The 5th topic
Can divers escape from fishfinder?

This time we'll write about fishfinder. Hope you will enjoy it.

Fishfinder is an unit to search fish with using ultrasonic wave. In these days compact ones are sold and especially essential for boat fishing.

In ultrasonic wave from fishfinder, there are two types, 50KHz and 200KHz. The reason for this is that there is a difference in searching ability. The lower frequency fishfinder beams, the larger area it search, but we cannot see the detail. On the other hand, high frequency enables us to see narrow area, but we can see the location of the fish and it's direction.
So usually first we search fish briefly with 50 KHz, and after we find them, we find the place with 200KHz.

Both fishfinder and Logosease use ultrasonic wave. Although fishfinder's ultrasonic wave does not correspond to that of Logosease, 32KHz, is there any influence?
The power of fishfinder is much stronger than Logosease. When fishfinder's ultrasonic wave is 200KHz, there might be no influence because frequency is much different, but when it is 50KHz there might be some influence.

Then we tested two units.

One is compact one made in oversea and costs around 100 doller. The other is made in Japan and costs more than one thousand doller. We put the antenna of the fishfinder in a pool and heard with Logosease.

The result was that we heard sounds "Tick Tuck" once in a second. The sound is that fishfinder beams ultrasonic wave to search fish in each second. The sound of 50 KHz is much bigger, but we can hear enough in 200 KHz.

Let's see more detail.

A figure below is that we decomposed 50KHz sound which we recorded with hydrophone into frequency with using spectrum analyzer.


Seeing the figure, certainly point of 50KHz is the highest. However surrounding of the point, we observed other frequencies such as 32 KHz. On the other hand, the sounds below 20 KHz, audible area, is inhibited, and divers cannot hear it directly.

When you hear sounds "Tick Tuck" in each second from Logosease, fishfinder works somewhere on surface.
Guys, please be careful not to be hooked!