听力原文：W1: I know youre all excited about our class project. But before we get started on the design of our mural, lets review some of the technical requirements for murals. What factors do we need to keep in mind? Yes, Lydia? W2: The mural has to be permanent. It has to last as long as the building. And it has to be sturdy enough to put up with conditions like the walls being washed. W1: Good points. Yes, David? M: It should have a flat finish so you can look at it from all angles without seeing any glare. And the design should be laid out so you can see it while youre walking rather than standing still. W1: Yes, those are all things we need to bear in mind. We want people to take in our mural—all of it—from a moving position, since theyll be observing it as they walk through the lobby to the elevators. And lets not forget that our mural is more than just a large-scale work of art done on a wall instead of a canvas. The term 'mural' implies a distinct feeling or nature, a quality thats appropriate to the architecture and the function of the room. A mural is an integral part of the building. If its painted in an existing building, like ours will be, it should be planned to fit into the architectural design, instead of giving the impression that its a surface decoration. Now, what are some of the technical requirements for our wall? M: Obviously, the wall shouldnt move. It shouldnt shrink or settle because that could cause cracks in the paint or plaster. W1: Yes, its vital that our wall be perfectly immovable. And for murals like ours—remember, our mural will be on the interior side of an exterior wall—another important requirement is that dampness should be excluded. Dampness can result from water seeping through porous materials or through joints, corners, or flaws in the building construction. When water leaks through a wall, it may dissolve the salts that are in the building materials. When the moisture dries, the salts are deposited in the form. of white, moldy-looking film on the surface of the plaster. This greatly weakens the plaster, which can crumble or crack, and, of course, its disastrous to the mural. W2: Can moisture make the paint peel off? W1: Yes, it can. When moisture penetrates a wall coated with paint, it causes the plaster to expand and the paint to peel off—either in small flakes or in big pieces. And we dont want that happening to our work, so well need to take measures to prevent it. Our goal will be to reduce expansion of the plaster and to lessen the condensation of moisture on the surface. So our first rule is to exclude dampness as much as possible. M: Dont we also need to guard the plaster from extreme changes in temperature? W1: Yes. So, how are we going to do this? M: We could create an air space between the outer wall of the building and the plaster. We could do this by building a screen or, urn, like a false Wall just inside the exterior wall, and then we put the plaster for the mural on this false wall. Thered be like an inch of air space between the false wall and the exterior wall. The air space would have an insulating effect. What I mean is, it would slow down any temperature changes on the plaster. W1: Yes, youre correct about the insulating effect of an air space. Now, to do this—its called 'furring' the wall—what we do is, we attach thin strips of wood to the exterior wall, and then we build another wall on top of this. The furring of the wall creates an air space of one-half to one inch between the outer and inner walls. Thats enough to protect our mural from moisture and temperature changes, as well as the mechanical strains and stresses of the wall itself.Art Murals Why is the class discussing the technical requirements of murals?