No experimental data have been published to date on the air/water/air acoustic waveguide with flat, sloped, or rough boundaries. In the conventional air/water/solid acoustic waveguide, the presence of the solid introduces the Scholte mode. The solid surface roughness causes not only energy leakage from the water waveguide to the solid, but it causes spreading of the energy over a wide time window. In order to study the normal modes interference and coupling phenomena in a rough waveguide, the problem would be greatly simplified if we could eliminate the Scholte mode and the energy leakage to the solid. A novel experimental technique is described which uses a thin stretched rubber membrane covered with a water layer from above. Controlled pressurized air below the membrane is used to counteract the weight of the water layer and obtain a flat surface at the bottom of the water waveguide. The water layer has a free top surface and a free bottom surface. Ultrasonic broadband pulses are propagated in the air‐suspended water waveguide. Theoretical and experimental results are compared for the free flat parallel boundaries waveguide ease. Scattering objects are placed on the surface of the membrane to form a rough or periodic boundary. Experimental findings are presented providing physical insights into the shallow‐water acoustic waveguide with rough boundaries.