Detecting Engine Vibration Using the X2-2 Logger
Someone's rocking my dreamboat, someone's invading my dream. We were sailing along, so peaceful and calm, suddenly something went wrong... (Otis Rene, Leo Rene)
...Is that the neighbor snoring in the next cabin...no, it's the engines cranking on this behemoth cruise ship heading for the Gulf of Mexico! For those who have experienced a cruise ship, the motion of the waves can be a soothing experience. Do you even notice the engines turning while on board?
Two X2-2 loggers were placed in an interior and an exterior cabin of a large passenger cruise ship. In each room, a logger was attached to a similar conduit found along the wall in both rooms. The vibration from an engine is narrow spectrum so this analysis focused on the low frequency range (<100Hz). The logger was configured to collect constantly at 512Hz, which allowed a 0-256Hz frequency band spectrum from the Fast Fourier Transform (FFT). Data collection started at approximately 7pm and ended at 8am three days later, when the loggers were recovered. An R script similar to the analysis used in the train vibration experiment processed all the data files.
The first plot summarizes the total RMS acceleration detected in the low frequency range (between 1-70 Hz) during the travel period in the exterior room. This method provided an easy indication of a when the engines were being used. Two peaks can be seen in the plot. The first peak at 10 Hz is likely due to the air handler for the room. This peak was apparent during the entire trip, even when docked without the engines operating. The second peak at 25Hz is the engine vibration, which became apparent each time the ship was underway.
The two plots below illustrate the base noise level reported by the X2-2 when the ship is at dock (left) and at sea (right). Click on the plots and an animated GIF will load showing the spectral response of the ship "At Sea" (be patient as this is a 3.5MB file).
Taking the experiment one step further, we compare the engine vibration detected at an interior cabin and an exterior cabin. The left plot below illustrates the sepectral response within the interior cabin and the right plot shows the exterior room at the same. Click on the plots and an animated GIF will load showing the spectrial response during transition from "Docked" to "At Sea".
The results suggest the exterior cabins are better coupled to the engine vibration than the interior cabins. Therefore, if silence you seek, then book an interior cabin. Although, we have limited data to support this claim....we may need to repeat this experiment with more cabin locations, different ships, and other cruise destinations ;-)