A single hyrophone cannot tune out interference from any
particular direction. An array of 2 or more hydrophones can be
processed by audio delay technology to 'steer' the directivity,
and tune out, or null audio noise from undesired directions. a
logarithmic response mixing circuit would enhance this effect. A
logarithmic amplifier is like a linear response noise gate:
louder signals are amplified more than weak ones.
With only hydrophones 2, a delay on one of them equivalent to the
transit distance between them will enable bidirectional steering
of the acoustic sensor array: ahead or behind. with no delay, the
array will be most sensitive abeam the array, with bipolar
response, equal on either side. Intermediate delays will enable
some inbetween steering capability, with some directional
ambiguity. With 3 hydrophones and 2 delay units, full steering
capabilities can be achieved, with unwanted signal noise
reduction of perhaps 3 db per additional hydrophone. Guitar
accessories called delay boxes, or similar 'echo boxes' can be
used to test this capability. They use 'bucket brigade' delay
line integrated circuits, and can be rented from music stores.
The vessels you might want to sense at sea would be large ocean
going ships, whose propellors turn at perhaps 5 - 10 hertz.
Audio filters can eliminate other frequencies, such as those
created by whales, shrimp and rain. Large arrays, perhaps with a
towed line hydrophone in a quiet capsule could benefit the arrays
sensitivity when sensing low frequencies.
SSB type technology can be used to transpose inaudible low or
high frequencies into a range your ears can hear. These musical
devices are similar to an 'octatone' which generally 'bends'
pitch to produce second harmony parts or tonic sub harmonics for
some instruments such as saxophones or to produce other
synthesised affects. They have been synthesised to produce
'yodeling' effects heard first on a vocal recording by Cher last
year or so, and more recently by other artists.
There seems to be some promise for the sailor in this method. As
a sailor, I have been awakened by the gearcase noise of the
passage of fast boats several hundreds of yards distant
transmitted through the water and my hull while I was snoozing at
anchor.
Just think what life must be like for whales and fishes who
depend on sound for sensing, especially when vessels use high
powered sonar devices. Passive methods could serve us well for
non-stealth vessels.
Terry K
Rufus wrote:
Several years back, I read of a fellow who set up a sensitive AM
receiver "wide open" on a freq not occupied by any standard broadcast.
This is similar to the cheap "storm detectors" which amplify distant
lightning noise. He reported that most boats and ships generate a
significant amount of RF noise which can be picked up and amplified to
provide audible warning. However, this doesn't help much in a busy
harbor with lots of noise around any way.
Rufus
Derek Weston wrote:
In a previous thread I sought advice on optical and hydrophone-based
ship detection systems. I have since done some experiments with
hydrophones which may be of interest to others.
I built the hydrophone described at
sonar-fs.lboro.ac.uk/uag/downloads/bender2.pdf
and connected it to a preamplifier similar to that used for the low
noise PIN diode laser receiver described at http://www.k3pgp.org/ This
was connected in turn to a simple LM386 audio amplifier driving a speaker.
Generally, the loudest noises were those from the clicking shrimps.
Ships travelling at low speed some 500 metres distant could often be
heard, and some travelling at higher speeds were audible up to about
1500 metres away. Rain on the water was suprisingly loud, and heavy rain
masked all other sounds.
Mulling over the impracticality of trailing a transducer, the
depthsounder transducer was connected to the same amplifier system. This
transducer proved far less sensitive to audible frequencies than the
"Bender" hydrophone, by an estimated 15 dB. It also seemed to have much
poorer response at lower frequencies. An additional disadvantage of the
depthsounder transducer as mounted in our yacht is that underway the
noise from even small waves striking the hull was very loud and masked
most other sounds.
Even with DSP I doubt anything I've tried so far could be part of a
useful ship detection system.
So far its looking like Larry, Armond and others' advice to just
generate more power and use the radar may indeed be the best way for me.
The hydrophone systems will probably be relegated to the role of
eavesdropping on cetaceans.
Derek Weston
Talking Depth Sounders http://www.alphalink.com.au/~derekw/mit/
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