Zen Buddhism, silent meditation
禪 centers on deep, silent meditation, especially in the context of Zen Buddhism. The idea is a quiet, focused state of mind.
禪 combines 示 (altar, religious ceremony) with 單, which likely contributes the sound. The character originally referred to a type of ritual or abdication, and later came to specifically denote meditative practices in Buddhism.
The left side 示 suggests a spiritual or religious context. The right side 單 looks like a person sitting in a quiet, composed posture. Together, they depict silent meditation.
For ゼン, picture a Zen garden with raked sand and a meditating monk: Zen -> ゼン, perfectly matching the silent meditation of 禪.