Service manual

Simrad EK 500 / EY 500
P2260/E 27
When the trial has been completed, the corresponding noise level, NL dB re 1
FPa can be calculated for both the recording procedures. The results from the
two procedures might differ somewhat because of the different ways they are
measured. The S
A
method gives the average echo level from a volume of
water during a specified time interval. The P
N
method is from one individual
power sample in each ping.
The equations for calculating from recorded data to NL is given below.
From S
A
: (based on the sonar equation)
NL = S
i
+ 10 " log (P
TX
" S
A
" T / Z " L) - 2TL + 10 log Q - 75
NL = Noise level dB re 1 FPa
S
i
= Transducer transmitting response dB re 1 FPa per A
P
TX
= Transmitter power W
Z = Transducer impedance ohm
2TL = Two-way transmission loss dB
L = Layer thickness m
10 log Q = Equivalent two way beam angle dB
I = Transmitter pulse length msec
S
i
, Z , and 10 log Q is from the data sheet for the specific transducer
For split beam transducers Z is for all four quadrants in parallel
P
TX
and I is from the sounder specifications 2TL and L from the table on
page 26.
From P
N
:
NL= P
N
- 20 log 8 - G + 192.8
8 = wavelength = c/f
c = speed of sound = 1500 m per sec.
f = frequency Hz
G = transducer gain dB
Read G in the Transceiver Menu/S
V
Transducer Gain
This noise level NL is comparable with the previous measurements on the
EK 400 when the EK 400 calibration procedure was followed.
The theory for the derivation of the above formula for NL from P
N
is given
here, now using linear quantities (not dB):
P
N
is received noise power W
I
N
is plane wave sound intensity
in front of the transducer W/m
2
A is the effective receiving area
of the transducer m
2
P
N
= I
N
.
A
G is the transducer gain = 4BA/8
2