Which tuning fork is most difficult to hear?

The tuning fork with a lower frequency is generally more difficult to hear, as lower pitches produce longer sound waves that can be less easily perceived by the human ear. For instance, a tuning fork vibrating at 256 Hz (middle C) is typically more audible than one at 64 Hz. Additionally, environmental factors and the listener's hearing ability can also affect the perception of sound, making some frequencies harder to detect.