Posts Tagged 'sound'

The effects of pH on acoustic transmission loss in an estuary

Increasing atmospheric CO2 will cause the ocean to become more acidic with pH values predicted to be more than 0.3 units lower over the next 100 years. These lower pH values have the potential to reduce the absorption component of transmission loss associated with dissolved boron. Transmission loss effects have been well studied for deep water where pH is relatively stable over time-scales of many years. However, estuarine and coastal pH can vary daily or seasonally by about 1 pH unit and cause fluctuations in one-way acoustic transmission loss of 2 dB over a range of 10 km at frequencies of 1 kHz or higher. These absorption changes can affect the sound pressure levels received by animals due to identifiable sources such as impact pile driving. In addition, passive and active sonar performance in these estuarine and coastal waters can be affected by these pH fluctuations. Absorption changes in these shallow water environments offer a potential laboratory to study their effect on ambient noise due to distributed sources such as shipping and wind. We introduce an inversion technique based on perturbation methods to estimate the depth-dependent pH profile from measurements of normal mode attenuation.

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Ocean acidification and its impact on ocean noise: Phenomenology and analysis

Ocean acidification has been observed since the beginning of the industrial era and is expected to further reduce ocean pH in the future. A significant increase in ocean noise has been suggested based upon the percentage change in acoustic absorption coefficient at low frequencies. Presented here is an analysis using transmission loss models of all relevant loss mechanisms for three environments experiencing a significant near-surface pH reduction of 8.1-7.4. Results show no observable change in the shallow water and surface duct environments, and a statistically insignificant change of less than 0.5 dB for all frequencies in the deep water environment.
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A computational assessment of the sensitivity of ambient noise level to ocean acidification

Low-frequency sound propagating through the ocean is partly attenuated by the pH-dependent boric acid relaxation process. Thus, the uptake of increased levels of atmospheric CO2 by seawater, leading to reduced pH, has potential to change ambient noise levels. An important question is: By how much? Here, changes in ambient noise level due to hypothetical changes in seawater pH have been calculated at three receiver locations for years 1960 and 2250. The calculations used a range dependent propagation model that was applied to realistic environments based on climatology. Model results indicate changes in noise levels less than 0.21 dB are anticipated.
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Modeling deep ocean shipping noise in varying acidity conditions

Possible future changes of ambient shipping noise at 0.1–1 kHz in the North Pacific caused by changing seawater chemistry conditions are analyzed with a simplified propagation model. Probable decreases of pH would cause meaningful reduction of the sound absorption coefficient in near-surface ocean water for these frequencies. The results show that a few decibels of increase may occur in 100 years in some very quiet areas very far from noise sources, with small effects closer to noise sources. The use of ray physics allows sound energy attenuated via volume absorption and by the seafloor to be compared.
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Future ocean increasingly transparent to low-frequency sound owing to carbon dioxide emissions

Low-frequency sound in the ocean is produced by natural phenomena such as rain, waves and marine life, as well as by human activities, such as the use of sonar systems, shipping and construction. Sea water absorbs sound mainly owing to the viscosity of the water and the presence of chemical constituents, such as magnesium sulphate, boric acid and carbonate ions. The concentration of dissolved chemicals absorbing sound near 1 kHz depends on the pH of the ocean, which has declined as a result of increases in acidity due to anthropogenic emissions of carbon dioxide. Here we use a global ocean model forced with projected carbon dioxide emissions to predict regional changes in pH, and thus sound absorption, in the years 1800–2300. According to our projections, ocean pH could fall by up to 0.6 units by 2100. Sound absorption—in the range between ~100 Hz and ~10 kHz—could fall by up to 60% in the high latitudes and in areas of deep-water formation over the same time period. We predict that over the twenty-first century, chemical absorption of sound in this frequency range will nearly halve in some of the regions that experience significant radiated noise from industrial activity, such as the North Atlantic Ocean. We suggest that our forecast of reduced sound absorption in acoustic hotspots will help in identifying target regions for future monitoring.

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