Simrad EM Technical Note

EM Technical Note

Date: September 20, 2005
Author: Erik Hammerstad

Subject: Sound Levels from Kongsberg Multibeams

The power output level of an echo sounder is normally specified by giving
its source level in dB relative to 1 μPa at a distance of 1 m from the
transmit transducer. However this is really a measure of the pressure
level of the output sound wave and is only directly applicable in the
farfield. The intensity (power per unit area) of a sound wave can be found
from the pressure level by the relation I = p2/c where  is the water
density and c the speed of sound. The quantity c is the acoustic
impedance of water and for sea water is nominally taken to be 1.5x106
kg/m2s. Thus a pressure level in sea water of 1 μPa is nominally equal to
an energy intensity of 0.667x10
example 210 dB corresponds to an intensity of 667 W/m2.

In the farfield the pressure level of a sound wave will fall off with the
square of the distance, this because of spherical spreading of the wave,
and the wave will be further attenuated due to absorption loss. In the
nearfield the pressure level will be nominally constant as there is no
spreading. If the transmit transducer generating the sound wave is
rectangular, there will be a transition region in which the pressure will
fall of proportionally to the distance due to cylindrical spreading in the
direction parallel to the shortest side of the transducer. It may be noted
that in the nearfield the pressure level will have large variations, with
peaks up to about twice the nominal level and also deep nulls, this effect
will however be ignored in this note.

The source level is given by SL = 170.8 + 10lgPAc + DI. PAc is the acoustic
power which is typically half the electric power applied to the transmit
transducer. DI is the transducer’s directivity index which for a rectangular
flat transducer can be approximated by DI = 46.2 – 10lg


are the transmit beamwidths in degrees along and across respectively.

y

The relation between beamwidth and transducer array length, L, depends
upon the applied shading and the number of elements in the array,
typically it would be  = 65/L where  is the wavelength. The nearfield
limit is conventionally given by R = L2/.

To derive the pressure levels in the nearfield from the source level of an
echo sounder one must first calculate the pressure level at the largest
farfield limit assuming spherical spreading from the 1 meter reference
level used in defining the source level. From the largest to the smallest
nearfield limit the pressure level will increase linearly with distance, and

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W/m2, and a pressure level of for

where x and

xy

1

from the smallest nearfield limit in to the transducer level the pressure
level will be constant.

If the source level of the echo sounder is not known, but both beamwidths
or transducer array lengths or even just area are, the maximum possible
pressure level may still to a good degree be estimated. Then one has to
rely upon the fact that there is a maximum acoustic power intensity that
can be applied to a transducer in the order of 2-5 W/cm2 to avoid cavitation
(lowest number is typical at say 12 kHz, highest at say 100 kHz). With
shading being applied in one direction the power will be reduced to about
50-60%, and for both directions to about 30%.

The calculations outlined above are for the on-axis direction (usually
straight down). Off-axis the pressure level will fall rapidly for a narrow
beam (alongtrack for a multibeam echo sounder), the level will be 20 dB
down at a little more than twice the beamwidth. Acrosstrack, the pressure
level will typically be 20 dB down for angles of more than 75-80° of the
vertical for flat arrays. At for example 45° the closest nearfield distance
will be half that of on-axis, leading to a 3 dB reduction in pressure level for
distances larger than the nearfield distance on-axis. At 60° the nearfield
distance will be reduced by another factor of two, and taking into account
the usual level reduction at large beam angles also, the pressure level
would typically be down about 8 dB compared to on-axis. At 70° the level
will be about 16 dB down due a further halving of the nearfield distance
and the beam pattern drop-off. For multibeams which use sectorized
transmission such as most current Kongsberg systems, beam defocusing is
applied in the central sector(s) in shallow waters which implies that the
nearfield will be shortened and the drop-off in pressure level starts earlier.
For curved transducers the nearfield limit and the pressure level will stay
fairly constant across the whole angular coverage angle.

Sonars may be transmitting horizontally and with a sound speed profile
where the sound speed lessens toward the surface the spreading will
cylindrical even in the farfield due to ducting causing a sound channel at
the surface. For multibeam echo sounders this is usually not the case,
except for tilted systems such as with the dual head EM 3002.

The following table shows the relevant parameters for the currently
available Kongsberg multibeam echo sounders. For each model the
alongtrack (transmit) beamwidth, the source level in dB re 1μPa at 1
meter, calculated nearfield distances and pressure levels in dB re 1 μPa at
the nearfield distances are provided. The effect of absorption loss is not
included in this table.

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