The sounds produced by desert dunes certainly vary, for travelers have compared them to a ship’s siren, a throbbing organ, the beat of drums, a trombone, and the twanging of a monster harp. In some instances, it seems that the softer tones are missing; others say that standing on the sand when it is singing is like resting on a huge stringed instrument while a bow is being drawn slowly across it. The note emitted by the desert sand is much lower than that of the beach sand and at a distance of 600 yards has been likened to the rumble of thunder.
Writing from Egypt a year or two ago, Lieutenant Colonel de Lancey Forth spoke of the following experience in the great sand dune country to the south of Siwa, Egypt: “I found, after a strong westerly wind had blown throughout the day and had banked the fine drift sand high up on the knife edged tops of the dunes, that sometimes in the evening, when the wind had died away, leaving a deep stillness in the air, this fine drift sand slid down in streaks over the coarse big-grained red sand which forms the steep slopes of the solid part of the dunes, and the friction of the one rolling over the other gave out a noise like distant rumbling thunder with a deep musical note as that of a cello in it.”
It is interesting that when Mr. St. John Philby was listening to the singing in the afternoon, one of his men referring to the desert spirits, said, “You wait, just wait till the evening and you will hear them letting off their big guns.”
Mr. Bertram Thomas, too, noticed the noise late in the afternoon, when the heat of the day was fading. Apparently, another factor also favors the close of day. During the day the wind blows the fine drift sand to the tops of the dunes, and toward sunset, when the wind usually dies down, it begins to roll down the slopes. Dryness seems an essential factor, for the ancient Chinese manuscript states that the Hill of Sounding Sand gave out notes only at the height of summer; and Mr. Philby likewise testifies that early in the morning, when the air was cool and the sand somewhat moist, he failed to elicit any response from it. And a few weeks later when there had been a little rain, there was no music in the sands.
Examination of the sand has not revealed any peculiarity linking the whistling sands of the beach with the booming sands of the desert. Samples from the dunes do not reveal any distinguishing features. The grains are no more uniform in size than those of many silent sands; and though clean sand sometimes seems to sing best, R. A. Bagnold heard it in a desert region where the sand was dirtier than usual and was wetted appreciably only once or twice in a decade.
A. D. Lewis found that when singing sand was taken from the Kalahari Desert to Pretoria, it lost its “voice” unless kept in airtight containers. But the quality could be restored by heating it to about 390° F.
Scientists generally agree that the sounds are caused by the rubbing of grains against each other, but as yet there is no real explanation of the mechanism by which they are produced. When further critical studies are made, the answers may be forthcoming. Meanwhile, when you are in the desert, keep an ear open for one of the strangest concerts ever to come from nature’s versatile music box.