curtain
幄 centers on a hanging fabric that encloses or screens a space: a curtain.
幄 combines 巾 (cloth) with 屋 (roof/building), suggesting a cloth enclosure within a building, i.e., a curtain.
A cloth (巾) hung inside a room (屋) to divide the space: that's a curtain.
For アク, imagine pulling back a curtain with a sharp 'ack' sound: ack -> アク, as the fabric slides open.