SONAR & underwater Acoustics
Oman Military College
SONAR is an acronym for SOund NAvigation and Ranging – a classic example of this acoustic application in air is the flying bat, and in water, dolphins, whales, ships, submarines and their weapons, such as torpedoes.
SONAR versus RADAR
SONAR is the underwater equivalent of radar, and like radar its functions are many and varied. These include the detection, identification, location and speed indication of ‘targets of interest‘.
There are basically two sonar modes of operation, ‘Passive‘ (receiving or listening) and ‘Active‘ (transmitting and receiving).
Radar operates on the principle of transmitting and/or receiving electromagnetic waves, travelling at approximately the speed of light
(3×108 m/sec) through space, or the atmosphere.
SONAR on the other hand, relies on sound or acoustic energy in water travelling at approximately 1,500m/sec, depending upon conditions such as temperature, depth, and salinity.
The ACOUSTIC SYSTEMS TRAINER(AST) is installed in a number of University Marine Engineering and Hydrography Departments.
The AST is an ideal platform for R&D and testing scenarios without the expense of seabound missions.
The AST can be seen at most of the more prestigious oceanology exhibitions.
A university MSc degree dissertation has already been written using the AST(Mk III) as a development platform at the University of Plymouth.
Why Sound Energy?
In the dull, dense and conductive environment of seawater, electromagnetic waves and light rays are severely attenuated. Acoustic energy however has proven to be more tolerant and controllable in this medium, but the sound is still influenced in many ways during its passage through the sea, consequently substantial acoustic energy is essential for many sonar applications.
The operational frequency of a specific sonar system is chosen to accommodate the required mode of application; this is critical to the system’s performance, and its receiving/transmitting transducers.
For example low frequency (e.g. 1 kHz) systems may be used for long range ‘listening’ and detection, whereas a higher frequency (>10 kHz) systems could be employed to resolve ‘target signatures‘, and to pinpoint the target location.
The accepted formula for calculating wavelength (λ) is –
λ= c/f meters
Where c = 1,500m/sec (approx. speed of sound in seawater),
and f = the operational frequency, e.g. 1kHz (1,000Hz).
Giving λ = 1.5m
“Can the speed of sound in water be measured at various temperatures?”
Yes, at room temperature and up to 50C, but also below room temp……………… by using ice…’on the rocks’!
Also by using a new ‘phase measurement technique’ – thanks to the Royal Navy at HMS Collingwood – ahead of the game!
“Is the PC/laptop spec demanding?”
Not really………Windows XP/Vista/Win 7, 8, 10…..1GHz, 20 Gb hard drive, 2 Gb RAM, USB2 port, colour monitor, sound card!
However, the old story…………..the faster, the better!
Note: A USB2 port is required to get the best results from the current BAE production AST.
“Is the water – salt water or fresh water?”
Either – it’s entirely your choice, the difference in results is minor with this water volume…….but the taste is different!
“Give me a list of the test instruments in the Sonar Signal Analyzer please?”
All two channel-
Oscilloscope, Spectrum Analyzer, Multimeters galore, Transient Recorder, Data Logger and a
single ‘channel’ AWG(Arbitrary Waveform Generator with Sweep and Burst……….dozens of features, including creative new MultiChannel software!
Note: A new 4 channel SSA is an option but does not have an AWG!
“Do you supply various profile active targets?”
Three Flat plates, Grill target and a Sphere, but we can consider other shapes on request……..within reason!
We also supply 3,4,5 (6 on request) bladed, miniature propellers for passive operation, vortex and cavitations demonstrations.
“Why is the AST Console so big?”
Instructor demonstrations, and Safety! This is a ‘two student’ platform: one to work in the water, and one to operate the controls,
also to make configuration patching on the panel easier………and somewhere to set the monitor!
Note: Most SONAR Instructors use the AST Console for classroom demos.
“What is the operational frequency…………………?
It is around 200kHz, but the transducers are tuned to perform efficiently at their natural resonant frequency……….you can also make them operate at a lower frequency……
……without any changes to the transducer!
“How about underwater communications between………………..?”
No problem! We have transmitted Morse, voice, music, and other ‘noises’ through our ‘vast’ volume of water!
“Do you have the ability to listen to submersed passive targets?”
Yes, uses specialized software and an audio frequency hydrophone!
Note: A Sound Library is also supplied.
“Does the Pulse Width change automatically?”
No, it has to be manually varied, or it can be set to the CAL(calibrated) position, as with the PRF, TX frequency, and Phase parameters.
Variable PW gives a nice demo of target range resolution.
“We are a USA University, Marine Engineering department, is the AST suitable for development work?”
The AST is very well suited to underwater development studies, and it is already installed at Texas A&M University at Galveston.
A degree dissertation using the AST, has been published by the Institute of Marine Studies at the University of Plymouth (UK). Contact us for a copy-
“Is your trainer applicable to the military, specifically Navy and ASW work?”
The draft outline for the design was originally created by the British Royal Navy, but the current Mk iX system is an order of magnitude technically superior to it’s great, great, great, grandfather!
Yes Sir! The system is currently installed in a number of ASW navy schools, and other military establishments…….we are proud to say!
“We are looking for a 12 bit resolution sonar signal analyzer……………?”
No problem! Our latest version of the SSA (Sonar Signal Analyzer) has 12/14/16 bit resolution, and connects to a USB 1.1, USB 2, USB 3 port.
“How about CTFM sonar operation?”
Thanks to the Royal Norwegian Navy we can now show CTFM (Continuous Transmission Frequency Modulated) ranging sonar operation on the AST.