How can ultrasonic waves be detected Kundt’s tube method?

Ultrasonic waves can be detected by Kundt’s tube. There at the nodes, lycopodium powder collects into the form of heaps. The average distance in between two adjacent heaps is equivalent to half the wavelength. Such type of method cannot be used when the wavelength of ultrasonic waves is extremely small that is, less than few mm. But a liquid medium, instead of lycopodium powder, powdered coke is utilized to detect the position of nodes.