Here's why:

* Frequency is the number of vibrations per second. Slow vibrations mean a low frequency.

* Sound waves are created by vibrations that travel through a medium (like air).

* Low-frequency sound waves correspond to low-pitched sounds. Think of the deep rumble of a bass drum or the low hum of a large engine.

So, the slower the vibration, the lower the frequency, and the lower the pitch of the sound produced.