Drum sand is a phenomenon of sound-emitting sands which have been found on many Neta-type planets, particularly those of the C and 50 to 60 subclassifications.
This name is used only on Arrakis and local nomenclature depends upon the nature of the sound emitted, e.g. that of an indigenous musical instrument.
Most musical sands produce a single tonal sound which decays exponentially with time after being stepped upon or otherwise impacted. A second step produces a similar sound with similar monotonic decrease in intensity. Drum sand, however, emits a series of low-frequency beats from a single step. These beats do die out exponentially, but the pulsating (drumming) character is quite distinctive.
It was known that four conditions are necessary for a sand to sing.
- sand grains must be of approximately equal size.
- the grains must be bonded together. This bonding is most commonly produced by the chemical deposition of water-soluble salts on the grain surfaces.
- the packing density must be uniform.
- the underlying bedrock must be essentially parallel to the sand surface.
Any impact produces vibrations, but when the above conditions are met this impulse can create resonance and a tone is produced. The sand layer vibrates as a unit. If the ratios of mean grain spacing to grain diameter and mean grain spacing to bed thickness fall within prescribed limits, the tone will be audible to human ears. These several conditions are not normally met and thus singing sands are not particularly common. But if these conditions are met then we have an excellent analog of a musical instrument, albeit without the marvelous tonal and expressive capabilities of true musical instruments (including the human voice).
Dorit Pachtra recognized that under certain conditions the ratios noted above could have values that resulted in: a pattern of constructive and destructive interference in the sound waves. This would cause a pulsating sound from a single impact or step.
Although he is generally given credit for discerning the mechanism, the record is a bit fuzzy here, but the academic discussion of who should receive credit is of little import and as Ghralic states, "It is the advancement of our knowledge that counts."
Drum sand was a surface feature of some dunes, a crystalline formation which caused them to amplify the footsteps of a traveller, attracting the attention of worms. It is also powdery and causes a traveller to sink into it, further increasing the danger. This is what happens to Ishanti in Sisterhood of Dune when she distracts a worm that is about to devour Griffin Harkonnen.