Sound control in timber buildings

How to complete this course

The course is open access and the content is available to anyone with internet access. This course should be completed in the sequence of the numbered sections. We make use of four components in our course (a) sections with text and illustrations, (b) video clips to explain some of the concepts, (c) a short online quiz after each section to test your acquired knowledge and (d) a test, available to learners who wish to obtain a certificate of completion. Please contact us at info@thewoodapp.com with the short course name to complete this test.

To make the course accessible to most people, we’ve tried to limit the time needed to complete it (an estimated 2.5 hours). This course will give you an overview of selected building materials impact and the environment, and design strategies to incorporate timber in a sustainable way. For in-depth knowledge on some topics you might require input from other sources such as national standards, material suppliers, selected textbooks, articles, etc.
If you have any questions or suggestions, please contact us at
info@thewoodapp.com.

Sound control in timber buildings Video 1

4. SANS building code

5. Timber construction

5.1 Walls
5.2 Floor-ceiling assemblies
5.3 Flanking paths
5.4 Design features
5.5 Quiz 3

6. Other sources

6.1 SANS codes
6.2 References

Disclaimer
In the compilation of this module, free use was made of published information such as text, figures, drawings, tables, graphs, etc. As the use of such material is subject to copyright considerations, and the suitability and relevance of this content is in the process of being assessed, the content of this module is only reserved for personal use and the purpose intended. To adhere to copyright regulations, any publication of the module or parts thereof considered, is subject to obtaining the necessary copyright agreement from the publishers by the author. Photographs taken and figures, drawings, tables and graphs generated by the author, are subject to copyright as well.

Whilst all and the utmost care has been taken to ensure the accuracy of the information contained in this module, no warranty can be given regarding the use, suitability, validity, accuracy, completeness or reliability of the information, including any opinion or advice.

1. Overview

 Sound is all around us. The human relationship to non-natural sound is integral to modern day living and sound can potentially affect our behaviour, performance, and health. The increase of man-made noise after the industrial revolution, has necessitated an active approach to noise control and the need for enhanced sound performance properties of buildings. Acoustic privacy has become a relatively well-known concept and is an essential requirement in office and living spaces these days. When design professionals consider acoustic privacy or sound control, factors such as the noise source, the sound propagation paths between the source and the listener, and protection of the receiver will be analysed.
In this short course the focus will be on the ‘path’ part of the source-path-receiver system of sound. The objective is to give designers and builders of timber structures an understanding of the relevant sound transmission paths in buildings. There will also be some background relating to sound propagation principles. Finally, this course looks at various design elements that can be used to modify or improve the sound performance of timber buildings.

Fun facts: Wood in music instruments
Wood use in music instruments are perfect examples of the sound transmission and sound propagation ability of the material – if used in the “right” way. Instruments are by design intended to amplify sound, whereas in most buildings, the opposite is true.
Recently, SU’s Department of Forestry and Wood Science began to conduct research on indigenous tone wood, and with the help of engineers, instrument builders and musicians, perhaps unlocked a new future for violins in South Africa.

Sound control in timber buildings Video 2

 2. The fundamentals of sound

Sound waves are longitudinal waves of pressure fluctuations in the air. The pressure fluctuations, which are usually generated by a vibrating object, cause the eardrum to vibrate.

This vibration is relayed to nerve cells in the cochlea which are connected via the auditory nerve to the brain where sound is perceived. Sound waves are characterised by frequency, wavelength and amplitude. A fixed relationship exists between the frequency, wavelength and velocity of sound and depends on the temperature and density of the propagation medium. Sound travelling through air is known as airborne sound, whilst the propagation of sound through other media such as a solid, is known as structure-borne sound.

In the Figure 1 below a speaker in the centre excites the air particles around it and the sound propagates outward.

Figure 1. A sound wave propagates in all directions (Meincken, 2023).

2.1 Frequency

The frequency (f) of a pure-tone, single frequency sound wave is perceived by human beings as a particular pitch and is the rate at which the pressure fluctuations occur in the air. The number of waves per second is the frequency and is measured in Hertz (Hz) (refer to Figure 2). The human ear can detect frequencies ranging from 20 – 20 000 Hz, which is recognised as the audible range of a person with normal unimpaired hearing. The ear is not uniformly sensitive to all frequencies, being insensitive at low frequencies and achieving optimum sensitivity around 3 000 Hz.

Figure 2. Sound waves are longitudinal waves, which are often represented as transversal density waves (Meincken, 2023).

2.2 Wavelength

The distance in space between consecutive maxima or minima in the pressure fluctuations, is the wavelength (λ) of a sound wave (see Figure 2). For a particular frequency the wavelength is determined by the velocity (c) of the sound in the transmission medium. The velocity of sound in air at a temperature of 21 ,1 °C is 344 m/s and in softwood and hardwood at the same temperature it is typically 3 300 m/s and 4 300 m/s respectively. Such different propagation velocities imply that the wavelength of a 100 Hz tone will change from 3,44 m to 33,53 m or 42,67 m as it is transmitted from air to either softwood or hardwood. Hardwoods tend to have a higher propagation velocity. In other words, propagation velocity (c) is a function of wood density.

Table 1. Speed of sound in various materials (NDE, 2023).

2.3 Amplitude

The intensity of a sound wave is perceived by human beings as the loudness or volume of the sound wave and is the power of the sound carried per unit area (W/m2). The sound level is represented by the amplitude of the wave as caused by the intensity of the air-pressure fluctuations (refer to Figure 2). The ear is sensitive to pressure fluctuations ranging from 20 µPa to 100 Pa. To simplify loudness measurements over such a large range and because the hearing organ does not react linearly to changes in sound pressure, a logarithmic scale, decibel (dB), was introduced to translate any sound pressure in the sensitive range to a value between 0 dB and 140 dB. A sound pressure level of 0 dB corresponds to a pressure fluctuation of 20 µPa, which is the threshold of hearing. The sound levels of some typical sound sources are presented in Table 2. In free space, sound waves travel from the point source in an increasing spherical pattern.

An important consequence of this “spreading out” of sound from a point source, is that the wave intensity, i.e. power/unit area, will diminish with increasing distance from the point source, since the affected area progressively increases. In air, a 6 dB reduction in sound intensity occurs for the doubling in distance between the point source and receiver. It is important to note that sound generally consists of a multitude of waves with different amplitude, frequency and wavelength. Additionally, the ear is not uniformly sensitive to all frequencies in the audible range. Therefore, when analysing the sound performance of timber constructed buildings, an analysis across the full audible range is necessary. Figure 3 depicts some variations of sound waves.

2.4 Quiz 1

3. Principles of sound control

The annoyance caused by noise is usually closely related to the loudness of the noise. Unwanted sound may be limited by (a) modifying the source to minimise the intensity of the sound generated, (b) by altering or controlling the transmission path and environment to reduce the efficiency of sound propagation between the source and the receiver, or (c) by “protecting” (screening) the receiver. The flow of sound energy between the source and the receiver is restrained through sound insulation and sound absorption, two different but complementary means of sound control. Sound insulation concerns the reduction of sound as it passes through a wall or barrier, primarily through reflection of the sound from the barrier. Sound absorption on the other hand, involves degradation of sound energy into heat along the transmission path.

The effectiveness of containing noise depends on the type and size of the source, intensity and frequency range of the sound, the nature of the environment and the ambient conditions. In a room, sound originating from a point source in the air, will propagate outwards as air-borne sound until striking any surrounding medium from where the sound energy is partially reflected, absorbed or propagated further as structure borne sound (refer to Figure 4). Due to the low absorption coefficient of air, the absorption of sound by air in confined spaces may be ignored.

3.1 Sound reflection

The measure of sound reflection or absorption occurring in a room depends on the nature of the surface of the obstructions struck by the sound waves, e.g. furniture and walls. Hard, rigid, non-porous surfaces reflect most of the incident sound whilst soft, porous surfaces, which can vibrate, absorb high percentages of the incident sound. In a room with low absorption and high reflection characteristics, most of the sound energy will be reflected, causing multiple reflections of the same sound wave(s) to occur. The result is an increase in the sound pressure level and an increase in the reverberation time of the room.
This increase in sound pressure level is due to the combined intensity of the primary energy and the energy stored in the reverberant field whilst the reverberation time of a room is a measure of the time it takes for the reverberant energy to decay. The reverberation characteristics of a room not only affects its sound pressure level, but when the reverberation time exceeds two seconds, it also affects intelligibility of speech.

It is evident that the sound pressure level of a room cannot be restrained by controlling the direct sound (apart from limiting or screening the source itself). It can however be controlled by limiting the intensity and reverberation time of the reflected sound through proper absorption techniques.

3.2 Sound absorption

The effectiveness with which materials absorb incident sound is rated by an absorption coefficient which ranges from zero to one, where one indicates total (100%) absorption. Good sound absorbing materials possess a cellular structure of interlocking pores. Within the interconnected open cells, sound energy is dissipated through viscous damping and through friction when conversion into thermal energy occurs.
Dissipation of sound energy depends on the thickness, density, porosity, flow resistance and fibre orientation of the material. Common porous absorption materials are made from vegetable or mineral fibres and various elastomeric foams and are normally available in prefabricated units such as blankets, fibre boards or tiles. It is important that the open cell structure be retained in the noise environment for the absorption to take place and it must not, for example, be covered with paint. Specifications for the absorption coefficients of general building materials are found in the product technical data sheets.

In most rooms of normal size, adequate sound absorption is achieved by the contents of the room, e.g. carpets, furniture and curtains and by the walls, floor and ceiling. When additional absorption is required, reflecting walls and ceilings can be lined with perforated fibre boards, plaster boards or various acoustic panels. By fixing these panels/boards on battens some distance from the wall or partition and filling the air gap with an absorption blanket (e.g. glass fibre) the sound absorption characteristics of the room will be increased even further.

When seeking to increase the absorption characteristics of a room, the dominant frequencies of the noise must be considered, since the effectiveness of any absorber is frequency related. High-pitched sounds with short wavelengths are normally easily absorbed whilst low frequency sounds with long wavelengths require thick and often impractical absorbers for efficient dissipation. If the frequency of the sound incident on the absorbing material is very low, the whole absorber vibrates and not the internal material fibres. Alternatively, high frequency sounds cause more relative motion of the internal material fibres and therefore more efficient dissipation of the acoustic energy. Total absorption is usually not possible or practical. However, application of high absorption coefficient materials will create a more comfortable acoustical environment.

3.3 Insulation

The sound pressure level inside a room is additionally affected by the level of externally generated sound penetrating the room boundaries. Sound propagation into or out of a room depends on the sound transmission or sound insulation properties of the floor, ceiling and walls. The sound insulation properties of timber walls and floor-ceiling assemblies are largely dependent on their structural design and ability to reflect and absorb sound energy.
Sound insulation is also frequency dependent, and therefore requires the sound insulation performance of a partition to be described across the full audible frequency range. This procedure is complex and has been simplified by introducing some single figure ratings to describe the sound reduction characteristics of partitions. Either the Sound Transmission Class (STC) rating system or the Weighted Sound Reduction Index (Rw) rating system may be used to rate the airborne sound insulation performance of partitions, whilst the Impact Isolation Class (IIC) rating system is used to rate the impact sound insulation performance of partitions. The numerical values of both systems are within ± 1 dB. In South Africa the Rw rating system is preferred. The higher the STC, Rw or IIC value, the more effective the structure is in reducing sound transmission. Generally, the sound performance of walls are specified by a STC or Rw rating and floor-ceiling assemblies by a STC or Rw and an IIC rating. Typical Rw ratings and their physical equivalents are listed in Table 3.

3.4 Quiz 2

4. Sound performance in SANS codes

Two codes of practice, as compiled by the South African National Standards (SANS)1, stipulate the sound performance of buildings in South Africa.

SANS 10103 covers the method of measurement of noise to determine the suitability of an environment with respect to possible annoyance and speech communication. This code also formulates acceptable residual sound and noise levels for different areas of occupancy/activity in non-residential indoor spaces and residential outdoor/indoor spaces. The residual sound level is the continuous A-weighted sound pressure level of a selected space over a specific period in the absence of intruding noise. These levels may be used as a guide for zoning and regional planning purposes and used to specify acoustic insulation procedures for buildings. Typical recommended comfortable residual sound levels for suburban residential indoor spaces vary from 40 dB in daytime to 30 dB at night.

SANS 10218 Part 1 covers the measurement of the acoustic properties of residential and non- residential buildings. Additionally, air-borne sound insulation requirements for buildings or parts of buildings are recommended. The minimum sound insulation levels against external sound as well as between adjacent rooms/ zones are provided for residential, educational, office, health and public accommodation buildings. Two grades, namely a standard and a higher-grade rating are prescribed for each option. All values are indicated using the Rw rating system. This code of practice also includes data tables which classify maximum internal sound pressure levels as caused by services in residential and public accommodation. As an example, these tables indicate that in a residential building, the kitchen and bathroom are the main sources of noise, having a Rw rating of 5 dB higher than other rooms. Table 2 and 4 (below) as obtained from the SANS 10218 Part 1 explains the minimum dBA values for airborne sound insulation of facades of residential buildings and between adjacent rooms within a dwelling unit.

SANS 10218 Part 2 covers procedures for the assessment of the acoustic properties of building plans and buildings regarding intruding and emitted noise, as well as the grading of buildings as prescribed in SANS 10218 Part 1.

Wherever SANS specifications are mentioned, the latest published version applies.

In the case of an auditorium or theatre (Figure 9), sound absorption is not the aim. In this instance optimal reflection is required to prevent any echoes, which is achieved by the uneven surface. The aim is to have an even loudness level in the entire room, although the source is on one side only.

5. Timber construction

In timber buildings, acoustic control through sound insulation is of primary concern. The sound insulation performance of traditional masonry and concrete structures are well appreciated, but the same cannot be said for timber structures. This section shows that minimal modification to and proper construction of conventional timber framed walls and floor­ceiling assemblies give sound performances that adhere to the recommended Rw and IIC levels.

5.1 Walls

Timber-framed walls and partitions are mostly subjected to air-borne sound, e.g. speech. In a conventional timber frame wall comprising of two 16 mm plasterboard layers separated by 79 x 38 mm studs at 600 mm centres (depicted in Figure 11), the sound incident on the plaster­ board lining causes it to vibrate, resulting in structure-borne waves created within the lining. These waves are propagated to the opposite lining via the studs as structure-borne waves and via the air cavity as air-borne waves. From the opposite plasterboard lining, the sound is propagated into the adjacent room as air-borne sound. Typically, such a wall has an Rw rating of 36 dB. The transmission loss is predominantly affected by the mass of the wall and to a lesser extent by the air-cavity between the two layers of plasterboard. The SANS code of practice recommends an Rw rating of 45 to 50 dB for air-borne sound insulation between rooms in adjacent dwelling units. It is therefore necessary to increase the Rw rating of the conventional stud wall. Let us observe how this is achieved through increased structure-mass, cavity insulation or through minimising rigid mechanical connections in the wall.

For single walls, the average insulation is almost entirely determined by its weight per unit area. The “mass law” states that for every doubling in weight 6 dB in air-borne sound insulation is gained. Adding another layer of plasterboard to each side of a timber stud wall doubles the mass with a subsequent increase of the Rw rating to 42 dB (see Figure 10). When increasing the mass of a wall to increase the Rw rating, it is important to consider the stiffness of the wall. If the sound incident on a wall compares in frequency with one of the natural frequencies of the wall, the wall will resonate, resulting in reduced sound insulation. This can be avoided by constructing a wall with less stiffness, which will have a lower natural frequency.

The SANS code of practice recommends an Rw rating of 45 to 50 dB for air-borne sound insulation between rooms in adjacent dwelling units. Single layer plasterboard walls typically have Rw ratings of 36 dB. A typical double-layer plasterboard stud wall (above) will have an Rw rating for airborne sound of 42dB. Adding insulation in the cavity will increase the Rw to 45 dB.
High-void materials are generally good absorbers of sound waves and when used to line wall surfaces, will reduce the sound pressure level inside a room. On the other hand, their sound insulation properties are poor, having a negligible effect to reduce sound transmission. Yet when used to fill the air-cavity in stud walls, the Rw rating can be increased by another 3 dB. This is explained by the property of the insulation blanket in the air-cavity to reduce resonance (standing waves) occurring in the cavity when the air-borne waves in the cavity have frequencies with wavelengths equal to or half the width of the air gap.
Doubling the mass and adding cavity insulation increase the Rw rating of a conventional stud wall from 36 dB to 45 dB. To acquire an Rw rating of 50+ dB, the mass of the wall could be further increased, but this would add significantly to the cost. Alternatively, propagation of the structure-borne sound waves can be attenuated by reducing propagation via the rigid connection in the wall, i.e. the studs. Propagation via the studs are affected by the nature of the mechanical connections between the studs and the plasterboards. When the connections are rigid, very little structure-borne sound attenuation occurs, resulting in sound short-circuits in the wall. These unwanted propagation paths can be reduced by using a staggered stud construction or by using a double row of studs on separate plates (see Figure 11).

Also visit “Timber frame building in South Africa” Figure 16 and Figure 29 for more on wall insulation elements and “Wooden flooring” Figure 12, 13 and 14 for more photos and details on floor and ceiling sound reducing elements; concrete slab, blanket insolation and sound absorbing rubbers.

5.2 Floor-ceiling assemblies

The structural, thermal insulation and aesthetic properties of wood make it extremely suitable for floor-ceiling assemblies in both domestic and industrial multi-storey buildings. Similar to timber walls and partitions, the sound performance of timber floor-ceiling assemblies are often questioned.
Floors are subjected to air-borne sound such as speech and music, as well as structure-borne sound caused by the impact of footsteps and other physical impacts. In most cases, poor sound performance of a timber floor-ceiling is caused by a design providing sound insulation for either impact or air-borne sounds and not for both.

The procedures to enhance air-borne sound insulation and to increase the Rw rating of a floor-ceiling, are similar to those applied for stud walls and partitions. As depicted in Figure 16, applying an insulation blanket between the floor­ceiling cavity and a resilient channel/bar between the joists and the ceiling board, will give an Rw rating of 49 dB. This is higher than the 44 dB rating of a 100 mm concrete slab, even though the slab has a mass of 260 kg/m3, which is far more than the 49 kg/ m3 of the timber assembly.

Control of impact noise (IIC of 50 is an acceptable minimum between multi-storey dwellings) requires reduction of the structure borne sound in the floor-ceiling assembly. Although a heavy structure provides excellent insulation against air-borne sound, its performance with impact sound is poor. Voids (air-cavities) and non-rigid connections (resilient channels) as depicted in Figure 16, obviously attenuate structure-borne sound, but the simplest method to control impact sound is to limit transmission to the structure in the first place. For floors, this is effectively achieved by covering the floor with a resilient material such as a carpet with underfelt. The IIC rating of a floor-ceiling assembly improves by over 20 points when a carpet-underfelt covering is used as opposed to a vinyl floor covering (Figure 17). At the same time the Rw rating is barely enhanced. Structure-borne sound is also propagated through a floor-ceiling assembly via the building framework to which it is attached. To avoid this propagation path and to prevent impact sound transmitting throughout the building, a “floating floor” construction can be adopted. In such a construction, the top floor sheet is allowed to float on a resilient layer, e.g. softboard, polystyrene, glass-wool or rock-wool, and it is not attached to the building framework at all.
Again visit “Timber frame building in South Africa” Figure 14 and “Wooden flooring” Figure 13, 14 and 15 for photos and details on floor and ceiling sound reducing elements.

5.3 Flanking paths

The previous sections show that proper constructed timber walls and floor-ceiling assemblies can attain Rw and IIC sound performance ratings of 50+ dB and 70+ points respectively.

Unfortunately, the improved sound insulation of a partition is greatly reduced by any single effective transmission path through the partition or by the existence of any indirect (flanking) path(s), which allow the sound to bypass the partition. A door or window installed in a partition has Rw ratings of 20 dB and acts as a direct transmission path through the partition. This causes the Rw rating of the partition to drop to below 30 dB, irrespective of its initial Rw rating.

The general rule is that a partition’s Rw rating is limited to 7 dB above the Rw rating of its most effective transmission path. Sound leaks around pipe and conduit penetrations and back-to-back installations of cabinets and electrical outlets in partitions also act as direct transmission paths. Critical flanking paths include common corridors, roof space, sub-floor space, ventilation grills, duct systems and any air-leaks around the partition. The combined effect of sound wave diffraction and reflection permits sound to propagate effectively via these flanking paths to adjacent spaces.

In addition to the flanking paths described above, sound can propagate throughout a timber building as structure-borne sound via its timber frame. If any part of the frame is subjected to sound, the rigid connections between the elements of the frame allow sound to propagate throughout the building with little attenuation. It is therefore necessary that major sound sources for example ventilation and plumbing systems, are insulated from the building frame. Where vibration and noise can be transmitted to the frame, insulation through vibration isolator pads, glass fibre, softboard or perimeter gaskets, are imperative.

5.4 Design features

Apart from achieving improved sound insulation by means of properly designed and constructed walls and floor-ceiling assemblies, and by eliminating noise leaks and flanking paths, the sound pressure level inside a building can be further reduced through correct design of the floor plan and through isolation of major noise sources.
The influence of external noise sources, e.g. traffic and industrial noise, can be minimised through proper positioning of the building on the plot and through external structures such as outside buildings and fences, which can act as noise barriers.
Inside a building it is important to isolate major noise sources and adapt the design to distance the noisiest areas from the quiet areas. In residential buildings, the kitchen and bathroom have been identified as the noisiest areas. This is primarily because most household appliances are operated in these areas and because of the customary hard reflecting surfaces such as tiled walls, table tops and cupboards, offering very little sound absorption. Kitchen noise can be more effectively isolated from the rest of the building by installing solid interior doors and acoustic ceilings in the kitchen. The noise level can also be reduced by isolating appliances and cabinets from the building structure with vibration isolators and rubber gaskets.
In a bathroom, the main noise source is plumbing noises. These normally result from excessive water velocity in an undersized piping system. This can be largely alleviated by using adequately sized water piping and limiting the water pressure through pressure reducing valves. Additionally, vibrating pipes can be isolated from the timber frame with resilient sleeves.
In summary, the acoustic properties of a timber frame structure can be improved by identifying major noise sources, determining whether they generate air-borne or structure-borne noise, and by isolating them in the correct way.

5.5 Quiz 3

6. Other sources

For further information on residential soundproof techniques, tips and product reviews please visit:
https://www.naturallywood.com/wood-performance/acoustic/
https://soundproofcentral.com/
Or contact your local sound engineering or acoustic and measurement professionals.

6.1 SANS codes

South African National Standards
SANS 10103: The Measurement and Rating of Environmental Noise with Respect to Annoyance and Speech Communication.
SANS 10218: Acoustical properties of buildings. Part 1: Grading criteria for the airborne sound insulation properties of buildings.
SANS 10218: Acoustical Properties of Buildings – Part 2: The Assessment of Building Plans and Buildings with Respect to their Acoustical.

6.2 References

Meincken M. 2023. Wood Products Science 264 class notes. Department of Forest and Wood Science, Stellenbosch University.

SALMA Timber Manual. 1995. Section 1.3: Noise control in timber buildings. Document available from dr. P. Crafford, pcrafford@sun.ac.za

Schoenwald, Stefan & Wenzke, Erik & King, Frances & Zeitler, Berndt. (2012). Effect of structural changes on acoustic performance of wood frame walls. 1-6.
NDE, 2023. Center for Nondestructive Evaluation Education, Iowa State University; https://www.nde-ed.org/Physics/Sound/speedinmaterials.xhtml