Acoustic Performance of Sandwich Panels

Noise pollution is a determining factor in the definition of environmental quality. In fact, not only does it represent a serious threat to human health and physical-psychic well-being, but it also has sensitive effects on the valuation of real estate, since a house or a building built in a noisy environment is much less attractive.

The development of acoustics in recent decades has been remarkable and, at the same pace, standards related to different types of acoustic measurements have been developed. This standardization has become necessary due to the exponential growth of noise levels in urban areas.

The standards regulating permissible levels of noise pollution take into account the following factors:

noise transmitted by industrial buildings to the environment;
noise produced by traffic and transmitted to surrounding buildings;
noise levels inside buildings and workshops;
sound insulation conditions between rooms in a building.

As is evident, the control of the characteristic and exponential noise of a building requires careful consideration, during the design phase, of every aspect of the construction or renovation. For this purpose, the following aspects must be verified:

acoustic insulation of exterior facades;
the acoustic insulation of vertical and horizontal dividers;
the insulation to the treading of the floors;
the noise emission level of the sanitary installations;
noise from service installations (elevators, air conditioning systems, autoclaves, etc.).

Therefore, it is during the design phase that it takes shape and enables the building to be truly protected against exterior and interior noise.

Nature of sound

Sound can be considered as a train of vibrations, produced by a vibrating source, which propagate through the air in the form of pressure waves and locally cause compression and rarefaction conditions. The air particles are excited by these waves and oscillate casually, transferring their energy to nearby particles, and so on. This leads to the moment when they reach a human ear, which perceives this movement as a pressure variation and, finally, as sound.

The fundamental parameters of sound are frequency and pressure: the measure of the intensity of sound when it reaches a human earThe unit of measurement for frequency is the hertz, which translates to number of cycles per second (vibration waves per second). Adults have an audibility range from 20 Hz to 20,000 Hz, although the human body can be influenced by sounds outside this range. A sound below 20 Hz is called infrasound, while ultrasound occurs above 20,000 Hz.

In any case, at a given frequency, the human ear responds to sound pressure, whose unit of measurement is Pa (N/m2). The lowest sound pressure that an average human ear is able to detect is approximately 2 x 10-5 Pa, while the upper limit at which the ear begins to feel pain is approximately 20 Pa.

Because of this wide range, sound pressure levels are usually expressed using a logarithmic scale. Therefore, since the energy of a sound wave is proportional to the square of the sound pressure, the following equation allows us to define the unit of measurement generally used for sound pressure, the decibel (dB):

N p2 p

= 10 log — = 20 log —

po2 po

where:Np = sound pressure level (dB)

p = effective sound pressure (Pa)

po = reference sound pressure: 2 x 10-5 Pa.

The sensitivity of the human ear to sound pressure varies with the various frequencies. The minimum level that can be perceived by the human ear at a given frequency is known as the threshold of audibility. The threshold of audibility differs from person to person and also changes with age. When the sound perceived by the ear becomes louder, it reaches a level at which the human ear begins to notice intense discomfort. This level is known as the pain threshold and has a value of approximately 140 dB. These limits, along with approximate typical levels that occur during conversation, or when listening to music, are given in Fig. 10.1.

Fundamental acoustic parameters

Sound can be absorbed, transmitted or reflected. When a sound wave strikes a divider, such as a roof or wall, some of the sound energy is reflected, some is absorbed within the material, and some is transmitted through the material, as shown in Figure 10.2.

Absorption

The percentage of incident sound that is reflected, absorbed or transmitted depends on the construction element, the material from which it is made and the frequency of the sound. Based on this, we can define three acoustic parameters:

absorption coefficient, ? = percentage of incident sound absorbed by the element;
reflection coefficient, ? = percentage of incident sound reflected by the element;
transmission coefficient, ? = percentage of incident sound transmitted by the element.

Sound insulation and sound absorption concepts

Sound insulation represents the ability of an element (e.g. a sandwich panel) to prevent incident sound waves from passing through it. This feature is of great importance in all applications where noise transmission from one environment to the adjacent one is to be reduced (Fig. 10.3). The sound insulation capacity depends on the sound frequency and the mass per unit area of the divider.

Sound absorption expresses the capacity of the insulating layer of the dividing element (for example, the mineral wool slab of a sandwich panel) to absorb the sound energy impinging on it. This feature is of extreme importance in all applications where the noise level in an environment is to be reduced (Fig. 10.4).

Acoustic insulation

Fundamental aspects of sound insulation

Airborne sound is the sound generated in a room or industrial facility from a source such as a loudspeaker, a conversation between several people, a television, etc. (not to be confused with impact sound, produced by forces applied directly to the structure, e.g. by footsteps). It can be transmitted to adjacent rooms or to the outside with different transmission paths, such as dividing walls, floors, the bearing structure of the building, windows, doors and conductors, as shown in Fig. 10.5.

The net reduction of airborne sound energy produced and transmitted through all these propagation paths is known as airborne sound insulation or simply as acoustic insulation.

In particular, it is possible to distinguish between two different propagation methods: direct propagation, in which the sound energy passes directly through the dividing element, and indirect propagation, in which the sound energy passes through the surrounding structure.

Direct propagation occurs when a wall separating two adjacent rooms begins to vibrate when a sound wave hits it, causing the sound to spread beyond the wall. To minimize this phenomenon as much as possible, it is necessary to limit the possibility of wall vibration, which implies a careful study of the elastic and dissipation characteristics of the construction materials. Such an analysis cannot do without laboratory measurements to determine the insulating properties of materials, to establish design data or to check the compliance of building materials with current standards.

Indirect propagation allows sound to reach the receiving room through more circuitous paths than those followed by direct sound. Walls or floors next to the dividing element are the main indirect propagation paths, but this phenomenon can also occur with doors, windows, suspended ceiling voids and other elements capable of reducing the acoustic insulation capacity of the environment.

In this case, the possibility of reducing or eliminating air leakage in the vicinity of the dividing element has a significant effect on its acoustic performance. Logically, it is highly unlikely that the material being tested will in practice demonstrate the same characteristics detected under ideal conditions at the test site. For this reason, in any sound insulation problem, it is essential to consider both propagation methods and to identify the weakest parts of the building.

The fundamental measure of sound insulation provided by a partition wall is called the sound reduction index, sound transmission loss or sound insulation capacity: it is indicated by the symbol R and is measured in decibels (dB).

The R-value is obtained in the laboratory by subjecting the test sample to a sound insulation test. Since no indirect propagation mechanism is taken into account, this value indicates the ability of the tested element to reduce the transmission of sound energy from one environment to another, and is characteristic of the physical properties of the element (Fig. 10.6).

When measurements are made on a real building and include the effects linked to indirect sound propagation, sound losses, etc., the sound insulation measurement parameter is called apparent sound reduction index, apparent sound transmission loss or apparent sound insulation capacity: it is denoted by R’ and is also measured in decibels (dB) (Fig. 10.7).

W1 = Sound energy incident on the wall,
W2 = Sound energy transmitted through the wall
W3 = sound energy transmitted laterally through the adjacent structure

The sound insulation power R varies depending on the surface density ? of the wall, i.e., depending on the mass per unit area of the divider (according to the law of masses) and the frequency f of the sound.

The law of masses expresses a clear relationship between the mass per unit area of a wall and its sound insulation properties: if the wall with the greatest surface mass is considered to have the greatest capacity to counteract the vibrating movements generally induced by incident sound waves, this law predicts that, each time the mass per unit area of a single-layer wall is doubled, the sound reduction index increases by about 6 dB, drastically reducing the sound pressure load physically absorbed by humans.

In practical building applications, the double-layer wall is the preferred solution when two adjacent rooms need to be sound insulated, since the gap between the two layers can be filled with a material suitable for the desired sound insulation values. The acoustic insulation capacity of a double-layer wall increases considerably when a mineral wool slab is inserted between the two walls.

It should be noted that, although sound insulation can be improved in a living space, low-frequency noise is very costly and difficult to stop.

Low frequency sound waves, due to their high dimensions, tend to envelop the entire environment, causing the whole structure to vibrate in unison. In this case, common acoustic insulation systems are not effective in blocking these waves, as they are too large.

Indirect sound propagation

Indirect propagation deals with sounds transmitted to adjacent environments through structures within the dividing element. This factor must necessarily be taken into account when using sandwich elements.

The following are examples of structural solutions in which indirect sound propagation can be obtained:

an external wall, constructed with sandwich panels, passes through a floor or an interior partition wall, capable of providing a high degree of acoustic insulation;
internal partition walls, characterized by high acoustic insulation values, connected to a ceiling constructed with sandwich panels.

In both cases, the sound is transmitted mainly through the structures around the dividing element (the outer wall and the ceiling), as this constitutes a preferred path through which the sound can propagate. Figure 10.9 shows two examples of solutions that can help reduce the risk of indirect propagation.

Sound reduction index for holes and cracks

The sound reduction index offered by holes and cracks is almost equal to 0 dB. Therefore, the influence of holes and cracks can be important, for example, in correspondence of connections between sandwich panels, doors and windows without the correct insulation and, ultimately, in any necessary openings present in partition walls.

If a sound-absorbing material is present in the cracks, it ensures a higher sound reduction rate in these cases. Thus, the connection between sandwich panels with mineral wool insulation layer is not as critical as the connection between rigid polyurethane foam panels, since it has a very low acoustic absorption capacity.

A recognized phenomenon in sandwich structures is the deflection of the panel, related to a temperature difference generated between its outer surfaces, which can have an unfavorable impact on sound reduction.

Sound insulation test for the determination of the sound reduction index

The test method for measuring the sound reduction index, R, of a sandwich panel is performed in accordance with the following international standards:

EN ISO 140-3:1995, “Acoustics – Measurement of sound insulation of buildings and building elements – Part 3: Laboratory measurements of airborne sound insulation of building elements”;
EN ISO 717-1:1996, “Acoustics – Classification of sound insulation of buildings and building elements – Part 1: Airborne sound insulation”.

The test method requires the use of two adjacent chambers: one of the chambers is used as the “source chamber”, producing a sound field in it (with the help of a sound source, such as an omnidirectional loudspeaker), and the other will be considered the “receiving chamber” (Fig. 10.10).

The chambers are separated by a common wall, equipped with an opening in which the sample to be tested is to be fixed, as shown in Fig. 10.11 and 10.12.

The test chambers in the laboratory are reverberation chambers, built with the purpose of avoiding any possible sound leakage, so that, during the test, all the energy reaches the receiving chamber exclusively through the test wall (Fig. 10.13).

If we indicate with L1 (dB) the average sound pressure level in the source chamber and with L2 (dB) in the receiving chamber, the sound reduction index between the two chambers is determined by:

R = L1 – L2 + 10 log S – 10 log A

where S is the area of the sample panel (m2), and A is the total absorption of the receiving chamber (m2).

At the end of the test, the certification body processes the results and issues a test report (Fig. 10.14) consisting of a graph of the sound reduction index at various frequencies.

The sound reduction index curves, with frequency variation, of PanelORG® Wall Sound panels of 100, 80 and 50 mm thickness, are shown in the following graph (Fig. 10.15)

Indicative sound insulation values for sandwich panels

The sound reduction index values obtained using sandwich panels are higher in cases where the insulation layer is mineral wool and not polyurethane foam.

PUR foam sandwich panels allow to obtain sound reduction index values always equivalent to 25 dB.

The sound reduction index values guaranteed by mineral wool panels are clearly higher and depend on the thickness of the panel. Approximate values are given in Table 10.2:

Thickness (mm)	Sound reduction index R (dB)
50	27
80	28
100	30
120	32
150	33

Table 10.2: Indicative values of the sound reduction index of mineral wool panels

Sound absorption

Fundamental aspects of sound absorption

Sandwich panels are generally used as walls and roofing in factories and workshops: these are generally characterized by noise levels that are often very high.

In case panels with metallic surfaces are used and no additional sound absorption system is provided, the acoustic quality of the building may not be satisfactory, since most of the sound is reflected around it.

To improve the quality of the acoustics it is necessary to install sound absorption materials inside the rooms, fixing them under the coverings or to the walls. Some common sound absorbing materials are ceiling tiles, soft furnishings or screens. These and other more specialized acoustic absorption materials are used in offices, customer service centers, cinemas, theaters, music and television studios, factories, workshops, vehicles, etc.

In any case, sandwich panels with metal surfaces and mineral wool core, in which one of the surfaces is perforated, have acoustic insulation and absorption properties and are well suited for use as partition walls (where a vapor barrier is not required) and in machine bodies. Unfortunately, the use of these panels is especially critical in heated building envelope and wall applications, when the outside temperature reaches particularly low values, as these panels do not possess vapor barrier properties and can cause condensation and dripping phenomena.

An example of a sandwich panel with the above characteristics is the PanelSandwich.ORG PanelORG® Wall Sound, shown in Fig. 10.16.

The material constant, which defines the ability of a material to absorb sound, is known as the sound absorption coefficient, ?. The sound absorption coefficient varies with the frequency of sound.

This coefficient indicates the ratio between absorbed energy and incident energy and therefore varies between 0 (total reflection) and 1 (total absorption). The sound absorption properties of objects, such as chairs or padding, are quantified by a parameter called equivalent sound absorption area, which is the area of a perfectly absorbent surface (? = 1), capable of absorbing the same amount of incident sound absorbed by the real object.

Sound absorption test for the determination of the absorption coefficient

A test method for measuring the sound absorption capacity of both flat materials such as acoustic mats or tiles, as well as single materials such as chairs, acoustic screens and padded pillows, is based on the use of a reverberation chamber.

The test is carried out in accordance with the international standard EN ISO 354:2003 “Acoustics

Measurement of sound absorption in a reverberant chamber”.

The method requires the creation of a sound field in an initially empty reverberation chamber. When the sound source is turned off, the reverberation time is measured: this is the time required for the sound pressure level inside the chamber to drop below 60 dB.

The test sample is then placed in the chamber and the reverberation time is measured again. Due to the sound absorption properties of the sample, the reverberation time should be shorter. Therefore, the two reverberation times thus detected allow the calculation of the equivalent sound absorption area of the sample.

The test specimen should be rectangular, with a ratio of width to length varying between 0.7 and 1 (Fig. 10.17). It should be placed directly against a surface of the chamber, usually resting on the floor and preferably with its edges not parallel to the sides of the chamber.

The edges of the specimen should be sealed or covered to prevent them from absorbing sound energy, with an acoustic reflection frame of steel, wood or gypsum board. In any case, if the edges of the test specimen are left uncovered in practical applications where it is normally to be used, they should also be left uncovered during the test.

If the test specimen is obtained from the joining of two or more pieces of material, it may be necessary to cover the joints with adhesive tape, with a suitable insulating material or with another material without sound absorption: this prevents the test specimen from absorbing sound energy in correspondence of the joints.

The result of the tests is a curve describing the variation of the sound absorption coefficient with frequency variations, made by the certification body in a final test report (Fig. 10.18).

The following graph (Fig. 10.19) refers to the variation of the sound absorption coefficient, ?, as the frequency varies, for a PanelORG® Wall Sound sandwich panel with a thickness of 50 mm.