Rough surfaces can introduce large losses in the measured signal of transmitted ultrasonic beams detected with phase‐sensitive transducers. It is shown that the ensemble‐averaged transmitted wave can be exactly described by a frequency‐dependent transmission coefficient for a plane wave normally incident on a single, planar, fluid–solid interface with spatially uniform, random roughness. One consequence of this is that the roughness‐induced, spatially averaged signal loss is largely independent of the experimental setup; such as, for example, the transducer’s size or shape, its distance from the rough interface or for weak focusing, the focal length. We demonstrate this independence from experimental setup by presenting measurements (for near‐normal incidence in a water‐immersion experiment) of the roughness‐induced frequency‐dependent (2–8 MHz) transmission loss of the spatially averaged signal for focused, planar and array probes at a variety of distances from a specially prepared plate that had been roughened on one side. The experimentally measured single‐transmission loss was found to be nearly independent of the transducer parameters described above and, for the sample studied, to be well predicted by the phase‐screen approximation to the rough‐surface transmission coefficient,TR≊exp(−Δk2h2/2), where Δkis the change in wave number at the fluid–solid interface andh≊54 μm is the root‐mean‐squared (rms) height of the surface.