An ultrasonic wave has a frequency of 50 kHz. If the speed of sound in air is 340 m/s, its wavelength is approximately:

We have λ = v / ν. Then λ = 340 m/s / 50,000. Then Hz = 0.0068 m. So Hz = 6.8 mm. Note that ultrasonic waves have very SHORT wavelengths, which is precisely why they are useful for high-resolution imaging — they can detect details on the millimetre scale (medical ultrasound, sonar, non-destructive testing of welds).