Geologist: There are 'singing' dunes in Słowiński National Park, but chances of hearing them are slim

According to scientists from the Polish Geological Institute, there are about 35 places on Earth where dunes 'sing' in a spectacular way - emitting loud, deep sounds that last for several minutes and can be heard from a distance of many kilometres. Some also compare it to whistling, moaning, squealing, screaming or barking.

This phenomenon is the result of specific atmospheric conditions and the structure of the sand, which cause resonance and vibration, leading to the formation of sounds. The sand must be hot and very dry to 'sing'. The grains must be of similar size, so the sand must be well sorted by the action of the wind. The shape of the grains should also be nearly spherical and their surface should be free of dust, impurities and organic matter.

'Sounds are generated when millions of grains flow in layers one above the other during +avalanches+, i.e. dune slope collapses. Each grain in an avalanche produces a local sound wave as it travels in a liquefied layer of sand, and when colliding with other grains, under the right conditions it generates vibrations that synchronize', the Polish Geological Institute - National Research Institute reports.

There is another theory regarding this phenomenon. Sound generation in dunes may be related to air escaping during sand packing on the leeward side of the dune. 'Such a sound resembles a rumble. It most often occurs when the process of grain packing is triggered by movement, for example walking on a dune', says Dr. Szymon Ostrowski from the Department of Engineering Geology of the Polish Geological Institute - National Research Institute.

According to the geologist, certain conditions must be met for a sound to be generated in this way. 'First, unpacked sands are only found on the leeward side of the dune; they are loosely piled here and potentially can be better packed. On the windward side, sand is compacted by the wind, so no further compaction occurs. Secondly, the sands must be very clean, without the clay fraction (good selection of material in dunes is typical). Thirdly, the material is extremely dry; even a small amount of water (humidity) will cause adhesion (causing sand grains to stick - PAP)', the geologist says. According to the researcher, the sound in the 'pack' of sand will appear only once - when it is packed.

Can the dunes in the Słowiński National Park 'sing'? 'The condition of dryness is met only in a thin layer near the surface. Theoretically, sound can occur when this layer packs up, but it is thin, so there is little air and the sound has very low intensity. Theoretically, the sound can occur when we have a long-term episode of a strong, dry wind, which will significantly remodel the dune (add a lot of sand). Then, an audible tone may appear on such a slope. In Polish dunes, the sound may be generated, but it will probably be too weak to make the effect spectacular (and perhaps even audible), although sensitive equipment would detect it. So the sound appears, but it is weak', concludes Dr. Szymon Ostrowski.

He emphasises, however, that in the Słowiński National Park you can only walk along designated trails that do not cover the leeward sides of the dunes, only the (already packed) ridges, so the potential for sand repacking disappears. 'And considering the intensity of tourist traffic, the chances of finding an untrodden leeward slope are close to zero', he believes.

'Singing' dunes in various places around the world can emit sounds exceeding 100 decibels, louder than most everyday sounds, such as typical city or traffic noise (about 80 decibels), a lawn mower (about 90 decibels) or a car horn (about 110 decibels).

According to the Polish Geological Institute, different dunes hum different melodies. 'At Sand Mountain, Nevada, dunes hum low B to C (60–70 Hz), two octaves below middle C of the piano. A little higher in the same octave, the Chilean Mar de Dunas sings low F (87 Hz). Ghord Lahmar in Morocco groans in low G sharp (105 Hz). The dunes near Al Ashkharah, a coastal city in Oman, harmonize with nine notes, from low F sharp to bass D (90-150 Hz)', the institute reports. (PAP)

Magdalena Barcz

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