Noise; Receiver Noise Versus Ambient Noise - Simrad EY500 Instruction Manual

It should be observed that both absolute accuracy and, more important,
consistency between TS and echo integration quantities are improved by
including these corrections.
The deep water performance of a sounder is determined by the system noise
level; the sum of receiver noise, local noise and ambient noise.
Receiver noise includes thermic noise from the receiver itself and pickup noise
from the digital circuitry of the sounder. The EY 500 utilizes a low noise
receiver input stage, and the digital noise pickup has been reduced to an
insignificant level by proper internal screening. Local noise includes propeller
noise, engine noise, flow noise and other locally identifiable acoustic noise
sources. Basically, this noise is related to vessel design and transducer instal-
lation and can be reduced significantly by taking the necessary precautions.
Ambient noise is the noise of the sea itself. It is that part of the total noise
background of the sea which is not due to some locally identifiable source. The
ambient noise level is subject to wide variations. Heavy shipping and strong
wind increase the noise level, and in shallow water the noise level is typically
higher than in deep water.
Noise can in most practical cases be considered "white" with a continuous power
Figure 6 Receiver noise versus ambient noise.
spectral density with respect to frequency. A quantitative evaluation of noise
requires that the various contributions are all referred to a common point
within the receiving system, and for most purposes it is convenient to use the
transducer terminals as the common point. Referred to this point the total
system noise is simply the sum of all individual noise power contributions.
P3403E/A

9 NOISE

Local noise
70
60
50
40
Receiver noise
30
20
10
1 10
Frequency (kHz)
Theory of operation
Ambient noise
100
(CD454)
17