**Question:** Is it possible to conceal one’s farts by walking?

**Short answer:** No.

**Long answer:** This week’s question comes from a reader who wonders whether walking fast could somehow change the sound of a fart, rendering it undetectable to the ear. The questions of both concealing and revealing farts have been the topics of previous posts.

The question is related to the famous Doppler effect, in which motion between a source and an observer causes changes in sound frequency. The effect was first described by Christian Doppler in 1842. Here is a handy illustration, courtesy of Wikipedia:

The change in frequency caused by the Doppler effect is well approximated by the following:

Here *f** _{0}* is the actual frequency,

*v*is the velocity of the observer relative to the source, and

*c*is the speed of sound. In previous posts, we have demonstrated that farts are characterized by peak sound frequencies in the range 200 – 350 Hz. Any sound outside that range is unlikely to be a fart.

Thus, for a typical fart with a peak frequency of *f** _{0}* = 275 Hz, we would need

*Δ*

*f*to be 75 Hz to shift the frequency outside the range of a normal fart sound. Rearranging the above equation, we have:

which yields a relative velocity of *Δv* = 93.8 m/s, which is 337.7 km/h. So, walking or biking would not be sufficient to distort fart sounds beyond the range of detectability. However, someone who farted out the window of high-speed train would be able to escape blame.