Originally Posted by
JamesCam
I find it extraordinary that the ping frequency apparently changes so dramatically with power supply voltage! Surely the device is crystal controlled? what use is a device like this if the frequency one is looking at is effectively unknown. To detect the signal at a distance one needs to use very narrow bandwidth filters of the order of only a few Hertz: here we are talking about a shift of 4.2KHz, well over 10%...
I don't believe this equipment performs this badly, given it's designed role. was the Ocean Shield following something else? can someone quote the source of the "manufacturer's" comments?
James
It's more of a "crystal" than "crystal-controlled", I think. It's an IC generating short square-wave "chirps", ~10 ms long, modulated with a 37.5 kHz wave, which then drives a piezoelectric crystal that's cut with the same resonance frequency. The whole device is very simple, can fit in the palm of your hand and weighs 200 grams including battery and casing. Narrow bandwidth is not a high priority target. Length of the chirp by itself spreads the spectrum by 100 Hz. Vibration response of the crystal to such a short chirp may be pretty complicated too. ULB manufacturers only guarantee frequencies up to +/-1 kHz, even for a battery at full charge.
My understanding is that underwater sounds can reflect back off the bottom and / or thermal layers in unpredictable ways. There are tales of submarines being tracked from more than a thousand miles away under the right acoustic conditions. I have no problem imagining that a pinger's signal could turn up three hundred miles away. Also, the only way I could imagine parts of the plane coming down 300 miles apart is if it was high up on re-entry al la space shuttle. I also can't imagine any fish making a love call on 37khz or so with a pulse repetition frequency similar to an underwater beacon.
Reflection can happen, but attenuation is attenuation and you can't get around that. At 37 kHz in seawater, it's ~5 db/km. A pinger that starts off at 160 db will be certainly below ambient noise even 30 miles away.
Submarines are a different story. Attenuation is orders of magnitude lower at lower frequencies.
Any repetitive source of white noise with the right repetition period would sound like a pinger to a narrowly-tuned pinger detector device.