The research was motivated by a persistent gap in the literature. Traditional dense mortars are mechanically stiff and thermally conductive but absorb almost no sound because of their compact microstructure, which drives up both energy demand and acoustic discomfort in buildings. Foamed mortars, whose densities can range from 400 to 1800 kilograms per cubic meter and whose thermal conductivities fall between 0.10 and 0.35 watts per meter-kelvin, offer a lighter and quieter alternative. Yet most previous studies examined a single aggregate within a single binder system, making it impossible to disentangle how aggregate shape, matrix architecture, and fiber inclusions jointly control multifunctional performance. The researchers, led by Elif Tuğçe Kocabeyoğlu and Fuat Köksal of Yozgat Bozok University together with Osman Gencel, set out to close that gap with a controlled benchmark of twenty mixtures.

The experimental design was deliberately systematic. Five fine aggregates in the 0–2 millimeter range were selected: crushed sand as the conventional reference, expanded clay, pumice, expanded vermiculite as lightweight mineral aggregates, and waste rubber granules as an eco-friendly damping additive. Each aggregate was combined with a normal mortar matrix and a foamed mortar matrix, and each of those ten combinations was produced with and without 15 kilograms per cubic meter of flax fiber. The flax fibers, used as received without surface treatment, had tensile strengths of 800 to 1500 megapascals, an elastic modulus of 50 to 70 gigapascals, and a density of just 1.54 grams per cubic centimeter. Foamed mixtures were produced by introducing preformed foam with a density of 80 grams per liter, generated from a plant-based foaming agent and compressed air. All specimens were water-cured for 28 days and then subjected to an extensive battery of tests covering physical, mechanical, thermal, acoustic, and microstructural behavior.

The physical results traced a clear hierarchy. Dry unit weights ranged from 2179 kilograms per cubic meter for the dense crushed-sand reference mortar down to just 562 kilograms per cubic meter for the flax-reinforced foamed vermiculite mixture, the lightest material in the entire series. Porosity followed the inverse pattern, climbing from 12.27 percent in the dense reference to 55.18 percent in the foamed vermiculite composite. Expanded vermiculite proved the most efficient single ingredient for reducing both density and thermal conductivity, a consequence of its lamellar, highly porous particle structure. Flax fiber addition consistently lowered unit weight and raised water absorption, capillary uptake, and porosity, because the fibers disrupted particle packing and introduced interconnected voids during mixing and drying.

Mechanical performance told a more sobering story. The crushed-sand reference mortar dominated every strength measure, achieving a compressive strength of 49.91 megapascals, a flexural strength of 10.6 megapascals, a splitting tensile strength of 5.32 megapascals, and an elastic modulus of 10.83 gigapascals. Foaming cut these values substantially; the foamed crushed-sand mixture reached only 21.17 megapascals in compression. Lightweight aggregates imposed further penalties, with vermiculite mixtures dropping to roughly 2.4 megapascals in compression. Intriguingly, the rubber mixtures recorded the lowest ultrasonic pulse velocities despite moderate porosity, because soft, hydrophobic rubber particles form weak interfacial transition zones that scatter and attenuate ultrasonic waves far more effectively than stiffer mineral pores. The study demonstrates that mechanical behavior cannot be predicted from porosity alone; the stiffness and interfacial compatibility of the aggregate skeleton matter just as much.

Thermal conductivity results delivered the study’s most dramatic numbers. The dense reference mortar conducted heat at 2.00 watts per meter-kelvin, but the foamed vermiculite composite with flax fiber achieved just 0.198 watts per meter-kelvin, a tenfold reduction and the lowest value of the entire series. This performance arises from three stacked mechanisms: the lamellar porosity of vermiculite particles, the air-void cellular structure of the foamed matrix, and the microstructural discontinuities introduced by fiber incorporation. Flax fiber alone reduced conductivity measurably even in dense mixes, dropping the crushed-sand mortar from 2.00 to 1.441 watts per meter-kelvin. Rubber aggregates also performed well thermally, at 0.371 watts per meter-kelvin in the best foamed variant, thanks to their low intrinsic conductivity and poor interfacial bonding with cement paste.

The acoustic findings overturned a common assumption. Noise reduction coefficients increased with specimen thickness across all mixtures, with 5-centimeter samples consistently absorbing the most sound, and foamed mortars outperforming their dense counterparts. But when the team moved from vermiculite to rubber mixtures, porosity dropped sharply while sound absorption rose. The flax-fiber-reinforced foamed rubber mixture, F-KAU-KL, achieved the highest noise reduction coefficient of the series at 0.5740 for 5-centimeter specimens, despite having lower porosity than the vermiculite foams. The researchers attribute this to the viscoelastic damping of rubber granules combined with the additional microstructural discontinuities created by flax fibers, which multiply internal friction and scattering pathways for sound energy. Porosity helps, but it is not the whole story.

Scanning electron microscopy and X-ray diffraction provided the microstructural evidence behind these macroscale trends. The dense reference mortar showed a compact, well-interlocked C–S–H gel network with minimal voids, explaining its strength and high conductivity. Fiber-reinforced samples revealed grooved flax fibers with partial debonding and interfacial gaps that created microchannels, reducing strength but opening pathways for thermal and acoustic insulation. The foamed rubber composite displayed large irregular voids between 50 and 390 micrometers, weak rubber-paste interfaces, and fragmented hydration products, a morphology that cripples load transfer but excels at dissipating sound and blocking heat. XRD confirmed the mineralogical signatures of each aggregate, from the quartz-rich expanded clay to the layered vermiculite and phlogopite of the GV particles and the amorphous rubber matrix with crystalline vulcanization residues.

The authors are careful about practical limits. The mechanically weak mixtures, particularly the foamed vermiculite and foamed rubber composites, are suitable only for protected non-load-bearing uses such as interior acoustic linings, insulation layers, lightweight infill, and sandwich-panel cores, and must never be used in load-bearing masonry, slabs, or impact-exposed elements. They also note that their comparisons are descriptive rather than statistically validated, since inferential analyses were not performed, and they recommend future work with confidence intervals, analysis of variance, dynamic mechanical analysis, and detailed pore evaluation. Within those constraints, the study delivers a rare integrated performance map: F-GV-KL is the material of choice when minimum density and maximum thermal insulation are the goals, while F-KAU-KL offers the best combination of low weight, high sound absorption, and moderate insulation for acoustic panels. By benchmarking five aggregates, two matrices, and a natural fiber in a single framework, the research gives designers of sustainable, acoustically enhanced buildings a quantitative basis for choosing the right mortar for the right job.

Subject of Research: Hybrid flax fiber reinforced normal and foamed mortars with five aggregate types for sustainable, acoustically enhanced non-load-bearing applications

Article Title: Comparative evaluation of hybrid flax fiber reinforced normal and foamed mortar using five aggregates for sustainable and acoustically enhanced non-load-bearing applications

Article References: Comparative evaluation of hybrid flax fiber reinforced normal and foamed mortar using five aggregates for sustainable and acoustically enhanced non-load-bearing applications. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: foamed mortar, flax fiber, expanded vermiculite, waste rubber, sound absorption, thermal conductivity, lightweight aggregates, porosity, sustainable construction, non-load-bearing applications, cement composites, acoustic panels

The study, published in the journal Ocean Dynamics, was carried out by researchers at the Naval Physical and Oceanographic Laboratory of India’s Defence Research and Development Organisation in Kochi, together with a colleague from the Kerala University of Fisheries and Ocean Studies. Their central finding is deceptively simple: the upper 50 meters of the Bay of Bengal has warmed over the past several decades, while the layer between 100 and 200 meters has, in contrast, cooled. That vertical seesaw matters enormously for acoustics, because the speed of sound in seawater depends on temperature, pressure, and salinity, and the fine-grained structure of those variables determines where sound bends, bounces, and fades.

Sound speed in the ocean increases with both temperature and pressure, so a warm, well-mixed surface layer typically acts as a duct that traps near-surface sound and channels it over long distances with comparatively little loss. Below that, the thermocline produces a minimum in sound speed, and the depth at which that minimum occurs defines the sonic layer depth, a critical parameter for anyone predicting how a sonar signal will behave. When the research team examined long-term changes in the isothermal layer depth, the mixed layer depth, the barrier layer thickness, and the sonic layer depth, they found substantial variability across the record. The in-layer and below-layer sound-speed gradients, the quantities that acousticians use to characterize how rapidly sound speed changes with depth, also shifted appreciably, signaling that the surface-duct characteristics of the bay are not fixed features but moving targets.

Perhaps the most striking physical result is the consistent increase in upper-ocean heat content. This echoes a broader pattern documented across the tropical Indian Ocean, which has emerged as one of the fastest-warming ocean basins on the planet, driven by a combination of greenhouse forcing, weakening of evaporative cooling, and changes in monsoon-driven mixing. In the Bay of Bengal, the peculiar stratification created by massive freshwater discharge from the Ganges-Brahmaputra river system already produces a thick barrier layer that isolates the mixed layer from deeper waters. Long-term warming of the surface and cooling of the 100 to 200 meter layer intensifies this stratification, which the authors note can sharpen the acoustic transitions at the base of the sonic layer and alter how much sound leaks out of the surface duct into the deeper ocean.

Temperature, however, is only half of the story. The researchers also assembled surface pH observations from the World Ocean Database and from the RAMA/BOBOA mooring array, a long-running buoy stationed in the bay that has tracked the carbonate chemistry of the surface ocean. Their analysis reveals a declining tendency in surface pH in recent decades, mirroring the rise in atmospheric carbon dioxide. As carbon dioxide dissolves in seawater it forms carbonic acid, lowering pH in a process known as ocean acidification, and the Bay of Bengal appears to be tracking this global signal, with regional studies suggesting that atmospheric pollutant deposition may even accelerate acidification along its coasts.

Why would a chemist’s measure of acidity interest an acoustician? The answer lies in the peculiar chemistry of sound absorption at mid-to-high frequencies. Below roughly 10 kilohertz, the dominant absorbers of sound in seawater are not water molecules themselves but two dissolved solutes: boric acid, which governs absorption below about 1 kilohertz, and magnesium sulfate, which dominates above it. The boric acid relaxation that converts acoustic energy into heat depends critically on pH. As pH falls, the boron chemistry shifts, and the absorption coefficient drops, particularly at frequencies above 1 kilohertz. This counterintuitive consequence of acidification, first highlighted by researchers at the Monterey Bay Aquarium Research Institute more than a decade and a half ago, means that a more acidic ocean is literally a noisier one, because sounds that would once have been absorbed over tens of kilometers now carry farther.

Using standard sensitivity calculations based on the widely applied Francois-Garrison absorption model, which was derived from careful laboratory and field measurements of sound absorption in natural seawater, the team showed that the observed decrease in pH reduces the calculated sound-absorption coefficient in the bay, with the effect most pronounced above 1 kilohertz. They also found that salinity variability alone produces appreciable changes in absorption, a reminder that in a bay whose surface salinity swings dramatically with the monsoon and river discharge, even the salt content of the water can leave a measurable fingerprint on acoustic losses.

To translate these chemical and hydrographic changes into a concrete acoustic prediction, the researchers turned to BELLHOP, a well-established ray-tracing propagation model used throughout the underwater acoustics community. Running the model under contrasting pH conditions representative of the observed long-term trend, they found lower transmission loss under the lower-pH scenario, meaning that sound would persist over greater distances before being absorbed. Critically, the difference between the two scenarios grew with propagation range under the prescribed acoustic environment, because absorption is a cumulative process: small per-kilometer changes compound over tens and hundreds of kilometers into substantial end-to-end differences in signal strength.

The authors are careful to emphasize the limits of what they have shown. All of the acoustic results in the study are model-derived, and the team states plainly that quantitative validation will require direct field measurements of acoustic absorption and transmission loss in the bay itself. Model predictions of how acidification reshapes absorption are only as good as the environmental inputs and the underlying absorption formulations, and the Bay of Bengal’s complex, strongly stratified water column, laced with internal waves and monsoon-driven variability, presents a formidable challenge to any propagation model. Nonetheless, the direction of the effect aligns with the global literature, and the bay’s particular combination of warming, intensified stratification, and falling pH makes it a natural laboratory for studying the ocean-climate-acoustics nexus.

The implications extend well beyond the technical literature. Navies and ocean-monitoring agencies that calibrate sonar performance using historical acoustic conditions may find their predictions drifting as the water column changes beneath them. Climate models and acoustic forecasting systems may need to incorporate carbonate chemistry alongside temperature and salinity to remain accurate. And for the whales, dolphins, and other sound-dependent animals that navigate and forage in these waters, a modestly more transparent ocean could subtly alter the reach of both their own calls and the growing din of human activity. What this study makes clear is that climate change is not only warming the ocean’s surface and souring its chemistry; it is also, almost invisibly, changing the very medium through which the sea speaks.

Subject of Research: Long-term temperature and pH variability in the Bay of Bengal and its implications for underwater acoustic propagation.

Article Title: Long-term variability of temperature and pH in the Bay of Bengal and its potential implications for underwater acoustics

Article References: Long-term variability of temperature and pH in the Bay of Bengal and its potential implications for underwater acoustics. (n.d.). https://doi.org/10.1007/s10236-026-01851-2

Image Credits: AI Generated

DOI: 10.1007/s10236-026-01851-2

Keywords: Bay of Bengal, ocean warming, ocean acidification, underwater acoustics, sound absorption, mixed layer depth, sonic layer depth, transmission loss, Indian Ocean, carbonate chemistry, EN4 dataset, BELLHOP model