Frequency of transistor oscillator is controlled by either of two lengths of nickel tubing, each looting inside form having two coil windings. One coil acts as driver and the other as pickup to provide feedback voltage for sustaining oscillation at control frequencies of 38. 5 and 41. 5 kc. Aluminum diaphragms on front ends of tubing increase acoust
ic output. -N. Frihart and J. Krakora, Ultrasonic Tones Select Tv Channels, Electronics, 31 :23, p 68-69.