In France, 34% of inhabitants complain about theacoustics of their home¹. Noise disrupts sleep, impairs concentration and is harmful to general health. It is a real social issue : the annual cost of noise pollution is estimated at more than 147 billion euros².
Imagine After a long day at work, you finally settle down on your couch with a good book. Just a few lines into your day, a dull thud sounds above you. Then another. And another. Heavy footsteps, chairs being dragged, objects falling... Every sound echoes through the ceiling, making it impossible to concentrate, let alone relax...
Among the various sources of noise pollution, le impact noise between dwellings – whether it's footsteps, falling objects or moving furniture – is one of the most common problems and the most feared when buying an apartment, whether new or renovated. A real scourge for residents, it deserves special attention in order to improve daily living comfort.
In a home, we are exposed to different types of noise, which have various origins, mainly air et solid.
It is therefore understood that the nature of the floor plays a key role in the intensity of the noise perceived in the rooms located below. Moreover, the insulation of the floor must also take into account the side transmissions, which do not pass directly through the floor but are transmitted through the walls or other parts of the home. In fact, lateral transmissions play a role in amplifying noise pollution.
A force is generated at each iimpact on a floor. This force depends of several factors: the mass and speed of the falling object, as well as the nature of the impact surface.
The force generated causes vibrations whose intensity varies depending on the characteristics of the floor. The lighter and more rigid the floor, the greater the vibrations can be.
Then, these vibrations propagate through the structure of the material and, upon contact with the air, they are transformed into acoustic waves : this is what we call the sound radiation of the floor. This phenomenon directly influences the perceived noise level in the room below, depending on its dimensions and the absorbent materials present there.
To measure the discomfort in a home or to test a product during the R&D phase, it is possible to measure performance of a system to impact noise. This measurement is carried out in the laboratory (ISO 140) or in real conditions
The principle of measurement is to position a shock machine which is a standardized device, with 500g weights falling from a height of 40mm at a frequency of 10Hz.
Impact machine – Impact noise measurement
The impact machine is positioned on the floor. Microphones are placed in the room below. The spectrum of the measured sound level is used to qualify the normalized impact noise level (weighted by the room's reverberation time).
In the laboratory, it is also possible to take a measurement with and without the flooring to be tested. Thus, by taking the difference between the “bare” floor and the floor with the flooring, the noise delta is calculated.
The measurement indices used for calculating impact noise are, according to ISO 717-2 standards:
These indices give a quick idea of the floor performance, which is the difference in performance between the bare floor and the floor with coating. The performance often sought is the ΔLw which is typically between 15dB and 25dB of impact noise reduction.
Be careful, though, there is a subtlety in these clues! They are not read the same way.
Tip: when the “w” is displayed, it is a global index, otherwise it is a spectrum (from 100Hz to 5000Hz)
It exists two main solutions to reduce impact noise. : the soft floors and the mass-spring systems.
The first is to use soft coverings (carpet, PVC flooring, etc.). Technically, these products will limit the force which is transmitted to the floor and therefore limit the propagation of vibrations. From a frequency point of view, this type of product will have an efficiency from a certain frequency depending on the characteristics of the materials. We can see this in the graph below, with an isolation which increases from 315Hz.
soft ground
Impact sound insulation of a flexible covering
The second solution is to create a mass-spring system. In concrete terms, a flexible material (spring) is positioned under a mass (a screed, a wooden floor).
Mass-Spring System, the mass being the yoke and the spring the resilient material (SCAM, insulating)
This “mass-spring” acts as a frequency filter which reduces vibrations from a certain frequency.
This frequency will depend on:
The most well-known “springs” are the thin acoustic underlays (called SCAM)Manufacturers such as SOPREMA, SIPLAST or ISOVER offer several types depending on the desired insulation performance. These underlays can also be placed under flexible floors to improve performance.
There are also materials combining thermal and acoustic properties such as polystyrene, glass wool, or wood fiber insulation. These materials, depending on their thickness and dynamic stiffness, have an impact on the overall performance of the floor. Some manufacturers include: STEICO, KNAUF, and PLACO.
On the side of the mass gain, the yoke also has an impact on performance. The higher its mass, the more effective the mass-spring system will be against impact noise (reduction of the resonance frequency).
More and more solutions with dry screeds (to be laid) are developed particularly in timber construction or renovation, with interesting performances. These solutions often integrate an underlay which also acts as a spring.
When it comes tosound insulation under screed (SCAM or other insulators), manufacturers seek to optimize the dynamic stiffness materials in order to obtain better performance. Dynamic stiffness corresponds to a material stiffness index and plays a key role in the effectiveness of impact sound insulation.
Dynamic stiffness determines the resonance frequency of the “screed + insulation” system:
To conclude:
In the diagram below, we observe that:
It is better to live in a building equipped with floors having an overall impact noise reduction index of ΔLw = 20dB!!
La dynamic stiffness is therefore an essential parameter for optimizing impact noise insulation. Measuring it allows manufacturers to ensure that their products meet performance standards.
Here is the formula for calculating dynamic stiffness:
The s' is the dynamic stiffness of the material and ms is the surface mass of the screed
With the rise of light-frame construction – in wood or metal –, le noise becomes a major issueWithout careful design and a judicious choice of materials, sound insulation can quickly become insufficient.
Why? Because impact sound insulation depends largely on the floor mass. However, on a light structure, low frequencies pass through more easily, making acoustic comfort more difficult to guarantee.
To limit climate change, the design of buildings and materials must be rethought. Optimizing resources involves:
But these developments have a direct impact on the dynamic stiffness of materials, and therefore on their acoustic performance. Reconciling lightweight structures and effective insulation is now a key issue for the future of construction.
See you soon for a new article on acoustics ♣
