When driving a caravan car, the driver could hear annoying noises from behind the driver’s cab in the living compartment. Even though the noises could be heard clearly, it was very difficult to discover the exact location and cause of the various annoying sounds due to overwhelming background noise.
When the car was standing still with the engine running, no extra noises could be heard, but especially two distinctive sounds that were different from ordinary engine and wheel noise were heard when the caravan was driving. The first sound had the characteristics of a loose screw sliding back and forth on a hard surface, so whenever the vehicle was turning, what sounded like a small piece of metal rolling from one side to the other on a hard surface could be heard. This was only heard when the caravan was in a turn, and not when driving straight. The second noise sounded like squeaking and creaking of wood, which made sense since a lot of the interior consisted of wood. This sound also appeared when the caravan was driving straight, and not only when turning. Since there were several pieces of the interior that consisted of wood, including the floor, it was not know what specific piece of interior made the noise. Based on previous listening experiments when the car was driving, it was thought that the indoor fridge which was positioned next to the entrance door might be root of of both noises.
The Norsonic Nor848A-10 1.0m acoustic camera with 256 microphones was used for the recordings. The camera was plugged into an external battery pack for easy transportation and mobility. Although the 40 cm Nor848A-0.4 acoustic camera would have been more mobile, at the time of the measurements it was unavailable. Nevertheless there was more than enough room in the caravan car to use the Nor848A-10.
The measurements were done with one person driving the caravan, and another placing the camera dish on a table and performing the measurements. As the array dish itself weighs in at only 11 kg, it was easy to control it with one arm, and starting and stopping a measurement in the software on the MacBook with the other arm. No tripod was used or needed for the measurements.
Initially the camera was pointed towards the fridge in the center of the living compartment of the caravan and measurements were made as this was thought to be the origin of the sources, In addition to looking at the coloring of the sources, it was also very useful to use the virtual microphone to listen to the sound field from a specific direction. In addition it worked very well to enable the band pass filter when listening in order to filter out background noise such as engine noise and wheel noise.
Seen in the image below is the inital result when filming at the fridge in the living compartment of the caravan with the caravan driving and turning, and when the distinct metal rolling sound was heard. As seen from the image there was no indication whatsoever that the sound originated from the fridge in the middle of the picture. Instead the coloring indicated that the sound originated from a different location. Also listening with the virtual microphone could more or less rule out the possibility that the fridge was the source origin. Instead the camera was positioned to aim to the left of the fridge where the coloring indicated a source.
The position of the true source was seen more or less instantly to be inside a cabinet positioned on the wall of the car. This true source location was also confirmed when listening with the virtual microphone on a recording, and even when holding the ear into the cabinet when driving.