A tuning fork of unknown frequency is sounded together with a standard fork of 384 Hz, and 6 beats per second are heard. When a small piece of wax is stuck to the unknown fork, the beat frequency drops to 4 per second. What was the original unknown frequency? Solution: The unknown frequency is either 384 + 6 = 390 Hz or 384 - 6 = 378 Hz. Adding wax lowers the fork's frequency. If the unknown were 378 Hz, lowering it further would increase the difference from 384 Hz and the beats would increase - but they decreased, so the unknown must have been above 384 Hz. Original frequency = 390 Hz.