The Wavelength of Doppler-shifted wave is found using  the following formula: `lambda'=(1-v_s/v)lambda`, where: 

• `v` = velocity of waves
• `v_s` = velocity of the source
• `lambda` = distance between pulses
• `lambda'` = wavlength

References

19.5 The Doppler effect by Benjamin Crowell, Light and Matter  licensed under the Creative Commons Attribution-ShareAlike license.