An apartment complex consists of five floors, with several apartments over four floors, and an attic on the top floor. In the attic an air circulation system is installed to provide circulation in the bathroms of all the apartments in the building. The circulation system is driven by an air fan distributing air through pipes going to all apartments. The air pipes are cemented in to the structure of the building itself, and in some apartments a low frequency structure born noise with the same frequency content as the frequency of the air fan can be heard.
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. Measurements were made both in the attic at the source location, and also in the bedroom and bathroom of one of the apartments three floors below. The Nor848A-10 was chosen for the recordings over the more compact and mobile 40 cm and 128 microphone Nor848A-4, mainly due to the low-frequency nature of the noise. An array that is larger in size will have better resolution for all frequencies, and will also be able to go lower in frequency content. Even though the Nor848A-10 has a diameter of 1.0 meter, it weighs in at only 11 kg with tripod mounting brackets, and could easily be mounted on a tripod for inspection of the air fan in the attic, or laid down on the floor, or a bed or similar for inspection of the roof in the bedroom and bathroom of the apartment.
Looking at the recordings from the attic, it was clear that the intake fan was the main culprit having a dominating fundamental frequency at 200 Hz. Also several harmonics of the fundamental frequency could be seen in the frequency spectrum.