Noise – especially traffic noise – is a major stress factor for local residents. However, the sound is difficult to tame. The usual conventional protection measures are thick insulating materials — in other words: huge, opaque, high-noise barriers surrounding roads and railways — or sound-absorbing acoustic resonators. However, both only solve the traffic noise problem to a very limited extent.
Tests in Austria
who – which Fraunhofer Institute for Structural Durability and System Reliability (LBF) The Austrian Autobahnen- und Schnellstrssen-Finanzierungs-Aktiengesellschaft (ASFINAG) of Austria has now found a solution to the ongoing struggle between the desire for lightweight construction and effective noise protection: ultrasonic materials. Thus, the researchers at LBF succeeded in turning even glass walls into good noise protection walls. A glass noise barrier equipped with its metastatic acoustic structures reduced sound transmission – at least in the lab – by up to 20 dB. If the sound pressure level drops by only six decibels, the human ear can only perceive half the noise as loud.
Like all metamaterials, vibroacoustic metamaterials are not materials in the physical sense. Metamaterials arise from an interaction between a substance and its structure. A conventional material, such as a glass noise barrier, becomes an acoustic vibratory dead wall by periodically arranging small resonator structures on its surface.
These resonators can be made of different materials, usually even from several materials. However, the deciding factor is not the material from which it is made. They only develop their effect when they are positioned less than half the wavelength of the air vibration they are meant to reduce. In the case of highway noise barriers, this is faint traffic noise, the average frequency of which is about 1000 Hz. Sound at 1,000 hertz has a good wavelength in air of 34 cm. Therefore, the resonators should be installed on noise barriers at a distance of less than 17 cm. Then they create what are called pause bands in the audio transmission. In these areas, sound waves cannot propagate further.
Portable structure to absorb noise
Ultimately, the acoustic vibratory metamaterial is an acoustic transducer. It converts airborne noise, which propagates through the air in sound waves, into structure-borne noise, which vibrates in solids. For example, in the case of earthquakes, the effect of noise in apartment buildings or vibrations in cars. This also includes ultrasound used to test materials. Then the noise barrier absorbs the noise waves carried by the structure.
Tests using vibratory glass bezel noise barriers were so successful that the laboratory prototype will soon be tested on Austrian highways and motorways. Since the sound-absorbing effect depends primarily on the structure, and not on the material of the resonators, the developers of Fraunhofer are confident in the possible service life. The material can be intentionally designed to be durable, strong and weather-resistant, and they commented on questions from the ASFINAG jury as part of the tender “Highways and Highways: Reducing Noise Pollution through Technology.” Likewise, nothing stands in the way of the increasingly important requirements of sustainability and recycling.
Once built, acoustic vibration noise barriers operate passively, they require neither maintenance nor technical support, it is even possible to modify existing noise barriers – more silence on the highway could be just a matter of proper installation of future resonators.
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