on surface
- Responses will severely change
- use detergent to break bubbles on hydrophones
- Ideally soaked before use to allow temperature to equalise
What is Mounting?
- Mounting affects hydrophone sensitivity
- Ideal mount is rigid
- Ideal mount doesn’t cause acoustic reflections
- Ideally, calibrate the hydrophone in the mount that will be used with the hydrophone
How does Depth variation affect hydrophones?
- temperature and depth(pressure) affect hydrophone performance
- “Simple” designs show greatest stability (e.g. simple ball hydrophone)
- Electrical impedance variation a good indicator
Why calibrate hydrophones?
- Can get sensitivity for pressure calculations using hydrophone voltage
- Sensitivity changes over time (aging)
- calibration curve supplied by manufacturer might not be valid
- Accurate measurements –> need accurate calibrated instruments
- For traceability
- To meet national/international standards
- Important for regulatory work
- So comparisons can be made
What do we need to calibrate the hydrophone?
- In free field
- Need steady state response
- lower frequencies require larger tanks
- Ring up and reflection free time limit the lower frequency
- Initial signal contaminated by start up transients
- worse for high Q transducers
- Q cycles need to reach steady state
Important things to remember
Choose the hydrophone for your measurement with care:
- Required frequency range?
- Sensitivity requirements?
- Is low noise important?
Avoid using long cables without a pre-amplifier
Use a pre-amplifier as a buffer to avoid loading
Understand what hydrophone sensitivity means (dB re 1V/ μPa)
- Don’t assume that high sensitivity means low noise
Make sure your equipment is calibrated
Which is most sensitive?
TC4040 with receiving sensitivity: -206 dB re 1V/μPa
TC4032 with receiving sensitivity: -170dB re 1V/μPa
TC4032
Has higher number
Most sensitive
