Sonar Coverage Help

[Crawdaddy]

Can anyone explain the difference in a tranducer that has a
24 @ -10db and one that has 20 @ -3db? Which one covers more area? The reason I ask is because I was looking at buying one of the new legend series fishfinders and I notice that they changed the coverage from the Zercom Z160. Is this a big deal or should I not worry about it?

Any help is appreciated
Crawdaddy

[GENETRON]

A GOOD QUESTION. THE THEORETICAL CENTER AXIS OF THE TRANSDUCER CONE ANGLE IS AN IMAGINARY LINE THAT EXTENDS STRAIGHT DOWN FROM THE CENTER OF THE TRANSDUCER. MEASUREMENTS OF CONE ANGLES ARE TAKEN AT DIFFERENT LEVELS OF SOUND RETRIEVAL FROM THE CENTER AXIS WHICH IS CALLED db. GENERALLY SPEAKING THE HIGHER THE db LEVEL THE FURTHUR THE DISTANCE FROM THE CENTER AXIS. THEREFORE A TRANSDUCER THAT IS 20 DEGREES AT - 3 db WOULD BE WIDER AT -10 db. ON THE CONVERSE THE TRANSDUCER CONE ANGLE THAT IS 24 DEGREES AT -10 db WOULD GENERALLY BE NARROWER AT- 3 db. ALSO WHAT A TRANSDUCER CONE ANGLE REALLY LOOKS LIKE IN REALITY MIGHT SURPRISE A LOT OF PEOPLE. DICK KNUTSON GENETRON

[SUPERTROLLER]

I understand what you're saying about the difference in reference points for the two transducers, but which one covers more area of the bottom at 20 ft. down? I think this was more his question.

[RANGER]

The 24 @ -10 db. The more -db, the more strength, therefore, the more coverage and larger more encompassing area. db is inversely proportional - in other words - -0db to -3db will double your signal, -3db to -6db will quadruple it, and so on.

[Gray Ghost]

Ok, here is the straight scoop. The xducer that is 24 degrees at -10 db is about 12 degrees at -3 db. The xducer that is 20 degrees at -3db is about 40 degrees at -10 db. Every xducer has side lobes, and the first side lobe shows up at about -10 db down and is about 2X wide of the value at -3 db. Even at 7 db less power the sonar system is still sensitive enough to detect fish and other objects out the edges of the first side lobe. Although, the main lobe, -3 db, half power point, is the real work horse that will get to the deepest depths. The first side lobe, -10 db, will perform to lesser depths. In shallower depths, some thing less than the one half of the maximum depth of the particular sonar the first side lobe should provide detection, assuming that the receive sensitivity is set to maximum and that the transmit power is set to maximum.

I have a Bottom Line NCC 6500, that has a 10 degree at -3 db and I will see the fish first with a halo around a bright return. The halo is due to the first side lobe detection and the bright return is due to -3 db beam. I also see returns at intermediate depths that are out of the main lobe, -3 db which are caused by the first side lobe, -10db.

Check out this site, Airmar is the leading manufacture of transducer for consumer sonar. http://www.airmar.com/

[The answer please]

If I understand correctly, the 20@-3db is covering more area?

[Mark]

That is correct.

[GENETRON]

TO FURTHUR COMPLICATE THE ANSWER ONE MUST ALSO TAKE FREQUENCY INTO CONSIDERATION. LOWER FREQUENCY GENERALLY WILL SPREAD THE BEAM WIDER FASTER THAN A HIGH FREQUENCY BEAM. FOR INSTANCE SOME SEARCHLIGHT BEAMS FOR SIDE SCANNERS HAVE A VERY NARROW MAIN LOBE AND SMALL SIDE LOBES WHILE SOME WIDE BEAMS LOOK LIKE THE TRANSDUCER IS SITTING ON TOP OF A BASKETBALL WHEN YOU LOOK AT A FIGURE OF MERIT. TO REALLY FIND OUT WHAT ANYONES PARTICULAR TRANSDUCER CONE ANGLE REALLY LOOKS LIKE IN THE REAL WORLD SOMEONE EARLIER SUGGESTED CONTACTING AIRMAR AND HAVE THE SPECIFICS OF YOUR TRANSDUCER AND THEN ASK FOR THE PICTURES(FIGURES OF MERIT) OF YOUR TRANSDUCERS CONE ANGLES. THERE ARE MANY DIFFERENT TYPES OF TRANSDUCERS THAT ARE DESIGNED FOR DIFFERENT PURPOSES! ALSO ONE MUST REMEMBER THAT THE SOUND NOT ONLY GOES DOWN BUT TO THE SIDES AS WELL FROM THE CENTER AXIS OF THE TRANSDUCER. DICK KNUTSON GENETRON

[Crawdaddy]

Thanks for the help guys.

Crawdaddy