The unit of frequency, kHz, is also equivalent to 1/kilosecond or 1/1000 seconds. The equation relating wavelength and frequency is:
c = wavelength*frequency, where c is known as the speed of light. But since we are dealing with ultrasound, we use the speed of sound. The speed of sound is dependent on the temperature. The correlation would be:
c = 331 + 0.6T = 331 + 0.6(20°C) = 343 m/s
Therefore,
343 = wavelength * 1/1000
wavelength = 343,000 meters