In nature, bats use ultrasonic echolocation to find their prey, and at the same time, prey has also evolved defenses – some moths can effectively absorb ultrasonic waves through fine structures on their wings to avoid sound reflections revealing their position. This is the first time scientists have discovered acoustic materials in nature. Although moth wings are aimed at ultrasonic waves (the vibration frequency is greater than 20,000 Hz), their sound-absorbing principles are consistent with all kinds of sound-absorbing materials we see in our lives, but the latter Adjust the similar design to the frequency band (20Hz-20000Hz) in line with human hearing. Today, let’s talk about NVH-related foam materials.
Sound originates from the vibration of an object, and is a wave phenomenon that propagates through a medium and can be perceived by the human auditory organ. NVH refers to noise (noise), vibration (vibration) and harshness (harshness), of which noise and vibration are the most directly felt by us, while the harshness of sound is mainly used to describe the human body’s subjective perception of vibration and noise. feeling of discomfort. Since these three appear at the same time in mechanical vibration and are inseparable, they are often studied together.
As shown in the figure below, when the sound is introduced into the material or the surface of the acoustic structural component, part of the sound energy is reflected, part of it penetrates the material, and part of it is absorbed by the material, that is, the friction between the sound and the surrounding medium during propagation or the impact of the component material. Vibration, the process by which sound energy is converted into heat and lost. Generally speaking, any material can absorb and reflect sound, but the degree of absorption and reflection varies greatly.
NVH materials are mainly divided into two categories: sound-absorbing materials and sound-insulating materials. When the sound wave enters the sound-absorbing material, it will cause the air and fibers in the material to vibrate, and the sound energy will be converted into heat energy and a part of it will be consumed, just like hitting a sponge with a punch.
The sound insulation material is the material used to block the noise, just like a fist hitting a shield and blocking it directly. The sound insulation material is dense and non-porous, and it is difficult for sound waves to penetrate, and most of the sound energy is reflected back, so as to achieve the effect of sound insulation.
Foamed materials with a porous structure have unique advantages in sound absorption. Materials with a dense microporous structure even have a good sound insulation effect. Common NHV acoustic foams include polyurethane, polyolefin, rubber resin, and glass. Foam, metal foam, etc., due to the different characteristics of the material itself, the effect of sound absorption and noise reduction will be different.
Polyurethane foam
Polyurethane foam material has its unique network structure, which can absorb a large amount of incoming sound wave energy to achieve a good sound absorption effect, and at the same time has high rebound and good buffering function. However, the strength of ordinary polyurethane foam is low, and the sound insulation effect is poor, and its sound absorption performance will decrease with the passage of time. In addition, burning will produce toxic gas, which is not friendly to the environment.
XPE/IXPE/IXPP polyolefin foam material
XPE/IXPE/IXPP, chemically cross-linked/electronically cross-linked polyethylene/polypropylene foam material, has natural sound absorption, thermal insulation, cushioning and environmental protection, and its internal fine independent bubble structure is good for sound insulation and noise reduction. Excellent performance.
rubber foam
Foamed rubber is an ideal NVH material, and materials such as silicone, ethylene-propylene-diene rubber (EPDM), nitrile-butadiene rubber (NBR), neoprene (CR), and styrene-butadiene rubber (SBR) are better than the previous two materials. , The density is higher, and the interior is full of small voids and semi-opened structures, which are easier to absorb sound energy, more difficult to penetrate, and attenuate sound waves.
melamine resin foam
Melamine resin foam (melamine foam) is an excellent sound-absorbing material. It has a three-dimensional grid structure system with sufficient openings. The vibration is consumed and absorbed, and the reflected wave can be effectively eliminated at the same time. At the same time, it has more multi-functional and balanced advantages than traditional foam materials in terms of flame retardancy, heat insulation, light weight and processing shape.
foam aluminum
Add additives to molten pure aluminum or aluminum alloy and send it to the foaming box, inject gas to form liquid foam, and solidify the liquid foam to form a metal material. It has good sound insulation ability, and the sound absorption performance is relatively long-lasting, the effective service life can reach more than 70 years, and it can be recycled and reused 100%.
foam glass
It is an inorganic non-metallic glass material made of broken glass, foaming agent, modified additives and foaming accelerator, etc., after being finely pulverized and uniformly mixed, then melted at high temperature, foamed and annealed.
In real life, there is often no material that can completely absorb sound waves in different frequency bands, and no material can perform flawlessly in applications. In order to achieve a better sound absorption effect, we often see the combination of the above acoustic foams and them with types of sound absorption/sound insulation materials to form a variety of foam reinforced composite materials, and at the same time to achieve the effect of material sound absorption and structural sound absorption, to achieve The sound absorption performance of materials in different frequency bands of high frequency and low frequency. For example, the composite process of acoustic foam and different non-woven processes can make full use of the unique three-dimensional structure of the latter to more effectively reduce the vibration of sound waves, creating infinite possibilities for sound absorption and noise reduction; ) foam sandwich layer composite material, the two sides of the skin are bonded with carbon fiber reinforced material, which has higher mechanical rigidity and stronger impact strength, thereby achieving better shock absorption and noise reduction.
At present, NVH foam materials are widely used in transportation, construction engineering, industrial noise reduction, vehicle manufacturing and other fields.
Transportation
my country’s urban transportation construction has entered a stage of rapid development, and noise disturbances such as automobiles, trains, urban rail transit, and maglev trains have attracted widespread attention. In the future, acoustic foam and its composite materials have great application potential in sound insulation and noise reduction of highways and urban traffic.
construction works
In terms of architecture and structure, in addition to good acoustic performance, materials have extremely high requirements on safety, and flame retardancy is a hard indicator that cannot be bypassed. Traditional foam plastics (such as polyolefin, polyurethane, etc.) are flammable because of their own flammability. When burning, they melt and produce droplets. The burning droplets will quickly cause the spread of fire. In order to make it comply with relevant flame retardant regulations and standards, it is often necessary to add flame retardants, many of which will decompose when exposed to heat at high temperatures, and emit a large amount of smoke, toxic and corrosive gases. cause secondary disasters and environmental pollution. Therefore, in the field of construction, acoustic materials with flame retardant, low smoke, low toxicity, and effective fire load reduction will face this great market development opportunity, whether it is commercial buildings such as sports venues, cinemas, hotels, concert halls, etc. residential buildings.
Industrial Noise Reduction
Industrial noise refers to the noise produced by the factory during the production process due to mechanical vibration, frictional impact and airflow disturbance. Due to the many and scattered industrial noise sources, the types of noise are more complex, and the continuous sound sources of production are also difficult to identify, which is quite difficult to manage.
Therefore, the noise control in the industrial area adopts a combination of measures such as sound absorption, sound insulation, noise reduction, vibration reduction, noise reduction, destruction of structural resonance, and pipeline sound absorption wrapping, so as to restore the noise to an acceptable level for people. degree, which is also the potential application area of acoustic materials.
vehicle manufacturing
The sources of automobile noise can be mainly divided into engine noise, body resonance noise, tire noise, chassis noise, wind noise and interior resonance noise. The reduced noise inside the cabin will greatly improve the comfort of the driver and occupants. In addition to improving the rigidity of the chassis and eliminating the low-frequency resonance area in terms of design, the elimination of noise is mainly eliminated by means of isolation and absorption. From the viewpoint of energy saving, the materials used are required to be lightweight. From a safety point of view, materials are required to have fire and heat resistance properties. The advent of acoustic foam and various multi-functional composite materials provides new possibilities for improving the noise resistance, safety, reliability, energy saving and environmental protection of vehicles.
Post time: Aug-17-2022