SUBMARINES & THE NOISE THEY MAKE
he early work on SOund Navigation And Ranging (SONAR), which
was named after RAdio Navigation And Ranging (RADAR), was done utilizing audio frequencies were an operator would listen
on headphones (for those systems that used stereo) or a loudspeaker. The first sonobuoys also used audio with a
human operator listening to the sound. This is called passive SONAR (code name Jezebel). There are no pings
and whoever is doing the listening is not giving away their presence or position. When it works it's the preferred
method and is by far the most common. In the 1950s the SOund SUrveillance System (SOSUS) which makes use of LOw
Frequency Analysis and Recording (LOFAR) rather than the use of human audible sound was put into service. This
worked on snorkeling diesel electric subs and on nuclear powered subs like the Soviet Hotel , Echo & November class
Active SONAR comes in two flavors, the most commonly known is
the active ping like in any movie involving submarines. A ping is sent out and the time measured until it
returns. The early pings were audible to humans, later ultrasonic pings were used and later still the frequency of
the ping changed it into a chirp. If the propagation speed is known the distance to the target can be calculated
and with the later types the radial speed of the target can be determined. The less well known active SONAR method
(code name Julie) involves setting off an explosion of a couple of pounds of TNT using either the Mk-15 (Mod-12) or
Mk-61 Signal Underwater Sound (SUS). The explosion generates a spike in the underwater pressure which is similar
to a ping at all possible frequencies, it's the most useful type of ping, but can only be used occasionally because it
requires a small bomb for each pulse.
Modern diesel electric subs are very quiet when running
on battery power underwater. The explosive type active SONAR is good at deteting these subs.
The word sono-buoy is based on sound and a floating
object. Sonobuoys have been around since about May 1941 when P. M. S. Blackett, head of the British
Admiralty committee for antisubmarine measures, proposed the idea. In June 1942, the AN/CRT-1 became the first operational sonobuoy, and on July 25,
1942, the first successful launch of a sonobuoy from an aircraft was made from a U.S. Army B-18 bomber.
They are nomenclatured SSQ-nn. They are part of
anti-submarine warefare. "All sonobuoys currently in inventory are normally launched from standard A-size
tubes via pneumatics, free fall, or a Cartridge Actuated Device (CAD). Shipboard personnel may also launch them by
hand or Over the Side (OTS). All are powered by either salt water activated magnesium or silver chloride, lithium
chemistry, or thermal batteries and are designed to scuttle at some point after usable or selected life expires".
Modern sonobuoys have an outside diameter of 4-7/8" (fit
4-15/16" launch tube commonly called 5 inch) and are 1 yard long (36"). The max weight is 39
pounds. This is a convient size for one man to handle on a P-3 Orion aircraft. The larger sizes are not easy
to handle. There are smaller sizes based on getting some interger number of them inside the A size outline.
3 each is called "F" and 2 each is called "G". The other sizes are pretty much not used in
A hydrophone is the common sensor for sonobuoys and is
typically deployed at 20 meters (65 feet: shallow) or 120 meters ( 328 feet: deep). A sonobuoy might have 50 depth
settings that can be set prior to ejecting it from an aircraft. This is important because of what's called the
thermocline which is where the temperature of the water changes rapidly. This changes the speed of sound
. This causes the sound to change direction just as light will be bent by a change in refractive index . And
just like light there's conditions where the bending acts like a mirror and all the sound (or light) is reflected off
the layer instead of just changing angles. So if the hydrophone is on the wrong side of the thermocline it may not
hear a sub that's on the other side. Bathythermograph This is a device that measures the water temperature as a function
of depth. For example the SSQ-36 might first be dropped and the temperature profile recorded. Then the ideal
depth for the hydrophone determined and programmed into the sonobuoy. Then the sonobuoys would be dropped.
The speed of sound in water depends on temperature so it
slows down as the depth moves from the warm surface to deeper depths, but at some point the pressure caused by deepth
will cause it to speed up again.
The service life can also be programmed prior to launch for
1, 3 or 8 hours.
A VHF vertical whip antenna is used. One feature of
this type of antenna is that there's a null directly above the buoy so when an aircraft directly overflys the buoy
there's a characteristic signal drop out. This allows confirming the buoy location. The 1 Watt transmitter
is FM modulated and covers an audio bandwidth of 10 Hz to 20 kHz (about the same as a Hi-Fi system or entertainment FM
COMMAND FUNCTION SELECT (CFS)
The aircraft can transmit to the sonobuoy to change the
commands while it's in the water which is much better than the old way of making the command decisions prior to launch.
DIRECTIONAL FREQUENCY ANALYSIS & RECORDING TYPE
ACOUSTIC SENSOR (DIFAR)
These buoys use directional hydrophones covering 5 or 10 Hz
to 2400 Hz combined with a magnetic bearing sensor and transmit this information. They can be used to passively
listen, to listen for reflected pings or the shock wave from a small explosion. Whale researchers use them.
Because the lower frequency limit is below Hi-Fi audio, consumer grade tape recorders could not be used so
instrumentation type recorders were used. With the advent of Digital Audio Tape (DAT) recorders the DIFAR signal
could be recorded on a DAT tapes