Airborne & mechanical noise transfer in buildings: An overview

Airborne and mechanical noise transfer is defined as the way sound moves through objects and the air.
As with any architectural acoustic issue, finding the source of the problem and treating it with acoustic solutions will help improve the quality of a space and how enjoyable it is for people who work or live in it.

In this article, we will give you an overview of airborne and mechanical noise transfer in buildings and the acoustic options that are available for architects and developers to avoid or treat these problems.

What is airborne and mechanical noise transfer?

Noise can transfer from outside to building interiors and vice versa, usually travelling through roofs, walls, windows, doors, eaves, HVAC systems, and any other breaches of the building envelope.

Every component of building construction has an impact on acoustical characteristics.
Limiting and controlling noise transmission from one space to another ensures speech privacy, noise consideration, room functionality, and overall makes environments more productive and pleasant to occupy.

The solution to noise transfer depends on the source, the noise type, and the path of transmission.

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Solutions for airborne noise transfer

When it comes to airborne noise transfer the sound waves are being transmitted into the air and travelling through it. The entire audio spectrum travels through the air, low frequencies just have enough energy contained within them to transfer to solid materials & impart mechanical vibration.

The solution for these airborne waves is through absorption. This takes the energy out of the wave. To do this, acoustic engineers need to know that sound waves follow the first law of thermodynamics, the law of energy conservation, which states that energy can not be destroyed and
can only be transferred/transformed.

Acoustic engineers utilise this law when designing acoustic absorption systems, transferring the kinetic energy of an inertial force (in this case a longitudinal sound wave) and transforming it into heat energy by forcing the sound wave to do some work, and hopefully, tucker itself out in the process! This results in a sound wave that is significantly less powerful and less likely to transfer into the surrounding solid materials within any given room or space.

Absorption can occur in a number of ways, acoustic consultants have to assess and define the appropriate type of absorption necessary to effectively dampen the soundwaves causing the noise transference issue. These methods change drastically over the full audio spectrum where different wavelengths of sound are propagating, in other words, no one solution suits all frequencies of sound.

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Airborne sound waves can travel through closed spaces, then outside, and then into neighbouring buildings. Therefore, it’s important to absorb these waves before it passes out of the source building.
To do this we recommend sealing air gaps in plaster joints, sockets, partition walls etc. Anywhere air can move, sound can too.

We also utilise porous absorption materials. Having a large surface area allows them to trap air, and therefore sound.

Finally, it is important to consider the Sound Transmission Class (STC) of the building materials used in your project. The STC rating allows you to gauge how effective this material is at stopping sound, for example, the plaster on the walls will have an STC rating. However, remember, STC ratings are not linear (don’t stack up), so you can’t just add two of everything and achieve twice the STC rating.

Solutions for mechanical or vibrational noise transfer

Mechanical (or vibrational) noise transfer is a sound that is capable of travelling through solid materials, such as a speaker system sitting on the floor, or footsteps (often referred to as impact) carrying through the ceiling to the room below.

Quite often low frequency sounds transfer themselves as vibrational noise, this is due to the fact that low frequencies carry more energy and can therefore move or agitate the heavier particles contained within solid materials. Sound also travels faster in solids, therefore reaching adjacent
rooms more rapidly than through airborne noise transfer.

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Absorbing vibrational sound is much harder than airborne sound due to this higher energy capacity, and therefore the main aim for treating these is to absorb via physical and mass induced dampening as much as possible. Acoustic consultants recommend the utilisation of heavy, more dense materials
to block the transference of low-frequency vibrations because a more dense material has a higher concentration of particles than a material of lower density, once again following the laws of physics explained earlier where the sound waves must do work in order to be effectively absorbed.

Dampening and decoupling sound requires you to separate the vibrating components as far apart as possible, allowing you to reduce the mechanical movement and energy (noise) transfer. To do this we recommend multileaf membrane absorbers. These are light and efficient mechanisms that work by isolating the vibrating components by intercepting them as a membrane between them.

The multileaf solution is effective in modern construction but depend on detaching the sound source (i.e. a loudspeaker) from the surrounding solid structures, whether the floor, walls, or ceiling. Mechanical dampers or decouplers work the same way. It’s all about isolating the source from any
structures it can transfer sound to.

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Solutions for both high and low-frequency noise

If a building is suffering from mechanical and airborne noise disturbance, treating both is a delicate game. Materials with high sound absorption are not great for stopping sound transmission, so both need to be treated accordingly.

As detailed above, the high and low frequencies have to be dealt with differently but when both in play must be managed and balanced.
In a space that suffers from high and low frequency, it will be quite uncomfortable for listeners. As the sound travels faster through solids, the vibrations will hit the ears first, and then the high-frequency waves afterwards, leaving the ear jarred and disordered.

Moreover, as a person moves around a space, they will experience the sound differently, further affecting the quality of the environment.
Both high and low frequencies need to be managed and balanced, if not sound pressure can be jarring to listeners, also different experiences of sound will be experienced depending on the place they are in the room The solution for this is appropriate construction materials that treat airborne transmission, and then the isolating and decoupling of any mechanical vibration.

Conclusion

Studying the acoustic needs of a building at the start of the planning process can help save time and money by reducing the need for any retrofitted acoustic treatment after construction is completed. 

Depending on the project, the acoustical properties of the building materials you specify and install will play a significant role in the end result, and that is why it is so important to take airborne and mechanical noise transfer into account from the very start.

If you are an architect or developer looking to achieve an improved acoustic environment in your next building project, then turning to Sound Zero for guidance and collaboration is the perfect option.

Our team of acoustic consultants is available to discuss your architectural acoustics needs to ensure you select the right acoustic treatment for the space you are working on. We have a range of products that can be completely customised to your colour, size, fabric, and other product requests.
Please do not hesitate to contact us today to go over our bespoke design-led acoustic solutions that can transform your architectural projects.

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