Underwater Noise Measurement Intercalibration Practices Experience 
Open-Sea Pontoon Facility 
To extend the frequency range and demonstrate calibration under more realistic field 
conditions, additional measurements were conducted at FOI’s open-sea pontoon 
facility in the Stockholm archipelago. With a water depth of 10 m, this site allowed 
free-field propagation down to 800 Hz and extended the usable range up to 20 kHz. 
The projector and the recorder were suspended at a fixed depth of 5 m beneath the 
pontoon, separated by 6-8 m. The acoustic pressure at the recorder positions was 
calculated from the projector’s known TVR and its applied drive voltage. 
Dimensioning and Facility Constrain(. 
The usable frequency range of a calibration setup is constrained by geometry and signal 
design. Levin (1973) described these constraints in terms of pulse duration 7, geometry 
of the tank (projector hydrophone distance d, depth A, and length L), sound speed c, and 
projector radius a. 
For the FOI indoor tank (d = 2 m, h = 4m, L= 8m, c= 1430 m/s and a = 0.10 m), 
these constraints imply a theoretical working area extending to low frequencies. How- 
ever, pulses of - = 1.2 ms contain too few cycles below about 2 kHz to provide stable 
sensitivity estimates. This Justifies restricting the usable tank range to a minimum of 
2 kHz during the workshop (Fig. 1). 
Ta 
zer 
E 
al 
rOl tank 
x x 10° 
an 
m 
CM 
3 
= ld 
al 
FOI pontoon 
m 
A 
ha 
Working area 
— direct path 
'— reflected from end wall‘ 
— surface reflection 
— =- pulse duration 
— = far field limit. 
On: m 
0 05 
1.5 2 2.5 3 35 
Distance d [m] 
> 
DD 
3 4 5 6 7 8 
Distance d [m] 
Fig. 1 Dimensioning constraints for free-field hydrophone calibration following Levin (1973). 
Curves indicate limits by direct path (blue), end-wall reflections (orange), surface-reflections 
(green), pulse duration (gray dashed), and the far field condition (magenta dashed). The shaded 
areas indicate the operational “working region” where direct signals can be separated from 
‚eflections and far-field conditions are fulfilled. Left: FOI tank (length 8 m, depth 4 m, and distance 
2 m) allowing a pulse duration of 1.2-1.8 ms. Right: FOI pontoon deployment in 5 m water depth 
and 6.4 m source-receiver spacing allowing a pulse duration of 2-3 ms