Old noise: new sound insulation will make flights twice as quiet for passengers

Perm Polytechnic University scientists have developed cone—shaped sound-absorbing panels for aircraft that reduce aircraft engine noise by 40 decibels, which is twice as efficient as existing materials. Currently, the average level in the cabin of the liner is about 80-85 dB, so such a decrease can significantly increase passenger comfort and reduce the harmful effects on health during flights. See the Izvestia article about how the new development can improve the flight experience for Russians.

How will the new development help

Scientists at Perm National Research Polytechnic University (PNRPU) have developed a new design of sound-absorbing panels for aircraft engines. It allows you to reduce the noise level and at the same time reduce the weight of the structure. Instead of traditional hexagonal honeycombs, the researchers used cells in the form of straight and inverted cones. Each element absorbs sound at its own frequency, and the combination of cells of different sizes allows you to suppress noise over a wide range.

After long flights, many passengers experience buzzing in their ears and fatigue. This is due to the fact that the noise level in the cabin reaches 80-85 dB — about the same as next to a busy highway. For pilots and flight attendants, these are working conditions that can affect hearing over time. Noise also worries people living near airports. According to the developers, the new technology is able to reduce these risks. The panel samples were printed on a 3D printer to accurately reproduce the complex shape, after which they were tested on a special installation.

— This is a soundproof laboratory installation, where a test noise of 130-150 decibels is created, approximately like that of a working aircraft engine. This is how we tested how the new design behaves in conditions close to real flight. First, the panel was tested without air flow, then at a speed close to flight speed. Microphones recorded how much noise she was absorbing. For comparison, an ordinary cellular panel was tested in the same way," said Karina Akhunzyanova, a junior researcher at the NIL PAKM Department of Mechanics of Composite Materials and Structures at PNRPU.

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According to her, the tests were carried out at frequencies from 1300 to 6200 Hz — it is in this range that the noise of aircraft engines is formed. Experts evaluated at which frequencies the panels work more efficiently and how their effectiveness changes with the appearance of an air flow.

Tests have shown that a conventional cellular panel without air flow reduces noise well at low frequencies by 51-54 dB, but at high frequencies its efficiency drops to about 14 dB. When there is an air flow, the indicators decrease to 13-23 dB.

— The flow-free cone design worked stably at all frequencies and reduced noise by 38-40 dB. This is a good result, which allows you to significantly reduce the engine volume. When there is an air flow, the indicators hardly change — from 32 to 44 decibels. Unlike a honeycomb panel, this design practically does not react to the air flow and remains effective in real conditions. The new panels make it possible to make airplanes quieter without increasing mass and increasing the cost of production," said Pavel Pisarev, Head of the NIL PAKM Department of Mechanics of Composite Materials and Structures at PNRPU, Candidate of Technical Sciences.

According to scientists, the new design is about one and a half times lighter than its multilayer counterparts. At the same time, the sound absorption level is on average 20 decibels higher than that of classic single-layer panels, and it shows the highest efficiency in the frequency range of 2000-5400 Hz.

The development is also less sensitive to small manufacturing errors. Conventional panels are configured for three engine operating modes: takeoff, landing and cruise flight, so even minor deviations can reduce their effectiveness. The new design is designed for a wide frequency range, so such errors practically do not affect the quality of noise reduction, Pavel Pisarev noted.

Value for the modern aviation industry

The development may be in demand if the stated characteristics are confirmed in practice, says Vyacheslav Bogachev, head of the laboratory at the NTI Center for Digital Materials Science: New Materials and Substances at the Bauman Moscow State Technical University.

— If everything is really as described, then the technology will be in demand. The hours-long hum in airplanes is very tiring for passengers. It would also be useful to refine the solution in terms of vibration reduction. In general, we don't have many specialists dealing with acoustics problems," he believes.

According to the expert, a 38-40 decibel reduction in noise will be very noticeable to human hearing. The decibel scale is not linear, it reflects subjective feelings, so we are not talking about a twofold, but about a fourfold decrease in perceived volume, he added.

The development is especially important in the context of import substitution, says Denis Melikov, General Director of the BAS Research and Production Center in the Tomsk Region, market expert at NTI Aeronet.

— Noise remains a serious problem when operating at high power, and in some cases it can act as an unmasking factor when using UAVs. In fact, this technology has no direct analogues: existing noise reduction solutions relate to other types of acoustic structures," he noted.

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According to Artyom Bogatyrev, market expert at NTI Aeronet, the key advantage is the use of 3D printing, which allows you to form a complex internal geometry of the panel in one operation.

— The main achievement of the development is not just a new panel, but an innovative way of manufacturing it. The use of a multi-height 3D-printed matrix allows you to form a complex internal geometry of the panel in one operation. This solves a long-standing production problem.: high cost and high percentage of defects when creating multilayer structures with cells of different heights, which are necessary for effective noise suppression in a wide range.

He added that similar solutions are already being applied in Russia. In particular, we are talking about fiberglass plastics developed by the Obninsk Scientific and Production Enterprise "Technology" and used for PD-8 and PD-14 engines.