Question

What will be the wavelength of ultrasonic waves in the air if the frequency of waves is 20khz

Answer 1

The wavelength of ultrasonic waves in the air

$$\lambda=\frac{\rm speed\; of\; sound}{\rm frequency}=\frac{c}{f}$$

$$\lambda=\frac{340\:\rm m/s}{20000\:\rm Hz}=0.017\:\rm m=17\: mm.$$