Soundproofing material

Types of soundproofing materials

In truth, all materials can absorb sound energy to some extent. However, materials that are specifically designed as soundproofing materials absorb most of the sound energy they are exposed to. These special materials are commonly referred to as “acoustic materials” and are designed to have excellent absorption properties. The main use of these materials is to reduce the sound pressure level of reverberant sound. This leads to a reduction of the overall echo in the space.
There are many sound absorbing materials. However, before discussing them, it is first necessary to understand the difference between sound absorption and sound insulation.

Sound insulation vs. sound absorption: what is the difference?

When it comes to acoustics, there are two different options:

• The ability to absorb the reverberant energy that creates sound in a space.
• It can prevent the transmission of sound energy.
  Both forms of sound manipulation use special materials and products to combat sound waves.
  Acoustic materials absorb most of the sound energy they receive and reflect very little. These properties make them useful in noise reduction within a space or enclosure. They are used in different places: near noise sources, along pathways or near receptors.
  Soundproofing a space involves four concepts: adding mass, attenuation, separation and absorption. In practice, acoustic absorption is a form of sound insulation. Materials used for soundproofing are often dense and heavy, allowing structures to be separated. Due to their density, sound waves that pass through the structure are reflected inside the panel.

Types of acoustic materials

Acoustic absorption is the process by which sound energy is dissipated and transformed into another form of energy: heat, mechanical or deformation.
From a scientific point of view, there are three main types of sound absorbers: porous, membrane and resonance.
There are many types of acoustic materials. Their ability to absorb sound waves depends largely on frequency, composition, thickness and mounting method.

• Porous absorbers – which absorb more sound as the frequency increases. This means they absorb high frequencies (high pitched sounds) more effectively. The more porous the fabric, the greater the absorption. Two types of material are commonly used: spongy, such as polyurethane foam, and fibrous, such as glass wool. The denser the material, the greater the absorption, up to a certain limit of course, from which it would act as a reflector, the average density of these materials being around 80 kg/m³. Another factor to monitor is the thickness used. The thicker the material, the lower the frequency at which the absorption becomes effective (theoretically, a porous absorber becomes effective at the frequency at which 1/4 of its wavelength coincides with its thickness). Its positioning alone improves its absorption at lower frequencies by separating it from a rigid surface (wall).
• Membrane/plate absorbers – which absorb low frequencies (bass) more effectively than high frequencies. The absorption mechanism of these materials is to convert sound energy into deformation energy.
• Resonance absorbers – which have the maximum absorption at a given frequency: the natural frequency of the acoustic fabric. The absorption mechanism of these materials is to convert sound energy into mechanical energy. Perforated or grooved panels are used, which are mounted on walls.

In summary, acoustic insulating materials are generally poor absorbers, as they are designed to reflect the sound they are exposed to. Acoustic insulators are generally denser and stiffer (lead, concrete, steel, etc.), while acoustic absorbers are more porous, designed to absorb and retain sound waves (rock wool, glass wool, polyurethane, etc.).

Note that not all thermal insulation materials are good sound absorbers.

Closed cells (expanded polystyrene, polyurethane foam, polyisocyanurate) do not insulate and do not absorb acoustically, so they have no acoustic function. Thus, fibres (rock wool, glass fibre, synthetic fibres, natural fibres, open cell foam, etc.) do not insulate per se, but improve the sound insulation of a multi-layer panel (veneer, double wall, cladding, laminated plaster). As the material improves the vibration separation between heavy layers and damps (absorbs) the resulting standing acoustic waves, it greatly improves the overall insulation. In addition, if one layer of a multi-layer panel is perforated, it helps to improve the sound absorption of the room towards which this perforated surface faces and greatly improves the internal acoustic environment, i.e. the “acoustic conditioning”.
If you want to reduce the noise in an industrial warehouse, room or home, you need to combine sound absorbing materials and soundproofing. Acoustic fabric placed in the enclosed space between two parallel partitions improve the insulation that these partitions alone would provide.

M.N.