How is sensitive flame method used for detection of ultrasonic waves?

Sensitive flame method:

A narrow sensitive flame is shifted along the medium. At the situations of antinodes, the flame is steady. But at the positions of nodes, the flame flickers since there is a change into pressure. Within this method, positions of nodes and antinodes can be determined in the medium. The average distance among the two adjacent nodes is equivalent to half the wavelength. When the value of the frequency of ultrasonic wave is identified, the velocity of ultrasonic wave propagated by the medium can be computed.