Sound shingle

Sound shingles were a Gallifreyan surveillance technology which resembled white gravel but amplified sound. They were placed along the Prydonian Wall to help alert the laser surveillance towers to intruders. (PROSE: Gallifrey: A Rough Guide)

Behind the scenes

 * Sound shingles originated in Dave Martin's Find Your Fate novel The Garden of Evil, where they were puncutated "sound-shingles".