The panel (1) includes a frame (4) with a fabric (5) stretchable over a lower side (6) thereof. An edge (8) of the fabric (5) is attached to a peripheral side (9) of the frame by means of a number of fastening brackets (11) distributed along the edge of the fabric. Firstly, each fastening bracket is attached to the peripheral side at a first lower position (15) in which the fabric is hanging loosely under the frame by hanging a hook-formed element (13) of the fastening brackets on a first lower protrusion (17) provided at the peripheral. Secondly, each fastening bracket is repositioned from said first lower position to a second upper position (16) in which the fabric is stretched over the lower side of the frame by hanging the hook-formed element of the fastening brackets on a second upper protrusion (18) provided at the peripheral side of the frame.