Sound Absorption In Sea Water Research Articles

Fred Fisher’s high-pressure work with eyewash and epsom salts

Starting in 1957 Fred Fisher led research programs devoted to high-pressure measurements related to the physical chemistry of sound absorption in seawater due to magnesium sulfate and other salts. As he put it, he spent his professional lifetime squeezing epsom salt. His interest in the low-frequency anomalous sound absorption in the ocean below 1 kHz led to the discovery of boric acid as the cause of the low-frequency relaxation. This paper is a short review of Fred Fisher’s contributions to our knowledge of sound absorption in seawater, based in part on his carefully hand-written lecture notes and numerous low-pressure discussions.

Long-range sound propagation in the northeastern Atlantic

Experimental data on the long-range propagation of explosion-generated signals are analyzed. The experiments were performed in the northeastern Atlantic under the conditions of a two-axis underwater sound channel. The sound field in the upper channel was governed by the vertical redistribution of the ray structure and sound energy under the influence of a smooth increase in the depth of the channel’s axis along the propagation path. The explosions were produced in the upper sound channel at a depth of 200 m, which was constant along the path. The time structure of the sound field is analyzed for the upper channel (a reception depth of 200 m) and for deeper layers lying somewhat below the boundary between the upper and lower sound channels (a reception depth of 1200 m). The deviation of the decay law obtained for the sound field level in the upper channel from the cylindrical law is used to estimate the attenuation coefficient. The low-frequency (several hundreds of hertz) attenuation coefficients experimentally determined with allowance for the sound field redistribution agree well with the calculated sound absorption in seawater. The attenuation coefficients determined by the differential method also agree well with the absorption calculated by the formulas proposed earlier. The analysis of the time structure of the sound field near the boundary between the upper and lower channels reveals a permanent insonification of this horizon by weak water-path signals propagating with the velocity typical of the signals traveling in the upper channel.

The pH of the North Atlantic Ocean: Improvements to the global model for sound absorption in seawater

At frequencies below 1 kHz, sound absorption coefficients in the ocean are a function of pH, and at higher frequencies they are dependent upon MgSO4. The pH dependent terms are attributable to relaxation of B(OH)3 and MgCO3 species, and the ensemble effect has been approximated (Mellen et al., 1987a) as α = α1(MgSO4) + α2(B(OH)3) + α3(MgCO3), where α is the absorption coefficient in decibels per kilometer, and αn = (S/35)An[ƒ2ƒn/(ƒ2 + ƒn2)], where S is the salinity, An is the amplitude of each component and depends on pH, ƒ is the frequency, and ƒn is the relaxation frequency. Overall accuracy of ±15% in a requires that pH be known to 0.05 pH units. The presently used oceanic pH field for sound absorption models is derived from a combination of Geochemical Ocean Sections Study (GEOSECS) data and Soviet data from the 1978 Gorshkov atlas for the North Atlantic where GEOSECS data are absent. We compare the North Atlantic fields with the well‐constrained Transient Tracers in the Ocean North Atlantic CO2 data set and find large differences. We further show that sufficiently strong correlations exist between CO2 system variables and other more commonly available hydrographic properties and that improved renditions of the pH field in other regions of the ocean are possible once equivalent local correlations are established, thus leading to greatly improved estimates of the sound absorption field.

A cylindrical resonator method for the investigation of low-frequency sound absorption in sea water

In this paper, a cylindrical resonator technique developed for the investigation of low-frequency sound absorption in sea water and synthetic media is described. The lowest usable measuring frequency has been extended down to about 3 kHz. Due to the low residual losses of the resonator system and to the high reproducibility of the energy decay time measurement, it was possible not only to measure excess absorption at normal sea water concentration and at low pH, but also, under certain conditions, to determine both the relaxation frequency and maximum absorption per wavelength of a single definite relaxation with acceptable accuracy, even though absorption was being measured mostly at two frequencies only. As examples, some experimental results are presented. An error analysis of the data is given.

Effect of pressure on sound absorption in seawater at 0°C

Earlier measurements of the effect of pressure on sound absorption by magnesium sulfate in seawater by Hsu at 0 °C in the laboratory were limited to pressures of just over 300 bars. The pressure was limited by the strength of the 100‐liter titanium sphere, a spare ballast tank from the Woods Hole submarine ALVIN. To obtain data at higher pressures at 0 °C, measurements were recently made in 0.5 M magnesium sulfate solutions in a cylindrical reesonator. Because the magnesium sulfate relaxation frequency is independent of pressure, measurements are necessary at only one frequency. This was done in the 100‐kHz region and a sodium sulfate solution was used to correct for wall losses in the cylinder. Mode interference problems as well as a shutdown in the building air circulation system, which gave rise to temperature control problems, degraded the data. Nevertheless, enough data were obtained at higher pressures (600 to 1300 bars) to be useful in assessing the effect of pressure on seawater sound absorption a...

A comparison between low-frequency sound absorption in seawater and high-frequency sound absorption in serum albumen

Although large scale (> 1000 km), low-frequency (<20000 Hz) sound absorption measurements in seawater [Mellen et al., J. Acoust. Soc. Am. 74, 987–993 (1983)] might appear to have nothing in common with high-frequency sound absorption in serum albumen [Barnes et al., J. Acoust. Soc. Am. 83, 2393–2404 (1988)], the results show some intriguing similarities that may be due to a common chemistry. Specifically, both show a pH dependence, a buffered reaction involving a carboxyl group, and the need of calcium to promote the principal reaction. However, boric acid, which is essential for seawater absorption, is apparently not a factor in serum absorption. A comparison is presented in the hope of stimulating a dialogue with the bioacoustics community to explore a possible linkage.

Sound absorption in Arctic regions

Sound absorption in seawater involves relaxations of magnesium sulfate, boric acid, and magnesium carbonate. Absorption varies with both region and depth, due mainly to the pH dependence of the boric acid relaxation. The nominal seawater pH range is 7.7–8.3 and magnitudes can vary over nearly a factor of 4 at low frequencies. A global model for World Ocean prediction has been developed, employing contour charts of the pH factor at several depths. Profiles for integration of loss along ray paths are generated by algorithm. The pH range in the Arctic is nominally 8.0–8.3 and similar contour charts are provided for the pH factor in these regions. Although long-range propagation is generally limited by under-ice scattering, absorption can be the dominant loss mechanism under ice-free conditions and for ranges less than 50 km at sonar frequencies with an ice cover. Predicted absorption spectra are presented and compared with the limited data available. [Work supported by NUSC/NL.]

Slow relaxations in sea water: T‐jump measurements

Sound absorption in sea water involves chemical equilibria with rather slow relaxation rates. The low‐frequency range of the resonator method is limited by practical considerations of size and experimental conditions must usually be altered to place the relaxation frequency within the measurement window. For relaxations involving acids or bases, the temperature‐jump pH method permits rate measurements below a limit set by indicator response (∼ 105/s). Recent T‐jump studies confirm unambiguously that the low‐frequency (∼ 1 kHz) relaxation mechanism in sea water is an acid‐base exchange between boric acid and carbonate [P. Mallo et al., J. Phys. Chem. (in press) (1984)]. Further investigations of the boric acid exchange in similar Lewis systems, e.g., ammonia, reveal single relaxations with comparable rates. The magnesium carbonate ion‐pair equilibrium—a minor component of sea water absorption at mid‐frequencies (∼ 10 kHz)—shows more complex behavior with several identifiable relaxations in the measurement range. The magnesium sulfate relaxation (∼ 100 kHz) is unsuited to the method because of both speed and lack of pH dependence. Visiting scientist; home address: 117 Boston Post Road, Old Lyme, CT 06320.

Investigations of the 1‐kHz sound absorption in sea water

Abstract : Using a combination of sound absorption data from a large spherical resonator and relaxation frequency data from the temperature jump technique, it has been demonstrated that there is an increased sound absorption in sea water below approximately 1 kHz. The absorption mechanism has been traced to a chemical relaxation of boric acid. Observed and calculated data are in general agreement when a two step, three state boric acid ionization reaction is postulated. The observed sound absorption is attributed to the unimolecular second step of this reaction, with the faster bimolecular first step being of low sound absorption step being of low sound absorption magnitude and under diffusion control. The magnitude of the observed sound absorption is approximately nine times greater than the absorption calculated by considering only the contribution from the high frequency magnesium sulfate effect. There is an effect of bicarbonate on the low frequency absorption presumably through the buffering effect on the hydroxyl ion.

Investigation of chemical sound absorption in sea water. Part IV

The low-frequency boric acid relaxation mechanism in seawater has been identified as an acid-base exchange between the B(OH)3/B(OH)−4 and HCO−3/CO2−3 equilibria. Ion pairing with Ca2+ is also involved and a coupled-system model is proposed. Supporting experimental data for the CO2 exchange mechanism are presented. Similar exchanges with ammonia and silicic acid are also investigated.

Effect of pressure on sound absorption in seawater and in aqueous solutions of MgSO4

Measurements have been made at 25 °C of the effect of pressure on sound absorption in Lyman and Fleming synthetic seawater from 30–300 kHz up to 307 atm. Similar measurements were made in 0.02M MgSO4 solution and in a 0.02M MgSO4-0.6M NaCl solution. The decrease with pressure of absorption per wavelenght (αλ) in seawater differs substantially from those reported by other investigators. The relaxation frequency, within experimental error, was found to be independent of pressure. For seawater, increasing pressure to 307 atm reduced absorption by 26.3%, very nearly the same (28.2%) as was observed previously for 0.5M MgSO4 solutions. The addition of 0.6M NaCl to the 0.02M MgSO4 solution decreased the absorption by a factor of 4 and the effect of pressure was found to be independent of the NaCl concentration. The Eigen and Tamm multistate dissociation parameters which best describe the pressure dependence of sound absorption and electrical conductance in a 0.02M MgSO4 solution predict a 21.4% decrease in absorption at 307 atm compared to the observed 19.6% decrease. However, these parameters yield an equilibrium constant which differs from conductance derived value by a factor of 3.

Approaches to a Quantitative Analytical Description of Low Frequency Sound Absorption in Sea Water

Abstract : The absorption of sound in sea water at low frequency (1-100 kHz) and intermediate frequency (50-500 kHz) ranges can be defined by a series of relaxation phenomena, each of which can be described mathematically by a Debye relationship. Below 5 kHz, the principal absorption mechanism is attributed to borate and magnesium/carbonate interactions, while in the range 5 to 50 kHz the absorption arises from magnesium sulphate ion pair formation in sea water. The dependence of the various relaxation modes on oceanographic variables is outlined in quantitative terms based on results from deep ocean sound channel experiments and laboratory experiments.

Investigation of chemical sound absorption in sea water: Part III

Chemical sound absorption in sea water cannot be considered the sum of independent relaxation processes because many equilibria have common ions. Effects of coupling on the MgCO30 and MgB(OH)+4 relaxations can be explained as simple buffer reactions that equilibrate rapidly and follow the slower reactions. The B(OH)3 relaxation, however, is explained by a different mechanism involving exchange with the HCO3−/CO32− system.

Investigation of chemical sound absorption in sea water: Part II

Resonator measurements of relaxational sound absorption by boric acid in artificial sea water exhibit pH anomalies similar to that observed in aqueous solution. Discrepencies between theory and experiment indicate that the simple two-step relaxation model is inadequate. However, the results are in good agreement with sea measurements in the sea water pH range.

Low‐frequency sound absorption in sea water: A new chemical relaxation mechanism?

Excess sound absorption in sea water arises mainly from chemical relaxations involving MgSO4 and B(OH)3. The high‐frequency (100 kHz) MgSO4 relaxation has been identified as a multistep ion‐pair process. The low‐frequency (1kHz) and B(OH)3 relaxation apparently involves more complex interactions with other constituents. To investigate B(OH)3 interactions in a simpler system, we measured absorption in NH3 solution using the resonator method. We have found αmax to be proportional to the product of NH4+ and B(OH)4− concentrations; however, the magnitude is much too large to be caused by the ion pair. The mechanism, probably similar to that in sea water, resembles catalysis, the absorption being governed by the large volume change of the faster NH3/NH4+ equilibrium and the relaxation frequency by the slower B(OH)3/B(OH)4− equilibrium. [Work supported by NUSC and DARPA.]

Sound absorption in seawater at pressures up to 307 atmospheres

Using a 100‐liter titanium spherical resonator sound absorption from 45 to 350 kHz has been measured in Lyman and Fleming seawater at 25° and at pressures of 1, 71, 188, and 307 atm. Assuming a single MgSO4 relaxation process, the data were analyzed for the values of the maximum absorption per wavelength, (aλ)max, and of the relaxation frequency fr At 1 atm (aλ)max = (61.65 ± 0.34) × 10−6, in good agreement with that measured by Wilson and Leonard [J. Acoust. Soc. Am. 26, 223 (1954)] and by Simmons (Ph.D. thesis, University of California, San Diego, 1975). The relaxation frequency of 133 kHz is lower than those reported by Wilson and Leonard and by Simmons. At elevated pressures the relaxation frequency remained the same contrary to the increase observed by Bezdek [J. Acoust. Soc. Am. 53, 782 (1973)] in field measurements. Absorption decreased linearly with pressure according to the equation a(p) = a(l)(1− 9.1 × 10−4P), where P is in arm. This decrease is 25% smaller than Bezdek found but nearly the same ...

Investigation of chemical sound absorption in seawater by the resonator method: Part I

Resonator measurements in synthetic media show that sound absorption in seawater is caused by chemical relaxations involving B(OH)3 and MgCO3 in addition to MgSO4. The MgCO3 relaxation can be modeled as a two-step MgCO03 ion-pair association in which a monomolecular step controls the absorption process. The B(OH)3 relaxation involves a similar two-step association of the B(OH)−4 ion in which interactions with the pH buffer system play a crucial role. A number of other relaxations are observed, but their contributions are negligible under seawater conditions.

Low-frequency sound absorption in sea water: A borate-complex relaxation

Resonator measurements of sound absorption in borate solutions show that magnesium, calcium, and carbonate are also involved in the low-frequency absorption in sea water.

Sound absorption in sea water by a third chemical relaxation

Evidence is presented for a third chemical relaxation producing sound absorption in the ocean. After accounting for sound absorption in the Atlantic and Pacific due to boric acid, magnesium sulfate, and water, there appears to be a third relaxation at 3 kHz (possibly higher) with αλmax=0.26×10−5 at 4°. The existence of this relaxation was determined by using the Fisher–Simmons equation for sound absorption based on Simmon’s laboratory work. For the Pacific data, the boric acid contribution was reduced to 0.44 that of the Atlantic. The third relaxation was deduced initially from the shot data of Morris (50–400 Hz) and the cw data of Lovett (750, 1500, and 3000 Hz) in the Pacific Ocean at about 30° N. The third relaxation is consistent with the Atlantic Ocean data of Thorp. The new relaxation may be the one predicted by Garland, Patel, and Atkinson for MgHCO3 for which αλmax=0.58×10−6 at 9 kHz at 25°.

Sound absorption in sea water: A third chemical relaxation

Sound absorption in sea water is dominated by chemical relaxations involving magnesium sulfate at higher frequencies (relaxation frequency fr∠100 kHz) and boric acid at lower frequencies (fr∠1 kHz). A third intermediate-frequency relaxation involving magnesium and carbonic acid has been found. Like the boric acid relaxation, the absorption increases exponentially with pH in the sea water range.