Measurements, analysis and modeling of wind-driven ambient noise in shallow brackish water

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School of Electrical Engineering | Doctoral thesis (article-based) | Defence date: 2012-03-09
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Authors

Date

2012

Department

Signaalinkäsittelyn ja akustiikan laitos
Department of Signal Processing and Acoustics

Major/Subject

Mcode

Degree programme

Language

en

Pages

54

Series

Aalto University publication series DOCTORAL DISSERTATIONS, 18/2012

Abstract

Underwater ambient noise measurements were conducted in shallow brackish water environments in the Gulf of Finland over a frequency range from 20 Hz to 70 kHz. The ambient noise levels are in fairly good agreement with average oceanic deep water levels for the highest wind speeds but the wind speed dependences differ markedly from each other. In shallow brackish water the wind speed dependence factor at 200 Hz is ~ 2.4 which is significantly higher than the typical factor of ~ 1.5 for the ocean environment. The high-frequency behavior of the spectra was resolved by modeling dispersion and noise in bubbly water. Absorption in brackish and fresh water, unlike in ocean water, tends to decrease above a frequency of 10 kHz due to the low proportion of small bubbles in bubbly mixtures created by breaking waves. The excess high-frequency attenuation in spectra above 10 kHz cannot therefore be attributed to the effects of absorption in a bubbly mixture. Bubble size distributions fitted to the brackish water spectra exhibit a distinctive maximum in the radius range 0.1 - 0.3 mm, and a substantial drop in bubble density below a radius of 0.1 mm. The brackish water bubble size distributions were tied to an oceanic spectrum with a spectral slope of 5.7 dB/octave obtained with a -3/2 power law dependence of bubble size density on radius. The measurements were carried out in all four seasons of the year but no significant seasonal effects were found in any parameter calculated from the spectra. This is probably due to the dominance of near field conditions in the measurements, where the role of long-range propagation effects was negligible. One should, however, be careful not to generalize the present brackish water results too much due to the inherent complexity of the coastal situation. Bubble densities are known to have considerable spatial variability depending on seasonal, biological, and even weather conditions.

Tuulen aiheuttamaa vedenalaista kohinaa tutkittiin Suomenlahden matalissa murtovesissä taajuusalueella 20 Hz - 70 kHz. Mitattujen kohinaspektrien taso yhtyy voimakkaimmilla tuulilla muutaman desibelin tarkkuudella valtamerien keskimääräisiin kohinatasoihin, mutta kohinan tuuliriippuvuudet eroavat merkittävästi toisistaan. Matalassa murtovedessä kohinatason tuuliriippuvuuden potenssi taajuudella 200 Hz on n. 2.4, kun se on valtameriympäristössä on tyypillisesti n. 1.5. Kohinaspektrin käyttäytymistä suuremmilla taajuuksilla (yli 1 kHz) mallinnettiin sekä kuplaisen nesteen dispersiomallilla että resonanssitaajuuksillaan värähtelevien kuplien mallilla. Toisin kuin valtameressä, makeassa ja vähäsuolaisessa vedessä absorptio alkaa laskea yli 10 kHz taajuudella johtuen murtuvien aaltojen synnyttämien pienten kuplien vähäisestä suhteellisesta määrästä. Kohinaspektreissä yli 10 kHz taajuuksilla havaittua jyrkkää vaimenemista ei siten voida selittää kuplavaimennuksella. Murtovedestä mitattuihin kohinaspektreihin sovitetuissa kuplakokojakaumissa on selkeä maksimi kuplan säteillä 0.1 - 0.3 mm ja huomattava pudotus alle 0.1 mm säteillä. Murtoveden kuplakokojakaumat sidottiin valtameren vertailujakaumaan, jossa jakauman jyrkkyyden potenssi oli -3/2, joka aiheutti spektrin laskevalle osalle jyrkkyyden 5.7 dB/oktaavi. Kohinamittauksia tehtiin kaikkina vuodenaikoina, mutta ajallista riippuvuutta ei havaittu spektreistä lasketuissa parametrissä. Tämä johtuu todennäköisesti mittausten lähikenttäolosuhteista, jossa kaukaa edenneen äänen osuus on vähäinen. Rannikon olosuhteiden luontainen vaihtelu on kuitenkin suurta ja kuplakokojakaumalla tiedetään olevan huomattavaa paikallista vaihtelua, joka riippuu vuodenajasta, biologisista tekijöistä ja jopa säästä. Tämän takia mittausten johtopäätökset ovat luonteeltaan suuntaa antavia, eikä niitä tule siten liiaksi yleistää.

Description

Supervising professor

Laine, Unto K., Prof., Aalto University, Finland

Thesis advisor

Kalliomäki, Martti, Dr. (ret.), Finnish Naval Research Institute, Finland

Keywords

underwater acoustics, ambient noise, bubbles, hydrophones, sonar, Baltic Sea, Gulf of Finland, vedenalainen akustiikka, vedenalainen kohina, kuplat, hydrofonit, kaikumittaus, Itämeri, Suomenlahti

Other note

Parts

  • [Publication 1]: Ari Poikonen and Seppo Madekivi. 2005. Recent hydroacoustic measurements and studies in the Gulf of Finland. In: John S. Papadakis and Leif Bjørnø (editors). Proceedings of the 1st International Conference on Underwater Acoustic Measurements: Technologies and Results (UAM 2005). Heraklion, Crete, Greece. 28 June - 1 July 2005. Heraklion, Crete, Greece. Foundation for Research and Technology - Hellas (FORTH), Institute of Applied and Computational Mathematics (IACM). 9 pages. ISBN 960-88702-3-2.
  • [Publication 2]: Ari Poikonen and Seppo Madekivi. 2007. Underwater noise measurements in very shallow coastal environment. In: John S. Papadakis and Leif Bjørnø (editors). Proceedings of the 2nd International Conference and Exhibition on Underwater Acoustic Measurements: Technologies and Results (UAM 2007). Heraklion, Crete, Greece. 25-29 June 2007. Heraklion, Crete, Greece. Foundation for Research and Technology - Hellas (FORTH), Institute of Applied and Computational Mathematics (IACM). Pages 647-655. ISBN 978-960-88702-5-3.
  • [Publication 3]: Ari Poikonen and Seppo Madekivi. 2010. Wind-generated ambient noise in a shallow brackish water environment in the archipelago of the Gulf of Finland. The Journal of the Acoustical Society of America, volume 127, number 6, pages 3385-3393. © 2010 Acoustical Society of America. By permission.
  • [Publication 4]: Ari Poikonen. 2010. High-frequency measurements of underwater ambient noise in shallow brackish water. In: Tuncay Akal (editor). Proceedings of the 10th European Conference on Underwater Acoustics (ECUA 2010). Istanbul, Turkey. 5-9 July 2010. 6 pages.
  • [Publication 5]: Ari Antti Poikonen. 2010. High-frequency wind-driven ambient noise in shallow brackish water: Measurements and spectra. The Journal of the Acoustical Society of America Express Letters, volume 128, number 5, pages EL242-EL247. © 2010 Acoustical Society of America. By permission.
  • [Publication 6]: Ari Poikonen. 2011. Analysis of high-frequency wind-driven ambient noise in shallow brackish water. The Journal of the Acoustical Society of America Express Letters, volume 129, number 4, pages EL128-EL134. © 2011 Acoustical Society of America. By permission.

Citation

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https://urn.fi/URN:ISBN:978-952-60-4514-6

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[diss] Sähkötekniikan korkeakoulu / ELEC