Understanding Coverage Area and Decibels

[Fishton]

We buy fish finders to find fish right?  So the coverage area of a cone is actually very important then, because the wider the beam the more area you will be covering to find fish.  If you have been using a dual frequency sonar such as the 83/200kHz for a while, you will know by now that the 83kHz covers 3 times the area of the 200kHz.



You probably thinking to yourself "Boring! Known that for years."  But what do we actually know about the relationship between coverage area and Decibels?  I have been looking into this for quite some time now and have come up with the following conclusion.

When you read the specifications of your sonar they will give you a power output (peak to peak and RMS), frequency (50, 83, 200 or a combination of these), cone angle and then either a -3dB or -10dB at the end.  What does this -3dB and -10dB actually mean?

The Decibel (dB) is the unit which the power or intensity of a sound wave is measured.  In order to determine cone angle, manufacturers generally use the -3dB range, because at this point, according to Airmar Technologies, the sound wave is already at half its full power*. 

Below is the 200kHz sound waves coverage area measured at -3dB
Pro: Wide coverage area = more fish
Con: More clutter


Below is the 200kHz sound waves coverage area measured at -10dB
Pro: Less clutter
Con: Narrow coverage area



*Just think about this statement by Airmar Technologies for a minute "At -3dB the sound wave is already at half of its full potential."  This got me thinking how important it is to have your transducer set up correctly.

[Mike Cork]

Great info! There is so much to getting our units dialed in. The transducer is where the unit gathers it's info so having it set up correctly is vital.

[bassinbob 1]

That is why an external mounted transducer is so much better than a transducer mounted thru hull. A thru hull is needed for high speed readings on high powered bass boats.So if you only have one unit on your dash I put a external and a thru hull and run them thru a switch box that allows you to choose the thru hull for high speed and a external once you slow down . You can adjust the angle of the transducer to the correct angle for your boat. Till you actually see the difference between the two transducers you don't realize what your missing.

[CatFan]

It's important to remember that just because the measured angle at the 3dB cutoff is provided, the unit doesn't stop working outside that angle. It's like a flashlight beam where most of the light is in the beam, but there is still some off to the sides.

The 3dB parameter on the sonar is a combination of the transducer output sound power and the sensitivity of the transducer in detecting echoes.

[Fishton]

It's important to remember that just because the measured angle at the 3dB cutoff is provided, the unit doesn't stop working outside that angle. It's like a flashlight beam where most of the light is in the beam, but there is still some off to the sides.

The 3dB parameter on the sonar is a combination of the transducer output sound power and the sensitivity of the transducer in detecting echoes.

100% CatFan, see this thread ... http://www.ultimatebass.com/bass-fishing-forum/index.php?topic=80058.0