A grass collection container (14) for a grass mowing machine (10) includes a bag or hopper attached to a support frame under the plastic top cover (25) with air permeable panels (18, 19, 20), and a flexible sound barrier curtain (28) attached to the support frame over the air permeable panels (18, 19, 20). The flexible sound barrier curtain (28) has a thickness of between about 1 mm and about 5 mm. An air passage is provided between the bag or hopper and the flexible sound barrier curtain (28).