The propagation of mechanical waves must have a medium. There are water waves and rope waves because their propagation media are different. The mechanical wave on the rope can only go back along the rope, while the wave in the water goes along the water surface, so the waveform has a great relationship with the medium. The waves on the beach are layer by layer, and the waves in the pond are round and round. If the vibration source that produces waves on the rope is placed on the water surface, it will produce waves round and round.

Let me explain to you that the spring goes through the slit. When water waves come to the slit, the short water surface of the slit will be used as a new vibration source, and new waves will be formed behind the slit, and the generated waves have the same frequency as the water surface at the slit.