daver@felix.UUCP (Dave Richards) (09/13/90)
In article <656@ghp.UUCP> jim@ghp.UUCP (Jim Stewart) writes: >I know they use sound, but at what frequecies? Some of the transducers >talk about different angles of scanning the bottom--is this effectively >just an average, or does it do some other function? How is the >transducer made? Is it some kind of crystal? Are there two parts to >it (i.e., one to generate a sound pulse and one to receive it)? How >difficult would it be to put something together to experiment with the >concepts (i.e., build a primitive transducer, or buy just the transducer >if somebody knew the pin-out, and hook it to a scope, or whatever just >to see how it works)? Heathkit (Benton Harbor, MI 49022) supplies depth finders and fish spotters. They used to have complete kits that came with schematics, etc. However, upon consulting the current catalog I find that they only offer assembled versions now. But you can buy the transducer only: Transom mount - MIA-2020-1 $54.95 or thru-hull type - MIA-2020-2 $129.95. Looking at the specs for the assembled units yields this information: Frequency: 200kHz. Pulselength: .25 to 6ms, depending on depth range. Xmit power: 85w RMS, 680w p-p from 0-250 feet, 150w RMS, 1200w p-p at the 500-1500 foot ranges. Xducer beam angle: 20 degrees Regarding the other questions: yes, it is some kind of crystal, and I have seen surplus sonar transducer crystals for sale. They are in the shape of a hollow tube, about 2 inches in diameter and about the same length. I don't know how these are packaged, or how the wires are attached. I don't think you would have much success trying to make your own tranducer. And even if you were successful, it still doesn't seem worth it when you can buy one for under $60. The transducers I have seen simply have a coax connector, so it would appear that the sending and receiving is done with the same element (at least for depth-finding. Active sonar uses one transmitter and an array of receivers, as I understand it.). So the pinout should not be a problem. These new units suggest that they include a water temperature sensor of some kind, in which case there may be extra wires for that. ______ Some other handy things to know: The speed of sound in water = \/ B/d Where B is the bulk modulus = 2.1e9 N/m*m and d is the density = about 1.025e3 kg/m*m*m in sea water I know that the density varies with depth, salinity, and water temperature. For all I know, the modulus may also. At any rate, this gives a Vw of around 1430 m/s, or about 4700 ft/s. This translates to 2.13e-4 sec/ft (don't forget that this is the time to go one way -- doesn't include the return trip). To "experiment with the concepts" I think it would be much easier to do it in air using a dynamic (coil) speaker as a transducer, a parabolic dish as a focussing device, and of course, much lower frequencies, say 1-2kHz. Vw in air is about 1100 ft/sec, or 9e-4 sec/ft. . . . . . . . . . . . . . . . . . . . . .| / ^ | | | | | \ | | | || spkr sound beam | building or other object | / | | | | | \ . . . . . . . . . . . .v. . . . . . . . .| dish | If you try this, let us know how it works. Dave
preece@sweep.enet.dec.com (09/14/90)
If you really want to play with the concepts of sonar, in air, try to get hold of one the "designer packs" that Polaroid Co put out a few years back. It was the "works" from the ultrasonic rangefinder they had on one of their cameras, but sold "loose", specifically for people to experiment with. I remember looking at it, and the cash in my pocket, and trying hard to think of a reason why I needed one! Have fun ! Ian "I wouldn't work for a company that had opinions like mine."
biocca@bevb.bev.lbl.gov (Alan Biocca) (09/15/90)
In article <151118@felix.UUCP: daver@felix.UUCP (Dave Richards) writes: :In article <656@ghp.UUCP> jim@ghp.UUCP (Jim Stewart) writes: :>I know they use sound, but at what frequecies? Some of the transducers :>talk about different angles of scanning the bottom--is this effectively :>just an average, or does it do some other function? How is the :>transducer made? Is it some kind of crystal? Are there two parts to :>it (i.e., one to generate a sound pulse and one to receive it)? How :>difficult would it be to put something together to experiment with the :>concepts (i.e., build a primitive transducer, or buy just the transducer :>if somebody knew the pin-out, and hook it to a scope, or whatever just :>to see how it works)? :Looking at the specs for the assembled units yields this information: :Frequency: 200kHz.... Xducer beam angle: 20 degrees There are some new models that use 455 khz, and beam angles vary from around 16 to 40 degrees. I suspect the narrow beamwidths are being accomplished by building essentially an active dish antenna with/on/attached to the crystal. :Regarding the other questions: yes, it is some kind of crystal. My understanding is that it is a quartz crystal. :To "experiment with the concepts" I think it would be much easier to do it in :air using a dynamic (coil) speaker as a transducer, a parabolic dish as a :focussing device, and of course, much lower frequencies, say 1-2kHz. I know someone who built some small systems of this type using disk ceramic capacitors as transducers. Apparently they (at least some of them) have the proper electro-mechanical characteristics. Alan K Biocca