1
Original Scientific Article • DOI: 10.2478/rmzmag-2020-0018 Received: March 08, 2021
 Accepted: March 09, 2021
Underwater noise in the Slovenian Sea
Podvodni hrup v slovenskem morju
Andreja Popit*
Institute for Water of the Republic of Slovenia, Ljubljana, Slovenia
*Corresponding author: E-mail: andreja.popit@izvrs.si
Abstract in English
Continuous underwater noise has been monitored in 
the Slovenian sea near the lighthouse foundation at 
Debeli Rtič since February 2015, according to the EU 
Marine Strategy Framework Directive (MSFD). An-
thropogenic noise sources (e.g. seawater densities, 
dredging activities and cleaning of the seafloor) and 
meteorological noise sources (e.g. wind speed and pre-
cipitation) were analysed in relation to the measured 
underwater noise levels using several graphical and 
statistical methods. The results of this study showed 
that average equivalent continuous underwater noise 
levels were, by 11 dB (L
eq,63 Hz
) and 5 dB (L
eq,125 Hz
), high-
er in the intervals when dredging activities took place 
than in the intervals when these activities were absent. 
Variation in underwater noise levels was partly related 
to the variation of the ship densities, which could be 
explained by the relatively small acoustic propagation 
in the shallow seawater. Precipitation level did not in-
dicate any significant association with the variations 
in continuous underwater noise levels, though some 
larger deviations in the wind speed were found to be 
associated with the larger fluctuations in continuous 
underwater noise levels.
Keywords: underwater noise, shallow sea, measuring  
equipment, natural and anthropogenic sound sources
Abstract in Slovene
V slovenskem morju izvajamo kontinuirne 
meritve podvodnega hrupa ob svetilniku pri 
Debelem rtiču od februarja 2015. Meritve 
potekajo v skladu z Okvirno direktivo o morski 
strategiji. Za analizo antropogenih virov hrupa 
(gostota ladij, poglabljanje in čiščenje morskega 
dna) in meteoroloških virov hrupa (hitrost vetra 
in padavine) v povezavi z izmerjenimi ravnmi 
podvodnega hrupa smo uporabili grafične 
in statistične metode. Rezultati te študije so 
pokazali, da so bile povprečne ekvivalentne 
ravni kontinuirnega podvodnega hrupa za 
11  dB (Leq, 63 Hz) in 5  dB (Leq,125 Hz) višje 
v času, ko so potekale dejavnosti poglabljanja, 
kot v času, ko so teh dejavnosti ni bilo. Nihanja 
ravni podvodnega hrupa so bila v manjši 
meri povezana z nihanji gostote ladij, kar 
lahko razložimo z relativno majhno akustično 
propagacijo v plitvem morju. Padavine niso bile 
veliko povezane z nihanji ravni podvodnega 
hrupa, medtem ko so bila nekatera večja nihanja 
hitrosti vetra povezana z večjimi nihanji ravni 
kontinuirnega podvodnega hrupa.
Ključne besede: podvodni hrup, plitvo morje, merilna 
oprema, naravni in antropogeni viri zvoka
Introduction
The background or ambient noise in the seas and 
oceans is composed of natural (i.e. meteorolog-
ical (wind speed, surface waves, precipitation), 
geological (tectonic processes) and biological) 
and anthropogenic (i.e. marine traffic) noise 
sources. It varies with the location and frequency 
of underwater sound. In regions with high 
shipping densities, the frequency band between 
10 Hz and 200 Hz is primarily associated with 
shipping activity, constituting the largest anthro-
pogenic contribution to the underwater ambient 
sound [1–11].
Most of the noise power radiated into the 
water by surface ships comes from propeller 
cavitation [1, 4, 12]. Propeller noise is generated 
through several cavitation noise mechanisms: 

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thermal noise spectral density at 100,000 Hz is 
20–25 dB re 1 m Pa
2
/Hz [7].
Rain can produce a peak in the ambient 
sound pressure spectral density (around 
60 dB re 1 m Pa
2
/Hz) in the vicinity of 15 kHz, 
corresponding to rain rates ranging from 
2 mm/h to 5 mm/h, measured at different 
wind speeds [7, 26].
Underwater ambient noise is generated 
not only by the combination of environmen-
tal sea state and anthropogenic contributions  
(e.g. shipping), but also by significant 
amounts of biological noise from fish, inver-
tebrates and whales. Biological noise may 
generate major background noise in some 
areas. Marine mammals, such as whales and 
dolphins, rely on sound to communicate with 
each other, locate their prey and find their way 
over long distances. All these activities, criti-
cal to their survival, are being interfered with 
by the increasing levels of noise from ships 
[1, 4, 27–33]. The European Commission’s 
Marine Strategy Framework Directive (MSFD) 
2008/56/EC [34] and International Maritime 
Organization( IMO) guidelines for the reduc-
tion of underwater noise from commercial 
shipping [35] have addressed underwater 
noise pollution from shipping, as well as the 
promotion of the use of the appropriate miti-
gation measures.
The EC MSFD 2008/56/EC [34] guidelines 
require the Member States to prepare a Marine 
Management Plan. These requirements were 
incorporated in Slovenian law by passing 
the Water Act [36] and by the Decree on the  
detailed content of the Marine management 
plan [37]. According to this legislation, Slovenia 
started to monitor continuous underwater 
noise near the lighthouse foundation at Debeli 
Rtič since February 2015.
The aim of our study was to analyse con-
tinuous underwater noise measurements from 
2015 until 2018. The measured ambient low-
frequency noise levels were most probably due 
to anthropogenic activities such as marine traf-
fic, dredging activities and cleaning of the sea-
floor, as well as to meteorological factors such 
as precipitation and wind. These levels were 
analysed through the proposed methodology 
and results of this study were discussed in this 
article.
tip vortex cavitation, different types of blade 
cavitation, hub vortex cavitation, pressure puls-
es due to wake inhomogeneity at the propeller 
plane, pressure pulses generated by the rotating 
propeller blades and singing due to resonance 
between blade natural frequencies and trailing 
edge vortices. Some vessels emit strong struc-
tural noise radiation arising from their hydrau-
lic systems, gears, compressors or other noisy  
machinery [4].
An increase in the low-frequency ocean 
ambient noise levels was observed between 
1963 and 2001 on the continental slope of 
Point Sur, California [7, 8, 13], between 1964 
and 2004, westwards of the San Nicolas 
Island, California [14] and between 1978 
and 1986 in the Northeast Pacific Ocean [15]. 
This was related to the shipping vessel traf -
fic. The number of commercial vessels in the 
world’s oceans approximately doubled and 
the gross tonnage quadrupled between 1965 
and 2003, with a corresponding increase in 
horsepower of the vessels. Increases in com -
mercial shipping are believed to account for 
the observed increase in the low-frequency 
ambient noise [14].
More recently, between 2006 and 2016, 
observations made in the Northeast Pacific, 
Equatorial Pacific and in the South Atlantic 
Ocean show a slightly decreasing trend in 
low-frequency ambient noise levels [16, 17]. 
This trend may be attributed to the fact that 
world vessel size and gross tonnage have in -
creased considerably over the recent years, 
while the number of vessels has decreased 
[18–21].
Wind-generated sea-surface agitation gov-
erns much of the ambient noise in the fre -
quency band between 200 Hz and 100,000 Hz. 
Wind-generated noise is largely the conse-
quence of bubbles created in the process 
of wave-breaking. At lower frequencies 
(<500 Hz),  the oscillation of bubble clouds 
themselves are considered to be the source of 
the sound [22, 23] while, at higher frequen-
cies (>500 Hz), the excitation of resonant os-
cillations by individual bubbles generates  the 
sound [7, 24, 25].
At very high frequencies, ~ 100,000 Hz, ther-
mal noise generated by the random motion 
of water molecules begins to dominate. The 

Underwater noise in the Slovenian Sea
3
The mathematical definition of the meas-
ured equivalent continuous sound level (Eq. 1) 
(also called time-average sound level), L
eq
, is 20 
times the logarithm to base 10 of the ratio of the 
root mean square sound pressure (p
rms
) during 
a time interval to the reference sound pressure 
(p
0
, which is 1 m Pa) [40]:
Lt
p
p
p
p
eq
rmsr ms
() =






=






10 20
10
0
2
10
0
logl og (1)
Root mean square of the sound pressure 
level (p
rms
) (Eq. 2) in Pascals (Pa) [41] can be 
represented as:
∫
()
()
=
−








p
tt
pt t d
rms
1
21
2
1
2
1
2
 (2)
where, P
rms
 or the mean square sound pres-
sure is the time integral of squared sound 
pressure over a specified time interval divided 
by the duration of the time interval; and t
1
 and 
t
2
 are the start and stop times of the time in-
terval over which the mean is evaluated. 
The RMS sound pressure is calculated by 
first squaring the values of sound pressure, 
averaging over the specified time interval and 
then taking the square root.
Materials and methods
Underwater noise measuring station and 
measured quantities
A permanent underwater noise measurement 
station was established on the concrete foun-
dation of a masonry lighthouse 300 m off the 
coast at Debeli Rtič, Slovenia in February 2015 
(Figure 1a). The coordinates of the lighthouse 
are Lat.: 45°35′ 28.2″ N, Lon.: 13°41′ 59.1″ 
E. The associated measuring equipment was 
composed of a spherical omnidirectional hydro-
phone (Type  8105, Bruel & Kjaer ) installed at 
a depth of 4 m (Figure 1b) (sea depth at that 
location was 5 m). The hydrophone is con-
nected to a sound analyser of Type 2250 Bruel 
& Kjaer, which includes a sound level meter and 
an octave-based frequency analyser, operat-
ing in the frequency band of 6.3–20 kHz. The 
hydrophone with a cable was installed through 
a metal pipe 1 m away from the lighthouse 
foundation to a depth of approx. 1 m above the 
seabed, as shown in Figure 1b [38]. A sound 
analyser was closed inside the lighthouse in a 
waterproof casing, according to the standard 
National Electrical Manufacturers Association 
(NEMA) IP65 protocols, and maintaining resist-
ance to water jets was ensured. The measuring 
system was connected to the batteries that were 
charged by a solar panel [38, 39].
Figure 1: Location of the permanent underwater noise measurement station near Debeli Rtič in the Slovenian Sea (A) and a 
sketch of the lighthouse at Debeli Rtič on which the measuring equipment is installed showing the hydrophone at a depth of 4 m 
(sea depth at the location is 5 m) (B) [38].
 

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Frequency analysis software enables deriva-
tions of the equivalent continuous sound levels 
in 1/3-octave band with centre frequencies be-
tween 6.3 Hz and 20 kHz, in the resolution of 
10 s. Daily arithmetic mean values were calcu-
lated and recorded on a hard disc of 1 Terabyte 
(TB). The memory capacity of the disc enables 
recordings for 75 days.
Measured data were transferred, displayed 
and analysed using BZ-5503 Measurement 
Partner Suite [40] software. This software can 
be used for data archival, data preview and data 
export, for post-process and export to other 
formats, online data display and remote access 
and operation, as well as for maintenance of the 
sound level meter software.
With BZ-5503 Measurement Partner Suite, 
daily equivalent continuous sound levels (L
eq
 val-
ues) in the 1/3-octave band with centre frequen-
cies between 6.3 Hz and 20 kHz were analysed.
Methodology used for processing 
continuously measured data
The first step in data processing was, in our 
case, done by the sound analyser of Type 2250 
(Bruel & Kjaer ), which calculates equivalent 
continuous sound levels in 1/3-octave bands. 
Then we proceeded with the second step in 
data processing, to calculate the annual average 
of the continuous sound level.
For monitoring and assessing anthropo-
genic continuous low-frequency sound in wa-
ter (D11C2) we used annual average of the 
squared sound pressure in 1/3-octave bands, 
one centred at 63 Hz and the other at 125 Hz, 
both expressed as a level in decibels in units of 
dB re 1 m Pa, according to the requirements of 
the Commission Decision EU/2017/848 [42]. 
The unit of measurement used for the criteria 
D11C2 is the annual average of the continuous 
sound level per unit area; proportion (percent-
age) of extent in square kilometres of the as-
sessment area.
For this purpose we used the arithmetic 
mean (AM) in time T [43] (Eq. 3), which shows  
compatibility with L
eq
 metric:
AM T
NT
pT
NT
() =
()
()
=
()
∑
1
1 n
n
 (3)
where N(T) is the number of snapshots of 
duration T in 1 year (Eq. 4) (assuming that the 
data are continuous, and contain no gaps for an 
entire year):
= NT
T
()
year 1
 (4)
where p
n
(T) is the mean square sound pres-
sure at the n-th snapshot of duration T.
The arithmetic mean is expressed as 
sound pressure level (SPL) (Eq. 5) in dB re 
m Pa [43]:
LT
AM T
p
AM
ref
() =
()
10
10 2
log (5)
where p
ref
 = 1 m Pa.
Annual averages of the continuous sound 
level and standard deviation (STDEV) for 
1/3-octave bands with centre frequencies of 
63 Hz and 125 Hz were calculated using daily 
averages, which were calculated using the 
sound analyser.
The results of the underwater noise meas-
urements from the measuring station at Debeli 
Rtič were analysed and reviewed using the 
BZ-5503 Measurement Partner Suite Software 
[39]. The equivalent unweighted continu-
ous noise levels within 1/3-octave frequency 
bands with centre frequencies of 63 Hz L
eq,63 Hz
 
and 125 Hz L
eq,125 Hz
 (in dB), according to the 
MSFD 2008/56/EC [34], were exported into 
an excel spreadsheet for further analyses. 
The underwater noise data were available 
at half-hour intervals for the following peri-
ods: from 13 February 2015 to 5 May 2015; 
26 September 2015 to 31 December 2015; 18 
August 2016 to 1 November 2016; 6 July 2017 
to 27 August 2017; and 18 August 2018 to 31 
December 2018.
Average hourly values of equivalent con-
tinuous underwater noise levels in 1/3-octave 
bands with centre frequencies of 63 Hz and 
125 Hz for each measuring period were pre-
pared and presented on diagrams.
Asymmetry (A) (Eq. 6) was used to 
test the normality of the distribution of 
underwater noise levels. Asymmetry was 

Underwater noise in the Slovenian Sea
5
used to indicate the direction of data 
asymmetry [44]:
= A
  (6)
where m
2
 and m
3
 are the second and third 
moments around the average. 
The j-th moment is calculated by the Eq. (7), 
represented below [44]:
m
xx
n
j
i
j
i
n
=
−
=
∑
()
_
1
 (7)
When A is 0, the data set is symmetric to its 
mean and the data are distributed symmetri-
cally or normally (Gaussian distribution). 
At A < 0 the data are asymmetric to the left 
and at A > 0 the data are asymmetric to the 
right. If A < −1 or A > 1 the distribution is very 
asymmetric. 
If A is between −1 and −0.5 or between 0.5 
and 1, the distribution is moderately asymmet-
ric. If A falls between −0.5 and 0 or between 
0 and 0.5, the distribution is approximately 
symmetric. 
The statistics were calculated in Excel 
(Microsoft).
Methodology for the analysis of anthropogenic 
sources (ship densities, dredging and cleaning 
activities) of the underwater noise in the 
canals of the Port of Koper
Marine traffic in the sea is monitored with the 
Automatic Information System (AIS). Obtained 
AIS data concerning locations of the ships were 
analysed in the North Adriatic Sea for 2015, 
2016, 2017 and 2018 to prepare hourly data on 
the ship densities in four different areas around 
the underwater noise measuring station at the 
lighthouse at Debeli Rtič, Slovenia. These four 
areas were namely within a radii of 2 nautical 
miles (NM) and 5 NM from the measuring sta-
tion, in the Gulf of Trieste and the Gulf of Venice. 
Data on ship densities were prepared for each 
period during which underwater noise levels 
were recorded.
Average hourly ship densities in all four ar-
eas around the measuring station, for each 
period in which underwater noise levels were 
recorded, were presented graphically in com-
bination with the average hourly continuous 
underwater noise levels in 1/3-octave bands 
with centre frequencies of 63 Hz and 125 Hz. 
Asymmetry (A) was used to test the normality 
of the distribution of ship densities. 
Dredging activities were carried from  
7 September 2015 to 26 October 2015 from 
7:00–21:00 h, while cleaning activities of the 
seafloor in the canals of the Port of Koper 
were carried out from 18 August 2016 to 31 
August 2016, and from 22 September 2016 to 
29 September 2016 from 8:00–16:00 h (Table 
1). Dredging was carried out in the sea with a 
dredger and a trailed harrow for levelling the 
seabed, while the cleaning work was carried out 
from the mainland with the help of the Link-Belt 
LS-108B excavator crane.
On the diagram concerning ship density in 
the four areas around the measuring station 
in combination with the average hourly con-
tinuous underwater noise levels in 1/3-octave 
bands with centre frequencies of 63 Hz and 
125 Hz, were drawn red arrows indicating 
dredging and cleaning activities.
Average equivalent continuous underwater 
noise levels during dredging and cleaning activ-
ities were analysed. Separately, average equiv-
alent continuous levels of underwater noise 
were analysed at the time when there were no 
anthropogenic activities (Table 1). These analy-
ses were performed to check whether the aver-
age values (AVE) of equivalent continuous un-
derwater noise levels, in 1/3-octave bands with 
centre frequencies of 63 Hz and 125 Hz at the 
time of dredging and cleaning activities, were 
higher than at the time when these activities 
were not being executed.
Methodology for the analysis of meteorological 
sources of the underwater noise
In this section, wind speed and precipitation 
were analysed as meteorological sources of 
underwater noise. Half-hourly data on wind 
speeds (m/s) from the Piran buoy (Lon.: 
13.5454°, Lat.: 45.5481°, altitude: 0 m) and 
half-hourly data on precipitation (mm) from 
the meteorological station in the Port of Koper 
(Lon.: 13.7448°, Lat.: 45.5645°, Altitude: 2 m), 
in the periods in which underwater noise 

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Table 1: Periods with and without the anthropogenic activity
Type of anthropogenic 
activity
Periods with the  
anthropogenic activity
Periods without any  
anthropogenic activity
Dredging
26.09.2015–26.10.2015 
(7:00–21:00 h)
26.09.2015–26.10.2015 (22:00–6:00 h)
Cleaning of the seafloor
18.08.2016–31.08.2016
&
22.09.2016–29.09.2016 
(8:00–16:00 h)
18.08.2016–31.08.2016
&
22.09.2016–29.09.2016 (17:00–7:00 h)
levels were recorded, were obtained from 
the Environmental Agency of the Republic of 
Slovenia (ARSO).
Average hourly wind speeds and precipi-
tation levels in the individual periods were 
calculated and presented graphically in com-
bination with average hourly continuous un-
derwater noise levels in 1/3-octave bands 
with central frequencies of 63 Hz and 125 Hz. 
Furthermore, asymmetry (A) was used to test 
the normality of the distribution of the aver-
age hourly wind speeds and average hourly 
precipitation data.
Results 
The average continuous underwater noise lev-
els in the 1/3-octave bands with centre fre-
quencies of 63 Hz (L
eq,63 Hz
) and 125 Hz (L
eq,63 Hz
) 
in dB re 1 m Pa, average ship densities in the 
four areas around the measuring station (r L,2 NM
, 
r L,5 NM
, r L,
 
Trieste.
 and r L,
 
Venice
), average wind speeds 
(v
v
) in m/s and average precipitation (h
p
) in 
mm in each measurement period are presented 
in Table 2.
The average L
eq,63 Hz
 and L
eq,125 Hz
 levels meas-
ured in the Slovenian Sea during the period 
2015–2018 (Table 2) were 82.8–101.1 dB re 
1 m Pa and 83.9–98.1 dB re 1 m Pa, respectively. 
The ship densities were 2–252. The average 
wind speed was 1.8–4.6 m/s and the average 
precipitation was 0.02–0.07 mm.
The L
eq,125 Hz
 data were distributed close to 
the normal (Gaussian) distribution in all meas-
uring periods (they were slightly asymmetric 
to the right or left), as the value of A was close 
to 0 (Table 3). The L
eq,63 Hz
 data were distrib-
uted moderately asymmetrically to the right 
(A = 0.5–1.1), except for the period from 18 
August 2016 to 1 November 2016, when they 
were distributed approximately symmetrically 
(A = −0.4) (Table 3).
The r L,2 NM
 data were distributed moderately 
asymmetrically to the right in all measuring pe-
riods and the r L,5 NM
 data and r L, Trieste
 were dis-
tributed very asymmetrically to the left in the 
first two periods, very asymmetrically to the 
right in the third and fifth periods and approxi-
mately symmetrical in the fourth period. The 
r L, Venice
 data were moderately asymmetrically 
distributed to the left in the first two periods, 
and moderately asymmetrically to the right to 
approximately symmetrically in the other peri-
ods (Table 3).
The v
v
 data were distributed very asym-
metrically to the right in all measuring periods, 
except in the period from 18 August 2016 to 1 
November 2016, in which they were distribut-
ed moderately asymmetrically to the right. The 
h
p
 data were distributed very asymmetrically to 
the right in all measuring periods (Table 3).
The relationship of the measured ambient 
low-frequency noise levels with the anthropo-
genic activities (ship densities, dredging and 
cleaning activities) is shown in the diagrams 
(Figures 2–6) of the average hourly ship den-
sities in the four areas around the underwater 
noise measuring station (r L,2 NM
, r L,5 NM
, r L, Trieste.
 
and r L, Venice
) in combination with the average 
hourly continuous underwater noise levels in 
1/3-octave bands with centre frequencies of 
63 Hz and 125 Hz. Blue curve presents L
eq,63 Hz
, 
black curve presents L
eq,125 Hz,
 violet curve pre-
sents ship density 2 NM from the measuring 
station, yellow curve presents ship density 
5 NM from the measuring station, green curve 
presents ship density in the Gulf of Trieste and 

Underwater noise in the Slovenian Sea
7
Table 3: The results of asymmetry (A) calculations of L
eq,63 Hz 
, L
eq,125 Hz 
, r L,2 NM 
, r L,5 NM 
, r L,
 
Trieste 
, r L,
 
Venice,
 dredging, cleaning activity,  
v
v
 and h
p
 in different measuring periods
Asymmetry From 
13.02.2015 to 
05.05.2015
From 
26.09.2015 to 
31.12.2015
From 
18.08.2016 to 
01.11.2016
From 
06.07.2017 to 
07.08.2017
From 
18.08.2018 to 
31.12.2018
A of L
eq,63 Hz
1.0 0.5 −0.4 1.1 0.6
A of L
eq,125 Hz
0.2 0.1 0.0 0.1 −0.1
A of r L,2 NM
0.8 0.7 0.6 0.6 1.0
A of r L,5 NM
−1.7 −3.0 1.1 0.4 2.0
A of r L,
 
Trieste
−1.6 −2.8 1.1 0.2 1.4
A of r L,
 
Venice
−0.7 −0.6 0.6 0.8 0.5
A of v
v
1.1 1.2 0.8 1.1 2.4
A of h
p
20.5 16.2 14.8 22.4 10.9
NM, nautical miles.
Table 2: The results of AVE and STDEV calculations of L
eq,63 Hz 
, L
eq,125 Hz 
, r L,2 NM 
, r L,5 NM 
, r L,
 
Trieste 
, r L,
 
Venice,
 dredging, cleaning activity,  
v
v
 and h
p
 in different measuring periods
Average of 
AVE and 
STDEV
From 
13.02.2015 to 
05.05.2015
From 
26.09.2015 to 
31.12.2015
From 
18.08.2016 to 
01.11.2016
From 
06.07.2017 to 
27.08.2017
From 
18.08.2018 to 
31.12.2018
AVE & STDEV 
of L
eq,63 Hz
83.0 ± 15.1 82.8 ± 10.8 101.1 ± 6.9 86.7 ± 7.7 88.6 ± 5.7
AVE & STDEV 
of L
eq,125 Hz
89.0 ± 13.1 83.9 ± 2.5 97.5 ± 6.8 85.2 ± 3.3
98.1 ± 3.9
AVE & STDEV 
of r L,2 NM
2 ± 2 3 ± 2 5 ± 3 5 ± 3 5 ± 3
AVE & STDEV 
of r L,5 NM
24 ± 9 37 ± 7 45 ± 6 52 ± 6 52 ± 8
AVE & STDEV 
of r L,
 
Trieste
35 ± 13 50 ± 10 58 ± 8 71 ± 9 70 ± 11
AVE & STDEV 
of r L,
 
Venice
117 ± 52 186 ± 51 252 ± 53 246 ± 48 247 ± 56
AVE & STDEV 
of v
v
4.6 ± 3.3 4.5 ± 3.8 4.6 ± 2.7 1.8 ± 1.2 2.0 ± 1.6
AVE & STDEV 
of h
p
0.02 ± 0.13 0.04 ± 0.35 0.07 ± 0.61 0.02 ± 0.31 0.05 ± 0.32
AVE, average value; NM, nautical miles; STDEV, standard deviation.
brown curve presents ship density in the Gulf 
of Venice (Figures 2–6).
Many gaps in the ship densities in 2015 
(evident in Figures 2 and 3) and one major gap 
(evident in October 2018 in Figure 6) were due 
to the reason that AIS System did not operate 
during these periods.
The red arrow on the diagram of aver-
age hourly ship densities (Figure 3) indicates 
dredging activities, which took place from  
26 September 2015 to 26 October 2015. The 
red arrows on the diagram of average hourly 
ship densities (Figure 4) show cleaning activi-
ties at the seafloor in the canals of the Port of 

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0
50
100
150
200
250
300
350
400
450
0
20
40
60
80
100
120
140
0 0 : 1 0 5 1 0 2 . 2 0 . 3 1
0 0 : 9 1 5 1 0 2 . 2 0 . 4 1
16.02.2015 13:00
18.02.2015 07:00
20.02.2015 01:00
21.02.2015 19:00
23.02.2015 13:00
25.02.2015 07:00
27.02.2015 01:00
28.02.2015 19:00
2.03.2015 13:00
4.03.2015 07:00
6.03.2015 01:00
7.03.2015 19:00
9.03.2015 13:00
11.03.2015 07:00
13.03.2015 01:00
14.03.2015 19:00
16.03.2015 13:00
18.03.2015 07:00
20.03.2015 01:00
21.03.2015 19:00
23.03.2015 13:00
25.03.2015 07:00
27.03.2015 01:00
28.03.2015 19:00
30.03.2015 13:00
1.04.2015 07:00
3.04.2015 01:00
4.04.2015 19:00
6.04.2015 13:00
8.04.2015 07:00
10.04.2015 01:00
11.04.2015 19:00
13.04.2015 13:00
15.04.2015 07:00
17.04.2015 01:00
18.04.2015 19:00
20.04.2015 13:00
22.04.2015 07:00
24.04.2015 01:00
25.04.2015 19:00
27.04.2015 13:00
29.04.2015 07:00
1.05.2015 01:00
2.05.2015 19:00
4.05.2015 13:00
Leq,125Hz (dB)
Leq,63Hz (dB)
Ship densities 2 NM from the measuring station
Ship densities 5 NM from the measuring station
Ship densities in the Gulf of Trieste
Ship densities in the Gulf of Venice
L
eq
(dB)
Date and time 
ρ
L
Figure 2: Diagram of the average hourly ship densities in the areas of 2 NM and 5 NM from the measuring station in the Gulf of 
Trieste and the Gulf of Venice in combination with the average hourly continuous underwater noise levels in 1/3-octave bands 
with centre frequencies of 63 Hz and 125 Hz (Leq) in the period from 12 February 2015 to 5 May 2015. NM, nautical miles.
0
50
100
150
200
250
300
350
400
450
0
20
40
60
80
100
120
140
0 0 : 1 0 5 1 0 2 . 9 0 . 6 2
0 0 : 3 0 5 1 0 2 . 9 0 . 8 2
30.09.2015 05:00
2.10.2015 07:00
4.10.2015 09:00
6.10.2015 11:00
8.10.2015 13:00
10.10.2015 15:00
12.10.2015 17:00
14.10.2015 19:00
16.10.2015 21:00
18.10.2015 23:00
21.10.2015 01:00
23.10.2015 03:00
25.10.2015 05:00
27.10.2015 07:00
29.10.2015 09:00
31.10.2015 11:00
2.11.2015 13:00
4.11.2015 15:00
6.11.2015 17:00
8.11.2015 19:00
10.11.2015 21:00
12.11.2015 23:00
15.11.2015 01:00
17.11.2015 03:00
19.11.2015 05:00
21.11.2015 07:00
23.11.2015 09:00
25.11.2015 11:00
27.11.2015 13:00
29.11.2015 15:00
1.12.2015 17:00
3.12.2015 19:00
5.12.2015 21:00
7.12.2015 23:00
10.12.2015 01:00
12.12.2015 03:00
14.12.2015 05:00
16.12.2015 07:00
18.12.2015 09:00
20.12.2015 11:00
22.12.2015 13:00
24.12.2015 15:00
26.12.2015 17:00
28.12.2015 19:00
30.12.2015 21:00
Leq,125Hz (dB)
Leq,63Hz (dB)
Ship densities 2 NM from the measuring station
Ship densities 5 NM from the measuring station
Ship densities in the Gulf of Trieste
Ship densities in the Gulf of Venice
L
eq
(dB)
Date and time 
ρ
L
Dredging activities in the Port of Koper 
Figure 3: Diagram of the average hourly ship densities in the areas of 2 NM and 5 NM from the measuring station in the Gulf of 
Trieste and the Gulf of Venice in combination with the average hourly continuous underwater noise levels in 1/3-octave bands 
with centre frequencies of 63 Hz and 125 Hz (L
eq
) during the period from 26 September 2015 to 31 December 2015. The red arrow 
indicates the period from 26  September 2015 to 26 October 2015, when dredging activities were done in the Port of Koper. NM, 
nautical miles.
Koper during the following periods: 18–31 
August 2016  and 22–29 September 2016.
The results presented on these diagrams 
(Figures 2–6) are interpreted and discussed in 
the subsection Discussion.
The average equivalent continuous under-
water noise levels in 1/3-octave bands with 
centre frequencies of 63 Hz and 125 Hz were 
higher in the intervals by ≈ 11 dB (L
eq,63 Hz
) and 
5 dB (L
eq,125 Hz
) when dredging activities took 
place than in the intervals when these activities 
were absent (Table 4). In addition, the average 
equivalent continuous underwater noise levels, 
were for 7 dB (L
eq,63 Hz
) and 7 dB (L
eq,125 Hz
), lower 
in the intervals, when cleaning activities took 
place than in the intervals when these activities 
were absent (Table 4).
The relationship of the measured ambient 
low-frequency noise levels with the meteorologi-
cal factors is depicted in the diagrams (Figures 
7–11) of the average hourly wind speeds and 
average hourly precipitation in each measuring 
period, in combination with the average hourly 
continuous underwater noise levels in 1/3-oc-
tave bands with centre frequencies of 63 Hz and 
125 Hz. Blue curve presents L
eq,63 Hz
, black curve 

Underwater noise in the Slovenian Sea
9
0
100
200
300
400
500
600
0
30
60
90
120
150
0 0 : 1 0 6 1 0 2 . 8 0 . 8 1
0 0 : 6 1 6 1 0 2 . 8 0 . 9 1
21.08.2016 07:00
22.08.2016 22:00
24.08.2016 13:00
26.08.2016 04:00
27.08.2016 19:00
29.08.2016 10:00
31.08.2016 01:00
1.09.2016 16:00
3.09.2016 07:00
4.09.2016 22:00
6.09.2016 13:00
8.09.2016 04:00
9.09.2016 19:00
11.09.2016 10:00
13.09.2016 01:00
14.09.2016 16:00
16.09.2016 07:00
17.09.2016 22:00
19.09.2016 13:00
21.09.2016 04:00
22.09.2016 19:00
24.09.2016 10:00
26.09.2016 01:00
27.09.2016 16:00
29.09.2016 07:00
30.09.2016 22:00
2.10.2016 13:00
4.10.2016 04:00
5.10.2016 19:00
7.10.2016 10:00
9.10.2016 01:00
10.10.2016 16:00
12.10.2016 07:00
13.10.2016 22:00
15.10.2016 13:00
17.10.2016 04:00
18.10.2016 19:00
20.10.2016 10:00
22.10.2016 01:00
23.10.2016 16:00
25.10.2016 07:00
26.10.2016 22:00
28.10.2016 13:00
30.10.2016 04:00
31.10.2016 19:00
Leq,125Hz (dB)
Leq,63Hz (dB)
Ship densities 2 NM from the measuring station
Ship densities 5 NM from the measuring station
Ship densities in the Gulf of Trieste
L
eq
(dB)
Date and time 
ρ
L
Cleaning of channels in the Port of Koper
Figure 4. Diagram of the average hourly ship densities in the areas of 2 NM and 5 NM from the measuring station in the Gulf of 
Trieste and the Gulf of Venice in combination with the average hourly continuous underwater noise levels in 1/3-octave bands 
with centre frequencies of 63 Hz and 125 Hz (L
eq
) during the period from 18 August 2016 to 1 November 2016. The red arrows 
indicate the periods 18–31 August 2016 and 22–29 September 2016, during which cleaning of the channels in the Port of Koper 
was performed. NM, nautical miles.
0
50
100
150
200
250
300
350
400
450
500
0
20
40
60
80
100
120
0 0 : 1 0 7 1 0 2 . 7 0 . 6
0 0 : 4 0 7 1 0 2 . 7 0 . 7
8.07.2017 07:00
9.07.2017 10:00
10.07.2017 13:00
11.07.2017 16:00
12.07.2017 19:00
13.07.2017 22:00
15.07.2017 01:00
16.07.2017 04:00
17.07.2017 07:00
18.07.2017 10:00
19.07.2017 13:00
20.07.2017 16:00
21.07.2017 19:00
22.07.2017 22:00
24.07.2017 17:00
25.07.2017 20:00
26.07.2017 23:00
28.07.2017 02:00
29.07.2017 05:00
30.07.2017 08:00
31.07.2017 11:00
1.08.2017 14:00
2.08.2017 17:00
3.08.2017 20:00
4.08.2017 23:00
6.08.2017 02:00
7.08.2017 05:00
8.08.2017 08:00
9.08.2017 11:00
10.08.2017 14:00
11.08.2017 17:00
12.08.2017 20:00
13.08.2017 23:00
15.08.2017 02:00
16.08.2017 05:00
17.08.2017 08:00
18.08.2017 11:00
19.08.2017 14:00
20.08.2017 17:00
21.08.2017 20:00
22.08.2017 23:00
24.08.2017 02:00
25.08.2017 05:00
26.08.2017 08:00
27.08.2017 11:00
Leq,125Hz (dB)
Leq,63Hz (dB)
Ship densities 2 NM from the measuring station
Ship densities 5 NM from the measuring station
Ship densities in the Gulf of Trieste
Ship densities in the Gulf of Venice
L
eq
(dB)
Date and time
ρ
L
Figure 5. Diagram of the average hourly ship densities in the areas of 2 NM and 5 NM from the measuring station in the Gulf of 
Trieste and the Gulf of Venice in combination with the average hourly continuous underwater noise levels in 1/3-octave bands 
with centre frequencies of 63 Hz and 125 Hz (L
eq
) during the period from 6 July 2017 to 27 August 2017. NM, nautical miles.
presents L
eq,125 Hz,
 brown curve presents wind 
speed and green columns on the x-axis present 
precipitation. The results presented in these dia-
grams (Figures 7–11) are discussed in the sub-
section Discussion.
Discussion
In this section, the relationship between the 
pressures in the Slovenian Sea that arise from 
anthropogenic activities (ship densities, dredg-
ing activities and cleaning of the seafloor) and 
the equivalent continuous levels of underwater 
noise in 1/3-octave bands with centre frequen-
cies of 63 Hz (L
eq,63 Hz
) and 125 Hz (L
eq,125 Hz
) (dB) 
is discussed. Furthermore, the relationship 
between the continuous underwater noise lev-
els and the meteorological parameters (wind 
speed (m/s) and precipitation (mm)) is also 
commented upon.
The average continuous underwater noise 
levels (L
eq,63 Hz
 and L
eq,125 Hz
) measured in the 
Slovenian Sea (Table 2) were similar to those 
reported in the literature, which were found 
to be associated with the shipping noise  
[1–21]. Large variations of the L
eq,63 Hz
 levels 
were highly related to variations of the L
eq,125 Hz
 

A. Popit
10
RMZ – M&G | 2020 | Vol. 67(3) | pp. 1–15
-50
50
150
250
350
450
550
650
0
20
40
60
80
100
120
140
0 0 : 1 0 8 1 0 2 . 8 0 . 8 1
0 0 : 3 2 8 1 0 2 . 8 0 . 0 2
23.08.2018 21:00
26.08.2018 19:00
29.08.2018 17:00
1.09.2018 15:00
4.09.2018 13:00
7.09.2018 11:00
10.09.2018 09:00
13.09.2018 07:00
16.09.2018 05:00
19.09.2018 03:00
22.09.2018 01:00
24.09.2018 23:00
27.09.2018 21:00
30.09.2018 19:00
3.10.2018 17:00
6.10.2018 15:00
9.10.2018 13:00
12.10.2018 11:00
15.10.2018 09:00
18.10.2018 07:00
21.10.2018 05:00
24.10.2018 03:00
27.10.2018 15:00
30.10.2018 13:00
2.11.2018 11:00
5.11.2018 09:00
8.11.2018 07:00
11.11.2018 05:00
14.11.2018 03:00
17.11.2018 01:00
19.11.2018 23:00
22.11.2018 21:00
25.11.2018 19:00
28.11.2018 17:00
1.12.2018 15:00
4.12.2018 13:00
7.12.2018 11:00
10.12.2018 09:00
13.12.2018 07:00
16.12.2018 05:00
19.12.2018 03:00
22.12.2018 01:00
24.12.2018 23:00
27.12.2018 21:00
30.12.2018 19:00
Leq,125Hz (dB)
Leq,63Hz (dB)
Ship densities 2 NM from the measuring station
Ship densities 5 NM from the measuring station
Ship densities in the Gulf of Trieste
Ship densities in the Gulf of Venice
L
eq
(dB)
Date and time
ρ
L
Figure 6: Diagram of the average hourly ship densities in the areas of 2 NM and 5 NM from the measuring station in the Gulf of 
Trieste and the Gulf of Venice in combination with the average hourly continuous underwater noise levels in 1/3-octave bands 
with centre frequencies of 63 Hz and 125 Hz (L
eq
) in the period from 18 August 2018 to 31 December 2018. NM, nautical miles.
levels (Figures 2–6). Average hourly continuous 
underwater noise levels (L
eq,63 Hz
 and L
eq,125 Hz
) 
presented in Figures 2–6 show that the levels 
of L
eq,63 Hz
 were, for most of the measured days, 
lower than L
eq,125 Hz
, which is in accordance with 
the data presented in Table 2. This can be ex-
plained by the fact that the propagation of un-
derwater noise in the shallow seawater at 63 Hz 
is lower than that at 125 Hz.
The results of this study showed that average 
equivalent continuous underwater noise levels 
were higher in the intervals by 11 dB (Leq,63 Hz) 
and 5 dB (Leq,125 Hz) when dredging activities 
took place, than in the intervals when these ac -
tivities were absent. Furthermore, the average 
equivalent continuous underwater noise levels 
were found to be lower in the intervals when 
cleaning activities took place, than when such 
activities were absent (Table 4). This finding in-
dicated that cleaning activities were not related 
to the underwater noise levels. This might be 
explained by the fact that cleaning of the sea -
floor was performed with an excavator from the 
mainland.
The lowest average ship densities were 
measured within the areas of the radii of 
Table 4: The results of AVE and STDEV calculations of L
eq,63 Hz
 and L
eq,125 Hz,
 in the periods with and without the anthropogenic 
activity
Type of the 
anthropogenic 
activity
AVE & STDEV of L
eq,63 Hz
 during the 
period of anthropogenic activity
AVE & STDEV of L
eq,63 Hz 
during the  
period without anthropogenic activity
Dredging 26.9.2015–26.10.2015 (7:00–21:00 h)
L
eq,63 Hz
 = 89.4 ± 16.9 dB re 1 µ Pa
L
eq,125 Hz
 = 88.6 ± 10.1 dB re 1 µ Pa
26.9.2015–26.10.2015 (22:00–6:00 hr)
L
eq,63 Hz
 = 79.5 ± 13.5 dB re 1 µ Pa
L
eq,125 Hz
 = 83.3 ± 7.2 dB re 1 µ Pa
27.10.2015–31.12.2015 (00:00–24:00 hr)
L
eq,63 Hz
 = 78.5 ± 12.9 dB re 1 µ Pa
L
eq,125 Hz
 = 83.5 ± 7.0 dB re 1 µ Pa
Cleaning of the 
seafloor
18.8.2016–31.8.2016 (8:00–16:00)
22.9.2016–29.9.2016 (8:00–16:00)
L
eq,63 Hz
 = 94.3 ± 12.3 dB re 1 m Pa
L
eq,125 Hz
 = 90.6 ± 8.6 dB re 1 m Pa
18.8.2016–31.8.2016 (17:00–7:00)
22.9.2016–29.9.2016 (17:00–7:00)
30.9.2016–1.11.2016 (00:00–24:00 hr)
L
eq,63 Hz
 = 101.4 ± 14.7 dB re 1 m Pa
L
eq,125 Hz
 = 97.7 ± 12.5 dB re 1 m Pa
AVE, average value; STDEV, standard deviation.

Underwater noise in the Slovenian Sea
11
0
5
10
15
20
25
30
0
20
40
60
80
100
120
0 0 : 1 0 5 1 0 2 . 2 0 . 3 1
0 0 : 9 1 5 1 0 2 . 2 0 . 4 1
16.02.2015 13:00
18.02.2015 07:00
20.02.2015 01:00
21.02.2015 19:00
23.02.2015 13:00
25.02.2015 07:00
27.02.2015 01:00
28.02.2015 19:00
2.03.2015 13:00
4.03.2015 07:00
6.03.2015 01:00
7.03.2015 19:00
9.03.2015 13:00
11.03.2015 07:00
13.03.2015 01:00
14.03.2015 19:00
16.03.2015 13:00
18.03.2015 07:00
20.03.2015 01:00
21.03.2015 19:00
23.03.2015 13:00
25.03.2015 07:00
27.03.2015 01:00
28.03.2015 19:00
30.03.2015 13:00
1.04.2015 07:00
3.04.2015 01:00
4.04.2015 19:00
6.04.2015 13:00
8.04.2015 07:00
10.04.2015 01:00
11.04.2015 19:00
13.04.2015 13:00
15.04.2015 07:00
17.04.2015 01:00
18.04.2015 19:00
20.04.2015 13:00
22.04.2015 07:00
24.04.2015 01:00
25.04.2015 19:00
27.04.2015 13:00
29.04.2015 07:00
1.05.2015 01:00
2.05.2015 19:00
4.05.2015 13:00
Precipitation (mm)
Leq,125Hz (dB)
Leq,63Hz (dB)
Wind speed (m/s)
L
eq
(dB)
Date and time
v
v
(m/s) and h
p
(mm)
Figure 7: Diagram of the average hourly wind speeds (v
v
) and average hourly precipitation (h
p
) in combination with the average 
hourly continuous underwater noise levels in 1/3-octave bands with centre frequencies of 63 Hz and 125 Hz (L
eq
) during the 
period from 12 February 2015 to 5 May 2015.
0
5
10
15
20
25
30
0
20
40
60
80
100
120
0 0 : 1 0 5 1 0 2 . 9 0 . 6 2
0 0 : 3 0 5 1 0 2 . 9 0 . 8 2
30.09.2015 05:00
2.10.2015 07:00
4.10.2015 09:00
6.10.2015 11:00
8.10.2015 13:00
10.10.2015 15:00
12.10.2015 17:00
14.10.2015 19:00
16.10.2015 21:00
18.10.2015 23:00
21.10.2015 01:00
23.10.2015 03:00
25.10.2015 05:00
27.10.2015 07:00
29.10.2015 09:00
31.10.2015 11:00
2.11.2015 13:00
4.11.2015 15:00
6.11.2015 17:00
8.11.2015 19:00
10.11.2015 21:00
12.11.2015 23:00
15.11.2015 01:00
17.11.2015 03:00
19.11.2015 05:00
21.11.2015 07:00
23.11.2015 09:00
25.11.2015 11:00
27.11.2015 13:00
29.11.2015 15:00
1.12.2015 17:00
3.12.2015 19:00
5.12.2015 21:00
7.12.2015 23:00
10.12.2015 01:00
12.12.2015 03:00
14.12.2015 05:00
16.12.2015 07:00
18.12.2015 09:00
20.12.2015 11:00
22.12.2015 13:00
24.12.2015 15:00
26.12.2015 17:00
28.12.2015 19:00
30.12.2015 21:00
Precipitation (mm)
Leq,125Hz (dB)
Leq,63Hz (dB)
Wind speed (m/s)
L
eq
(dB)
Date and time
v
v
(m/s) and h
p
(mm)
Figure 8: Diagram of the average hourly wind speeds (v
v
) and average hourly precipitation (h
p
) in combination with the average 
hourly continuous underwater noise levels in 1/3-octave bands with centre frequencies of 63 Hz and 125 Hz (L
eq
) in the period 
from 26 September 2015 to 31 December 2015.
2 NM and 5 NM from the measuring station, 
while higher ship densities were observed in 
the Gulf of Trieste; the maximum ship densi-
ties were observed in the Gulf of Venice, as 
expected (Table 2). The most likely reason un-
derlying the fact that variation in underwater 
noise levels was partly related to the varia-
tion of the ship densities (Figures 2–6), could 
be the relatively small acoustic propagation in 
the shallow sea [45, 46].  Acoustic propagation 
in shallow water environments was reported 
to be complex because of interference due to 
seafloor and sea surface sound reflections and 
sound transmission losses [47, 48]. Shallow 
water channels do not allow propagation of 
low-frequency signals due to the wave-guide 
effect; this implies that there would be a lower 
cut-off frequency below which sound waves 
would not propagate, since the sound propa-
gates into the sea bed [49, 50]. This phenom-
enon leads to the less significant contribution 
of shipping to underwater noise.
Figures 7–11 demonstrate that precipita-
tion is not greatly associated with the fluctu -
ations in continuous underwater noise lev -
els, while some larger deviations in the wind 
speed are associated with the larger fluctua -
tions in continuous underwater noise levels. 
This could be explained by the fact that wind 
blowing over the sea generates waves that, 

A. Popit
12
RMZ – M&G | 2020 | Vol. 67(3) | pp. 1–15
0
5
10
15
20
25
30
0
20
40
60
80
100
120
140
0 0 : 1 0 6 1 0 2 . 8 0 . 8 1
0 0 : 6 1 6 1 0 2 . 8 0 . 9 1
21.08.2016 07:00
22.08.2016 22:00
24.08.2016 13:00
26.08.2016 04:00
27.08.2016 19:00
29.08.2016 10:00
31.08.2016 01:00
1.09.2016 16:00
3.09.2016 07:00
4.09.2016 22:00
6.09.2016 13:00
8.09.2016 04:00
9.09.2016 19:00
11.09.2016 10:00
13.09.2016 01:00
14.09.2016 16:00
16.09.2016 07:00
17.09.2016 22:00
19.09.2016 13:00
21.09.2016 04:00
22.09.2016 19:00
24.09.2016 10:00
26.09.2016 01:00
27.09.2016 16:00
29.09.2016 07:00
30.09.2016 22:00
2.10.2016 13:00
4.10.2016 04:00
5.10.2016 19:00
7.10.2016 10:00
9.10.2016 01:00
10.10.2016 16:00
12.10.2016 07:00
13.10.2016 22:00
15.10.2016 13:00
17.10.2016 04:00
18.10.2016 19:00
20.10.2016 10:00
22.10.2016 01:00
23.10.2016 16:00
25.10.2016 07:00
26.10.2016 22:00
28.10.2016 13:00
30.10.2016 04:00
31.10.2016 19:00
Precipitation (mm)
Leq,125Hz (dB)
Leq,63Hz (dB)
Wind speed (m/s)
L
eq
(dB)
Date and time
v
v
(m/s) and h
p
(mm)
Figure 9: Diagram of the average hourly wind speeds (v
v
) and average hourly precipitation (h
p
) in combination with the average 
hourly continuous underwater noise levels in 1/3-octave bands with centre frequencies of 63 Hz and 125 Hz (L
eq
) in the period 
from 18 August 2016 to 1 November 2016.
0
2
4
6
8
10
12
14
16
18
20
0
20
40
60
80
100
120
0 0 : 1 0 7 1 0 2 . 7 0 . 6
0 0 : 4 0 7 1 0 2 . 7 0 . 7
8.07.2017 07:00
9.07.2017 10:00
10.07.2017 13:00
11.07.2017 16:00
12.07.2017 19:00
13.07.2017 22:00
15.07.2017 01:00
16.07.2017 04:00
17.07.2017 07:00
18.07.2017 10:00
19.07.2017 13:00
20.07.2017 16:00
21.07.2017 19:00
22.07.2017 22:00
24.07.2017 17:00
25.07.2017 20:00
26.07.2017 23:00
28.07.2017 02:00
29.07.2017 05:00
30.07.2017 08:00
31.07.2017 11:00
1.08.2017 14:00
2.08.2017 17:00
3.08.2017 20:00
4.08.2017 23:00
6.08.2017 02:00
7.08.2017 05:00
8.08.2017 08:00
9.08.2017 11:00
10.08.2017 14:00
11.08.2017 17:00
12.08.2017 20:00
13.08.2017 23:00
15.08.2017 02:00
16.08.2017 05:00
17.08.2017 08:00
18.08.2017 11:00
19.08.2017 14:00
20.08.2017 17:00
21.08.2017 20:00
22.08.2017 23:00
24.08.2017 02:00
25.08.2017 05:00
26.08.2017 08:00
27.08.2017 11:00
Precipitation (mm)
Leq,125Hz (dB)
Leq,63Hz (dB)
Wind speed (m/s)
L
eq
(dB)
Date and time
v
v
(m/s) and h
p
(mm)
Figure 10: Diagram of the average hourly wind speeds (v
v
) and average hourly precipitation (h
p
) in combination with the 
average hourly continuous underwater noise levels in 1/3-octave bands with centre frequencies of 63 Hz and 125 Hz (L
eq
) in the 
period from 6 July 2017 to 27 August 2017.
0
5
10
15
20
25
30
0
20
40
60
80
100
120
140
0 0 : 1 0 8 1 0 2 . 8 0 . 8 1
20.08.2018 23:00
23.08.2018 21:00
26.08.2018 19:00
29.08.2018 17:00
1.09.2018 15:00
4.09.2018 13:00
7.09.2018 11:00
10.09.2018 09:00
13.09.2018 07:00
16.09.2018 05:00
19.09.2018 03:00
22.09.2018 01:00
24.09.2018 23:00
27.09.2018 21:00
30.09.2018 19:00
3.10.2018 17:00
6.10.2018 15:00
9.10.2018 13:00
12.10.2018 11:00
15.10.2018 09:00
18.10.2018 07:00
21.10.2018 05:00
24.10.2018 03:00
27.10.2018 15:00
30.10.2018 13:00
2.11.2018 11:00
5.11.2018 09:00
8.11.2018 07:00
11.11.2018 05:00
14.11.2018 03:00
17.11.2018 01:00
19.11.2018 23:00
22.11.2018 21:00
25.11.2018 19:00
28.11.2018 17:00
1.12.2018 15:00
4.12.2018 13:00
7.12.2018 11:00
10.12.2018 09:00
13.12.2018 07:00
16.12.2018 05:00
19.12.2018 03:00
22.12.2018 01:00
24.12.2018 23:00
27.12.2018 21:00
30.12.2018 19:00
Precipitation (mm)
Leq,125Hz (dB)
Leq,63Hz (dB)
Wind speed (m/s)
L
eq
(dB)
Date and time
v
v
(m/s) and  h
p
(mm)
Figure 11: Diagram of the average hourly wind speeds (v
v
) and average hourly precipitation (h
p
) in combination with the 
average hourly continuous underwater noise levels in 1/3-octave bands with centre frequencies of 63 Hz and 125 Hz (L
eq
) in the 
period from 18 August 2018 to 31 December 2018.

Underwater noise in the Slovenian Sea
13
when they are large enough, break and pro -
duce underwater sound. This phenomenon 
is well described in several previous studies 
[7, 9, 22–25].
Conclusion
The results of our study have indicated that 
the underwater noise levels in the Slovenian 
Sea are related to dredging activity in the Port 
of Koper and are partly related to variations 
of the ship densities. Some larger deviations 
in the wind speed were found to be associ-
ated with the larger fluctuations in continuous 
underwater noise levels, while precipitation 
was not related to the underwater noise. Use 
of larger data sets is suggested to ensure that 
it becomes possible to further study and evalu-
ate underwater noise levels in relation to man-
made or natural sound sources.
Acknowledgements
The current study was funded by the Ministry 
for the Environment and Spatial Planning, 
Slovenia. The AIS data from the year 2015 were 
obtained by the BALMAS project partnership. 
The AIS data from 2016 to 2018 were obtained 
by the Slovenian Maritime Administration in 
the frame of the Ministry for Infrastructure.
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