Venturi

That part of a tube, channel, pipe, etc., so tapered as to form a smaller or constricted area. A liquid, or a gas, moving through this constricted area will speed up and as it passes the narrowest point, a partial vacuum will be formed. The taper facing the flow of air is much steeper than the taper facing away from the flow of air. The venturi principle is used in the air horn throat of the carburetor. See air-valve carburetor, carburetor venturi, variable venturi.