A child’s train whistle replicates a classic conductor’s whistle from the early 1900s. This whistle has two open-open tubes that produce two different frequencies. When you hear these two different frequencies simultaneously, you may have the perception of also hearing a lower note, called a difference tone, that is at the same frequency as the beat frequency between the two notes. The two tubes of the whistle are 12 cm and 11 cm in length. Assuming a sound speed of 350 m/s, what is the frequency of this difference tone?