PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Evaluation of range standards for underwater radiated noisein beam aspect
Hans Hasenpflug(1), Anton Homm(2), Stefan Schäl(2), Layton Gilroy(3)1 Defence Materiel Organization, E-Mail: h.hasenpflug@mindef.nl
2 Wehrtechnische Dienststelle 71, Eckernförde, E-Mail: stefanschael@bundeswehr.org
2 Wehrtechnische Dienststelle 71, Kiel, E-Mail: antonhomm@bundeswehr.org
3 Defence Research and Development Canada, E-Mail: layton.gilroy@drdc-rddc.gc.ca

ABSTRACTThe underwater radiated noise levels of a small naval vessel were measured at the Heggernes deep watersound range. The data evaluation and analysis were performed in accordance with the NATO standard
STANAG 1136 and the ISO 17208-1 standard. Both standards describe procedures for the determination ofunderwater radiated noise levels of ships in beam aspect on a deep water sound range.
To assess and compare the results of both standards, the hydrophone configuration at the Heggernes range
was utilized at various depth settings. Measurements were made of the ship in two different ship conditions(machinery states) and a variety of sailing speeds. Some of these measurements were repeated a large numberof times. The resulting 3rd octave based spectra are used to compare the two procedures.
Keywords: Underwater Sound, Ship Noise, Acoustic Signature

1. INTRODUCTION
The ISO 17208-1 (1) is a relatively new standard for measuring the underwater sound of ships. The
standard describes the Underwater Acoustics Quantities, the required hydrophone layout, themeasurement procedures and the analysis specifications. The standard is developed for thedetermination and evaluation of the underwater radiated noise levels of ships in relation to sea life
which is jeopardized by increasing world-wide ship traffic and noise pollution. Accurate measurement
procedures are needed to compare the noise levels produced by ships.
In addition to the ISO standard, there is the STANAG 1136 standard for measuring the radiated noiselevels in beam aspect of naval platforms in relation to sonar detection.
This paper is meant to investigate the practical and technical use of the ISO 17208 -1 in comparison
with the STANAG 1136 (2), evaluated and compared to the actual procedures and available
infrastructure at the Heggernes deep water sound range.
For this evaluation, the acoustic data of a measurement trial with a small naval vessel at Heggernes
was made available. A number of runs with speeds of 5 and 12 kts were carried out in accordance withthe requirements of the different procedures.
Only the main differences with respect to the hydrophone layout and the data window length (DWL)
for the determination of the radiated underwater noise levels will be addressed here.

2. SOUND RANGE
The Heggernes sound range is situated in Norway near the city of Bergen. The main objective of therange is to determine the underwater radiated noise levels as function of all relevant ship parameters,such as speed and diving depth without the interference of background noise and reverberations due
to reflections of walls and the sea bottom. An evaluation of the performance of the range can be foundin (4). The Herdla fjord was chosen as a suitable location to fulfil these requirements . The fjord has adepth of almost 400 meters and a width of more than 1200 meters at the measurement location. The7425

length of the Herdla fjord is about 16 km. The wet end of the range equipment consists of three cables
operated by individual winches. Each cable is equipped with 3 hydrophones at fixed positions. The
depth of the hydrophones is adjustable by the hydraulic winch system ashore. The hydrophones areconnected through a junction box to the range house by fiber and copper cables. The cables are kept
vertical by a main buoy and small buoys between the hydrophones.
One of the cables is positioned at the track line and is used to measure the radiated noise levels under
the ship in keel aspect. The other cables are positioned North and South from the track at a distanceof approximately 100 meters. These hydrophones are used to measure the ra diated noise levels inbeam aspect.
The ranging can be performed in two directions (East and West). A Differential Global Positioning
System (DGPS) is used for the determination of the exact position of the ship in relation to the trackline and the hydrophones.

Figure 1: Sound range hydrophone configuration

3.

STANDARDS VERSUS HEGGERNES

Both standards describe a procedure to measure the 1/3 Octave Root Mean Square Sound Pressure
Level re 1 μPa, measured in the far field and normalized to a reference distance of 1 m by means ofspherical propagation correction. The results of these measurements is called ‘Radiated Noise Levels’(RNL).
3.1 Hydrophone configuration
The standards require that only calibrated omnidirectional hydrophones are to be used. The Heggernes
sound range is equipped with regularly calibrated hydrophones according to IEC 60565:2006 (5).
According to the ISO standard, the passing distance between the ship and the hydrophone at theClosest Point of Approach (CPA) should be at least 100 m, in order to realize far field conditions
which assumes that the radiating sound sources on the ship can be considered as a point sources. The
beam aspect hydrophones deployed at the Heggernes sound range are positioned at 100 meters fromthe track line.
The STANAG 1136 requires only one hydrophone at a depth between 9-36 m. The underwater radiatednoise levels in accordance to ISO 17208-1 should be measured at 3 different aspect angles whichmeans that the required water depth of the range location is determined by the lowest hydrophone ata nominal depth of 100 meters. The required minimal water depth is then 150 meters.
For the measurement trials the South and North hydrophones were positioned at a depth of 30, 62.5
and 95 meters. This is almost exactly in line with the requirements of both standards.
Table 1 shows that the differences between the required and realized hydrophone configurations areless than 5 meters for the ISO standard.

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Table 1: main differences between different standards

The results of the three hydrophones are averaged in order to smooth the variability caused by Lloyd’s
mirror surface image coherence effects. Figure 2 shows the ISO hydrophone specification along withthe prescribed averaging equation (1).

Figure 2: ISO hydrophone layout௅ೃಿ ሺ௥ǡ௛భ ሻൗ௅ೃಿ ሺ௥ǡ௛మ ሻൗ௅ೃಿ ሺ௥ǡ௛య ሻൗଵ଴ௗ஻ ൅  ͳͲଵ଴ௗ஻ ൅ ͳͲଵ଴ௗ஻ͳͲ‫ܮ‬ோே ሺ‫ݎ‬ሻ ൌ ͳͲ݈‫݃݋‬ଵ଴ ቌ͵ቍ(1)Figure 3 shows the influence of Lloyd’s Mirror on the radiation of a point source (180 dB) in a water
depth of 4 meters for the different hydrophone depths at the sound range. The average of the threehydrophones in different depths is comparable with the levels at a hydrophone depth of 60 meters.
Calculations are done by assuming a constantly moving point source near an ideal pressure releaseboundary, according to [3].

Figure 3: Influence of the “Lloyd’s Mirror-Effect”

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3.2 Data window length
The data window length (DWL) is defined by the distance at CPA and the data window angle (DWA)
of ± 30°. Port and Starboard aspect underwater RNL are the energetic levels of all samples within
DWL during CPA of the ship reference point. The ships reference point is located at ¼ ship length infront of the ship’s stern.

Figure 4: DWLThe STANAG 1136 does not clearly define a DWL. The radiated underwater noise levels should beanalyzed over a DWA of ± 45° arc, with a passing distance of 100 meters this results in a DWL ofapproximately 209 meters. This large DWL is not useful for ships with an overall length between 50and 150 meters. Range experiences have shown that a large DWL can lead to an underestimation ofthe RNL. The results of the RNL calculated with different DWL’s for the hydrophones at a depth of30 meters showed that a DWL related to the length of the ship is more suitable. The average
differences calculated over the complete frequency range between the RNL on the basis of STANAG
and the procedure at Heggernes or ISO is approximately 1 dB, see Figure 5.

Figure 5: effect of DWL
3.3 Noise measurements
The available naval vessel carried out a large number of runs in the east and west directions with fixed
operational conditions at 5 and 12 kts. The number of runs, average speed and track deviation arepresented in Table 2. The average (ground) speed in the west direction was slightly higher for bothconditions due to the local current and wind direction and influenced the radiated noise levels. Thus,
average noise levels for port and starboard aspect where determined for both headings, respectively.
The average track deviation was approximately 2.5 meters for the two conditions and headings.
Table 2: Run statistics
Heading
East runs

West runs

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Run statistics
Number of runs
Average speed (kts)Average track deviation (m)Number of runs
Average speed (kts)Average track deviation (m)

5H01
64.6
2.4
7
5.9
1.9

12H08
9
11.8
2.9
10
12.4
2.3

The sea state and background noise levels were sufficiently low during the trial program. The signal
to noise ratio in most frequency bands was at least 10 dB.
On-board sensors located near the main machinery and hydrodynamic noise sources of the ship
confirmed steady state and repeatable conditions for all runs in east and west directions. Also, theunderwater noise data at the different single hydrophones show repeatable results with an average
standard deviation of 2 dB.
The noise distribution during the track shows that the maximum levels are reached inside all used
DWL’s. Starting recording at a large distance before CPA is useful to confirm that a sufficient signal
to noise ratio was established as shown in Figure 6.

Figure 6: Used DWL’s and noise distribution during runs with 12 kts
3.4 Evaluation and comparison
Table 3 gives an overview of the hydrophone settings of the range in relation to the different
procedures and DWL’s used for the determination of the RNL’s
Table 3: used hydrophone depth and DWL’s

The differences between the RNL based on ISO and STANAG are presented in Figure 7. The ISOaverage RNL levels of 3 hydrophones are in all frequency bands higher compared to the RNL of asingle hydrophone based on the STANAG standard. In the lower frequency bands this difference isrelated to the Lloyd’s mirror effect and in the higher frequency bands only to the large DWL inSTANAG.

Figure 7: RNL differences between ISO and STANAG

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These differences reduce to almost zero (see Figure 8) when measurements are carried out with asingle hydrophone at a water depth of 60 meters, which corresponds to an aspect angle of 30° , and asmaller DWL conforming the ISO standard or the procedure used at Heggernes.

Figure 8: RNL differences between ISO and modified STANAG procedure

4. CONCLUSIONSThe Heggernes sound range has a hydrophone layout which complies with the spe cifications of therelatively new ISO 17208-1 standard for vessels up to an overall length of 100 meters.
The DWL suggested by STANAG is too large for small ships and leads to 1 dB lower RNL comparedto the DWL defined by ISO and used at Heggernes. Thus, for small ships, the DWL should be related
to the ship length.
RNL measured with a single hydrophone at shallow water depths, as specified by STANAG are lower,compared to the ISO 17208 standard due to the Lloyd’s Mirror effect. RNL measured with a singlehydrophone at 60 meters depths are comparable to the RNL of ISO.
Both, the ISO and STANAG procedures should be reconsidered with respect to the definition of theDWL for small ships. A DWL related to the ship length, especially for small ships, is a more suitableapproach. In order to achieve comparable RNL with ISO, measurements with a single hydrophoneshould be carried out at an aspect angle of 30° which corresponds to a measurement depth of circa 60
meters at a CPA distance of 100 meters.

REFERENCES
1. ISO 17208-1, Underwater acoustics — Quantities and procedures for description and measurement ofunderwater sound from ships —Part 1: Requirements for precision measurements in deep water used forcomparison purposes, 2016
2. STANAG 1136 Standards for use when measuring and reporting radiated noise characteristics of surface
ships, submarines, helicopters etc in relation to sonar and torpedo acquisition risk.
3. Junger, M. C., Feit D., Sound Structures and Their Interactions, ASA 1993, p. 52-54;
4. Homm, Schäl, Hasenpflug, "Understanding radiated underwater noise levels measured at different soundranges", Proc. Internoise August 2018, Chicago, Il, USA
5. IEC 60565:2006, Underwater acoustics – Hydrophones – Calibration in the frequency range
0,01 Hz to 1 MHz7430