Scientists have made active noise control and tested it in the layout of the room with full-size appliances window. It turned out that in typical urban transport frequencies with its help it is possible to mute up to 10 decibels of the average sound pressure level in room with open window. Further developments of such systems are likely to combine natural ventilation and noise reduction in the city. A study published in the journal Nature.
From the point of view of physics, sound is a wave of mechanical vibrations in any environment — e.g., in the air. The source of the noise causes a shift in the nearest particles of the medium, and those, in turn, pass it on to neighbouring particles — outrage spread, reaching the ear that causes us to hear sound. The sound volume is determined by the amount of displacement of the particles — that is, the wave amplitude: the stronger the vibrations, the louder they will sound.
Thus, to suppress the sound, it is necessary to reduce the amplitude of the oscillations — this is typically done using two main approaches: passive and active noise reduction. At passive noise cancelling on the path of wave propagation is a kind of obstacle — an object that somehow prevents the sound and thereby reduce noise. In the case of active noise control situation is quite different — this method involves the creation of a second sound wave in the opposite — that is, so that when overlaid on top of each other fluctuations, quenched each other. To completely drown out the noise, you need to exactly copy extinguish the outrage, but in practice to achieve this is quite difficult — you need to have time to write the suppressed sound or track is already reduced noise and adjust the parameters of the generated wave so to minimise it.
Scientists from Britain, Singapore and Japan under the leadership of Whang Lam (Bhan Lam) from Nanyang technological University have built a system of active noise reduction, which aims at drowning out external sounds in the room with the window open. To test it, the researchers used the prototyping room of the cubic form, the sides of which was a square plywood Board with a thickness of about 30 millimeters and a side length of 2.1 metres. In one of the walls of this design, the authors established a two-pane sliding window size of 1×1 meter and attached to the lattice (the presence of such characteristic of home Windows in South-East Asia).
On the window grille at a distance of 12.5 cm from each other, the authors fixed the noise canceling system of 24 speakers, each of which could generate the sound wave apart from the rest. External noise of different kind created with a major dynamics at a distance of two meters from the window, and in the middle between them is a reference microphone that capture outstanding sound vibrations and transmit them to the controlling system. At the training stage of tests of physics also used a system of 24 of the microphones 18 inches from the window inside the room — the algorithm used was configured noise-cancelling speakers so that the system has caught a minimum sound pressure. During the main measurements, this adaptive mechanism has not been used, and the parameters of generation of sound based on the signal from a reference microphone was recorded (based on data from the training stage).