When we build or renovate to achieve acoustically important properties, we use specific materials. It is important to know their properties, where to use them and for what purpose.

Building acoustics

Sound insulation

A key concept in building acoustics is sound insulation, which evaluates the performance of a separating structure in preventing sound transmission. In addition to increasing mass, the number of layers can be increased to limit the transmission of sound. The insulating layer in the interstitial space is formed by porous materials. Most commonly this is the mineral wool. These can further limit the resonance phenomena that occurs in the interstitial space in the form of sound waves. The addition of such a filler – we speak of acoustic insulation – can significantly increase the insulating capacity of the wall. Choosing a material that has more intrinsic damping is also important for insulating performance. Alternatively, damping can also be increased by adding layers of high-damping material, such as cork, to the base material. Similarly, the choice of filler can be optimised, and should in particular offer high sound absorption. Of course, increasing the weight of the filling can also increase the sound insulation performance, but it should be ensured that this does not make the filling too dense. This can lead to the filling forming a rigid contact between the layers, which in turn lowers the sound insulation performance.

Impact noise

Impact noise is noise generated as a result of direct mechanical action on the building components. Acoustically relevant are, for example, walking and the movement of objects on the floor. Impact noise is considered to be one of the more disturbing noises in multi-apartment buildings. To limit impact noise, the mass of the separating layers is considered to increase the sound insulation. In addition, the use of multi-layer assemblies is also a measure to limit the impact noise of floor separations. These can be implemented as an additional ceiling in the lower room, but a much more effective measure is to implement a floating floor. This is a solution where a layer of relatively soft (elastic) material is added on top of the load-bearing layer and a layer of high mass material is added on top of that. This is an effective solution based on preventing rigid contacts between the layer being excited and the rest of the structure. The effectiveness of the solution increases as the mass of the top layer increases and as a soft insulation layer is selected. Basically, the most important property of the latter is that it is elastic and soft even under high loads – that it has a low dynamic stiffness. For this purpose, materials such as EPS, mineral wool, polyurethane and polyethylene foams are commonly used.

Total noise protection

When we are building, we are always interested in the overall noise protection contributed by all layers in the separation structure. However, their contribution is not easily summing up, as the whole structure acts as a system in which the layers interact. Knowledge of the individual layers’s material properties is not sufficient to know the final noise protection, and it is therefore difficult to simplify what is happening to the point where absolute and simple relationships in the sense of the higher the density, the greater the insulation performance.

Some acoustic solutions are also very sensitive to the correctness of the design. Common design faults are, for example, rigid joints between layers, or installation penetrations that interrupt the air paths between rooms. It is also worth noting that, in physical terms, defects are very small, but can significantly reduce the achieved sound insulation performance. This again highlights the need for a good understanding of the process of sound propagation, its interaction with different materials and implementation details.

Room acoustics

Spatial acoustics deals with the sound within a space [1] that is produced as a result of a source. In this context, sound may be unwanted, i.e. noise, and we want to limit it; conversely, sound may be welcome and we want to amplify or otherwise enhance it with space.

Noise limitation

Noise is any unwanted sound, which means that noise sources can be different: devices in the room, music or even a colleague. By managing room acoustics appropriately, noise can be limited, which can be achieved by introducing barriers – for example between workstations in an office – or by introducing sound absorption into the room. The former limits the direct path of noise propagation from the source, while the latter limits so-called reverberant noise.

This is caused by the unnecessary repetitive bouncing of sound around the room, which can be limited by materials with a high absorption coefficient. Such materials are mainly porous materials such as wools, fabrics and foams. As the name implies, the key property of porous materials is their porous structure. This is because sound propagates into them and then loses energy through viscous losses, which is converted into heat.

Speech intelligibility

For spaces such as classrooms, lecture theatres and theatres, we expect the acoustic design of the space to allow for a good quality speech transmission between the speaker and the listener. The key here is that the speech energy received by the listener is sufficient (so that the speech is audible). Additionally, the background noise level must be low, as otherwise speech may be masked by noise. As already mentioned, background noise is reduced by adding absorption into the room. In fact, by reflecting sound on peripheral surfaces, the speaker’s voice can also be reflected and, if the designers are skilful, redirected to the listeners. Usually, the rear rows are the most targeted with reflections, where the speech is also the least audible due to the distance. Sound is reflected by using rigid, high mass materials, which in particular must have low absorption.

Musical performances

Historically, spaces used for musical performances have developed complex acoustic requirements that vary depending on the genre of music being performed.  In general, it is difficult to generalise and summarise these requirements, but it is worth noting that diffuse sound fields are often a requirement in music performance spaces. This means that we want a sound field that is the same between the bays and directions of the room. The conventional approach to increasing diffusivity is by introducing diffusers. These are acoustic elements that prevent specular sound reflections, i.e. the sound wave is scattered in different directions of the room when reflected at the diffuser.

Diffusers are generally constructed of rigid acoustically reflective materials and are characterised by their richly articulated geometric structure. They are thus recognisable as deliberately uneven surfaces. How well they scatter sound can be evaluated by the diffusivity coefficient [2], which is rarely given for commercially available materials.

In the light of the above written it is difficult to describe materials in general terms as acoustic, because this can lead to considerable confusion as well as misuse. In addition, it is worth noting that some material properties are given as material specifications, which are available at the time of material selection (e.g. density). Others (e.g. attenuation, diffusivity coefficient) are generally not given, and must be determined according to the relevant laboratory measurement.

References:

[1] Heinrich Kuttruff, Room Acoustics,‎ CRC Press; 6th edition (October 13, 2016)

[2] ISO 17497-2:2012, Acoustics — Sound-scattering properties of surfaces — Part 2: Measurement of the directional diffusion coefficient in a free field

Author: doc. dr. Rok Prislan, InnoRenew CoE