Thread: A Questioning Mind

Originally Posted by larryb227

Ricky,
Does the HB have a depression angle on SI or does it pick up surface clutter.

Explain "depression angle"...I haven't heard this term before...

Rickie

Originally Posted by rnvinc

Explain "depression angle"...I haven't heard this term before...

Rickie

The angle produced by the surface of the water and the edge of the side scan beam from the side scan ducer.
(I tried to add a PNG drawing but the site hangs up and I've had to start over several times. It freezes when I try to change the size of the thumbnail)
It is usually done to avoid the surface clutter.

Originally Posted by larryb227

The angle produced by the surface of the water and the edge of the side scan beam from the side scan ducer.
(I tried to add a PNG drawing but the site hangs up and I've had to start over several times. It freezes when I try to change the size of the thumbnail)
It is usually done to avoid the surface clutter.

It would be my guess (and maybe Greg can shed light here)...that the SI image processing has a filter (similar to Surface Clutter under the Sonar tab) to help with any near field reflective targets in the area of close proximity of the SI piezoes...

Each filter in the unit (Surface Clutter, Noise Rejection, Switchfire, Fish ID, etc)...has it's own specific algorithm criteria that it "filters" out of the displayed image...(the echo data received is "the echo data received" ....each specific filter {working within its own criteria} ...just says "Nope- not gonna show that echo, and not gonna show this echo" ...and so on)...

If there is a filter specifically for near field clutter in the SI image processing... it's not available to users because I think the present "Surface Clutter" setting under the Sonar tab only affects the 2d image processing...

Rickie

Ok Rickie if that is how the HB works.
Generally surface clutter is from 0 depth to half the depth the unit is set at with the attenuation gradually coming back to full sensitivity. (mostly on 2D sonar) In SI your surface clutter is present to the end of your range. Near field in not the only problem.
Basically you are saying the beam is not depressed.

I agree the near field is not the only problem (because the beam form is going to be in this area of possible surface clutter for a greater distance than a "down pointing" beam form)...

And from the "calculated" beam form depiction (see below)...I can't see where there is any measured "depression angle"...because the sound pulse (in both frequencies)...reaches all the way to the water's surface...albeit at "ever-decreasing pulse strength"...

The echoes are there (if the beam form is there)...the processor (and/or it's associated filters)...decides which of the total echoes are painted into the SI image..



I'm only guessing that there is a specific filter used for omitting "surface clutter" type echoes in the processing of the SI image...(there is no setting for SI surface clutter...but that doesn't mean one doesn't exist...we just don't have access to it)...

Maybe I'm mis-understanding your meaning of "depression angle" (as relating to an SI image)...(or it's delving into an area of sonar technical data I have yet to learn)...

Because in SI sound pulses thru water ...there is no measured angle (that I know of)...from the water's surface to the "edge" of the SI beam...

There is no "edge" in any sound pulse beam form...only the brand's own criteria for how they measure the "ever-decreasing strength" of the sound pulse before it reaches 0db ...or "Absolute Silence"...

And of course there is the measured angle at 0db "Absolute Silence"...but that angle is way out past the "advertised beam angle"...(brands have no need in advertising this 0db angle...so we never see it in advertising)...

Rickie

I found this ...that speaks of "Depression Angle" as applied to sonar...

NAVAL SONAR - APPENDIX - DEFINITIONS

I have no idea if this relates to any Sonar Beam form scenarios that are fixed direction "side pointing"...

Rickie

Originally Posted by rnvinc

I found this ...that speaks of "Depression Angle" as applied to sonar...

NAVAL SONAR - APPENDIX - DEFINITIONS

I have no idea if this relates to any Sonar Beam form scenarios that are fixed direction "side pointing"...

Rickie

I've always understood that the depression angle I'm talking about is of course, theory. Side lobes will change much of that but the dicer is cut to have a certain angle i.e. 30 degrees or 20 degrees or what ever. In side imaging the beam is very narrow like a pencil beam therefore helping the resolution along with frequency.
Bottom line I'm sorry I mentioned it.

Originally Posted by larryb227

I've always understood that the depression angle I'm talking about is of course, theory. Side lobes will change much of that but the dicer is cut to have a certain angle i.e. 30 degrees or 20 degrees or what ever. In side imaging the beam is very narrow like a pencil beam therefore helping the resolution along with frequency.
Bottom line I'm sorry I mentioned it.

Heavens no ...don't be sorry about mentioning anything here...we learn by discussion...and your topic has me motivated to study more about "Deflection Angle"...

I like learning..

I will mention ...you are absolutely correct that piezoes are formed to a specific size and shape to achieve the desired beam angle (and beam shape)...that each brand desires for their specific sonar beams....

http://www.marinop.it/pdf_doc/Transducer%20Design.pdf
http://www.airmartechnology.com/uplo...Operations.pdf

But how the brands "measure" that beam angle ...compared to "cartoon depictions" of brand's advertised beam angle is misleading (to say the least)...

The sound pulse doesn't just go to the edge of the depiction beam line and stop (as it seems in the depiction below)...the sound pulse continues past the "measured beam angle" in an ever-decreasing strength until 0db....no sound....



This very topic of the advertised SI beam angles was 1 of the most difficult discussions to hash out when HB SI hit the market...

That the "cartoon depictions" of the individual left and right SI ...did not touch at the bottom of the HB depiction...(see depiction above)...

It was discussed for months before it was understood that there was no gap in coverage of the SI beams...but just the way HB measured the "advertised beam angle" ...and their assigned measurement numbers didn't add up to ...no gap...

Here is some of that initial discussion and testing that proved the SI beam angles depicted in the HB SI advertisement were merely "measurements" ...and not where the sound pulse actually stops...

The Proof of Humminbird Down Imaging

It wasn't until much later that Rickard and some of the other wave theory experts at sideimagingsoft dissected the HDSI xducer and calculated the mathematical beam forms to show just how far past the "advertised beam angle" that the sound pulse actually travels ...

Rickie