Ultrasonic memory is a form of acoustic delay-line memory. Acoustic delay-line memories were developed as rapid-access data storage devices in the mid-1940s, before affordable electronic memory in the form of vacuum tube arrays or transistors existed.

The acoustic delay memory principle is a simple one. A tube one or two meters long and about an inch across is filled with warm (liquid) mercury and capped at each end with a piezoelectric quartz crystal, one for sending waves through the tube and the other for receiving them. (Piezoelectric crystals expand or contract when a voltage is applied across them, and produce a voltage when stretched or squeezed.) Data to be stored in the delay-line memory are transmitted along the mercury in the tube as a series of sound waves. When these waves arrive at the receiving end, the crystal there translates them into electronic pulses. (Only...