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Most tone burst sonar units today come with dual frequency sonar capabilities...
The most common are ...:
*83kHz/200kHz for 2d sonar...
*455kHz/800kHz for imaging...
Looking at the wave length depiction below...
Which wave length frequency (high or low)...will give the most detailed display image...??
Why...??
Attachment 273024
Rickie
Based on what I've seen higher gives better detail. As for why I'll stick around for that if I'm even close.
I am going to guess that the Higher will give the more detailed "picture".
As to why again a guess, but I figure that the higher frequency will provide for a greater amount of returns in the "bounce back" phase of collection, therefore with more "returns" of data in a given period, compared to the lower frequency, that should equal a more detailed view of what is in the beam.
Of course I could be 100% wrong and it would not be the first time.
High
High is my guess also.
Just a swag, it seems from the drawing, the high freq. waves are closer together, which I am guessing means more info. Less time elapsed between waves should get you a better image.
Now I need to look up MHz and learn what it means.
I have to be different. I say low.
High = better detail but less range......Hope range wasn't your next quiz ;)
You guys know more about Sonar than you realize....
Now, the boring "technical explanation"...
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The sound pulse travels thru the water in a sine wave...(Below are 2 examples of sine waves) ...
Attachment 273025
Sine wave - Wikipedia, the free encyclopedia
Within a sine wave ...there are measured sections called wave lengths...
1 wave length is defined as the distance over which the wave's shape repeats...see below...
Attachment 273026
Wavelength - Wikipedia, the free encyclopedia
The tighter these individual wave lengths are together - the higher the frequency of the sine wave becomes...
Also, the tighter these individual wave lengths are together - the more of the actual sine wave is able to hit the "target"...
Thus...the higher the sine wave - the more of the sine wave that hits the target - the more reflections that can be returned = the more data returned = the more detailed the image....
Rickie
Shoot learned something new right there Rickie! Thanks!!
Why does the 800 not reach as far as the 455?
i.e. is the same amount of power used to make a 455 vs an 800? I am not grasping why they don't reach the same depth or distance. Is the same power used peak to peak, or over a different duration?
Don't laugh, Does the wave spread as it travels or is it more like a laser and remains the same size?
In the case of 800Khz on SI, it is largely a function of the beam angle from transducer....the beam angle of the 800Khz is less than the 455Khz. I don't know anything about why the transducer does this.
I also think that the higher frequencies are more readily "absorbed" by the media....the media in this case is water.
If you are old enough to remember VHF and UHF TV frequencies, it is the same thing when transmitting thru the air. The higher the frequency, the more "line of sight" the signal is. AM is lower frequency that FM, and can consequently transmit further.
The US Navy uses VLF (Very Low Frequency) radio transmission that have a VERY long wavelength that will almost transmit around the world. These VLF radio frequencies will bend with the curvature of the earth, and even penetrate into the water. These are the frequencies they used to communicate with submarines.....higher frequencies just wouldn't penetrate far enough into the water.
Maybe more trivia than you wanted to know.....I am sure Rickie knows a lot more about the transducer lobes from the various frequencies.
Bob gave a good description...
Basically...sound has natural tendencies...
Given the same amount of "push" (from the transmitter in this case)...low frequencies will travel further ....
This is also part of the reason why the edges of the 800kHz HB SI image is darker out to the sides of the display ...the higher frequency just will not travel as far as the 455kHz...(the beam form is also part of this reason)...
Conversely, the 455kHz HB SI image is always fuzzier (than the 800kHz)...because the lower frequency sine wave is "missing" minute targets that the 800kHz frequency sine wave will "hit"...
Attachment 273027
Rickie
Thanks Bob and Rickie! Do you have a drawing of a transD with a drawing of how the waves come out of it? I remember something about the piezos being tilted at an angle inside the ducer?
I don't have a depiction of the actual sine waves that come out of the xducer...(I don't recall ever seeing a pic of the actual sine wave as it emits out from the piezoes)...
Here is the HB depictions of the 455kHz SI beams (lower left image)...and the 800kHz SI beams (right image)...
(These depictions also show the 2d "cone" beam right under the xducer)...
Attachment 273028
Rickie
Thanks!
If you check out the frequencies in most military applications they are low for the improved ranges. Minesweeping sonars are usually higher frequencies for better detail and shorter range. Fathometers are normally very low frequencies. Oops...guess we're talking fishing here...LOL But amazing how the same basic principles still apply.