Identifying the cause of an annoying tone in a cruse shit restaurant

Three weeks after the maiden voyage of a cruise vessel, the operator reported an issue: an annoying noise was affecting several decks at the very aft of the ship. One of these decks housed a restaurant where guests had to endure a persistent tonal noise. The other decks contained premium cabins with spectacular views but were also affected by the sound disturbance. Unsurprisingly, guests were dissatisfied, feeling that the reduced comfort did not justify the high price of their cabins. Alarmed by these complaints, the ship operator contacted us for assistance. A remote analysis ruled out simple explanations, leading to the decision to conduct an on-site measurement investigation.

To identify the source of the disturbing noise and resolve the issue efficiently, we began with a structured, step-by-step investigation onboard the vessel. Through detailed measurements, analysis, and design review, we were able to trace the problem back to its root cause and recommend practical solutions.

The on-site investigation

Two days after the initial contact, our acoustic engineer boarded the vessel, which was docked in a very pleasant and sunny port. Initial interviews with the Chief Engineer, the Superintendent, and the Hotel Manager confirmed that the disturbing noise had been present since the vessel’s maiden voyage. Despite previous attempts by the shipyard, the source of the issue had not been identified or resolved.

The noise had a tonal quality that remained constant regardless of the ship’s speed or other operational conditions. It started at around 6 knots and was unaffected by steering, sea state, loading conditions, temperature, or which generator sets were in use. Notably, the sound appeared to come from the large windows in the cabins and restaurant areas.

Comprehensive measurements

With this information in hand, our engineer conducted a thorough series of measurements throughout the vessel. It quickly became clear that only the aft section of the ship was affected and that the noise levels were beyond acceptable limits. Airborne and structure-borne noise measurements were taken inside the suites, near the windows, throughout the restaurant, and at technical points including the electric motor, the structure above the shaft, and above both rudders and propellers.

Key findings

The data revealed that the noise was strongest near the windows and carried a distinct tone at approximately 270 Hz. On the port and starboard sides, the tone differed slightly—by 0.7 Hz—indicating a mechanical source rather than environmental variation. The tones remained stable regardless of operating conditions and did not match the frequency of any rotating equipment onboard. The highest noise levels were recorded directly above the rudders and propellers.

These findings led to a clear conclusion: the source of the noise was resonance caused by either the rudder or the propeller. Vibrations were being transmitted through the ship’s insulated steel structure, with the large windows acting as amplifying surfaces that allowed the sound to enter the occupied spaces.

Identifying the root cause

To verify this hypothesis, we requested design drawings of the rudder and propeller. Analysis showed that the rudder's geometry closely matched the measured tone, and the 0.7 Hz difference was likely due to manufacturing tolerances between the two sides. Visual inspection of the rudder confirmed the problem: its shape differed slightly from standard designs, encouraging the formation of a Kármán vortex. This vortex generated resonating vibrations strong enough to excite the rudder structure itself.

Tailored solution options

Our acoustic consultant presented two potential solutions aimed at disrupting the formation of the vortex behind the rudder. One option was to redesign the rudder shape during the next scheduled dry-docking. The second, more immediate solution involved installing a few 20 cm steel rods on the rudder by a diver during the next port stop. Both options were effective, and the operator chose to proceed with the implementation.

The implementation of our solution completely eliminated the annoying noise, restoring the calm and comfort expected in the premium cabins and restaurant.

Passengers could once again enjoy the serene ambiance and luxurious experience they paid for, without any disruptive tonal sounds. For the ship operator, this meant not only resolving immediate passenger complaints but also preserving their reputation for high-quality service.

Additionally, addressing the issue quickly and effectively minimized potential revenue loss from dissatisfied guests and reinforced trust in their ability to deliver top-tier experiences. The solution showcased how tailored, expert acoustic intervention can have a significant positive impact on both customer satisfaction and operational success.