Each single measurement is then validated by means of the Reduction Factor calculation. The sound reflection measurement method has been improved using a square 9-microphone grid not rigidly connected to the loudspeaker, an optimized alignment algorithm of free-field and reflected impulse responses, including fractional step shifts and least squares estimation of the best relative position, and a correction for geometrical divergence and sound source directivity. After some years and a large number of barriers measured, the original method has been significantly enhanced and validated in the frame of the EU funded QUIESST project, WP3. The in-situ measurement of sound reflection and airborne sound insulation characteristics of a noise barrier in Europe are currently performed following the CEN/TS 1793-5 European standard guidelines (last revision published in 2003 ). Processing the measured impulse responses between the loudspeaker and the microphones, global images of the local airborne sound insulation can be obtained in each frequency band. The entire surface of a 9 m wide, 4 m high sample barrier was analysed. On the opposite side of the barrier (noise source side) a loudspeaker was held in axis with the microphones, at fixed height, for each step. Preliminary measurements were made by moving in the horizontal direction a linear antenna of microphones, kept vertical, along the entire width of the barrier in 0.25 m steps. This work presents a preliminary study of a quick method for evaluating airborne sound insulation along the entire length of a noise barrier and finding weak points, like assembly errors. For this purpose it is necessary to carry out measurements along the entire extension of the noise barrier and not just at few positions taken as representative. (Sub-wavelength focusing in the near field, where different wave behavior dominates, has already been demonstrated.)īy showing that a simple Coke can array can focus sound waves beyond the diffraction limit, the study could have applications in providing energy for tiny electromechanical devices, among other uses.EN 1793-6 allows measuring the in-situ sound insulation of installed noise barriers at selected positions, but it would be desirable to check the quality of the installation or the decrease in performance over time over the whole length of a noise barrier. "Without being too enthusiastic, I can say is the first experimental demonstration of far-field focusing of sound that beats the diffraction limit," Lerosey told Nature News. Such focus is significantly beyond the diffraction limit. That’s enough time to allow the evanescent-like waves to build up into a highly focused spot of just a few centimeters, or about 1/25th the space of the meter-long wavelength of the original acoustic wave. While the normal sound waves scatter and disappear quickly, the evanescent-like waves take longer - about a second - to scatter out of the can. The resulting sound waves amplify the sound above the can from which the original sound came from, and cancel out the sound everywhere else.Īs this single can continues to resonate, sound waves inside the can become scattered. Here, the researchers figured out a way to amplify and capture the evanescent-like waves coming from the soda cans using a method called “time reversal.” They recorded the sound above a single can with a microphone, and then played this sound backwards through the speakers. Previously, scientists have used acoustic metamaterial lenses to amplify the evanescent waves in order to make them easier to capture. However, evanescent waves only exist very close to an object’s surface because they fade very quickly, making them difficult to capture. If researchers can capture evanescent waves, they can beat the diffraction limit. The small waves are similar to evanescent waves, which can reveal details smaller than a wavelength and be used to focus sound. As a whole, the lens generated a variety of resonance patterns, some of which emanated from the can openings, which are much smaller than the wavelength of the sound waves. When they turned the speakers on to play a single tone, the sound waves traveled around and inside the cans, causing the cans to collectively oscillate like organ pipes. Then, the scientists surrounded the Coke can array with eight computer speakers. To build the acoustic lens, physicists Geoffroy Lerosey, Fabrice Lemoult, and Mathias Fink at the Langevin Institute of Waves and Images at the Graduate School of Industrial Physics and Chemistry in Paris (ESPCI ParisTech) assembled a 7x7 array of empty Coke cans with the tabs pulled off.
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