Sounds right

This Issue This is a part of the Noise in buildings feature

By - , Build 149

The interior acoustics of a space can have a big impact on people’s general wellbeing and health, so it’s important to get it right. A great starting point is understanding how sounds react with materials.

Figure 4: Sound absorption and attenuation properties of various building materials.
 Figure 1: How sound interacts with materials.
Figure 2: Acoustic frequency ranges.
Figure 3: Reverberation design times.

CREATING THE DESIRED environment in a commercial space is dependent on getting the acoustic design right. It not only affects what we hear and how we perceive our environments but can also affect mental and physical wellbeing.

Good interior acoustic design can reduce excessive and distracting noise, create appropriate reverberation, reduce echoes and provide clarity where noise is distorted and unclear.

Sound energy interaction with materials

Sound energy, like all energy, can neither be created nor destroyed. It can only be changed from one form to another. Whenever sound waves encounter an obstacle, part of the sound energy is reflected, part is converted to heat through friction and part is transmitted.

The relative amounts of acoustic energy reflected, changed and transmitted greatly depend on the nature of the material. Getting the acoustics right involves predicting how sound energy will interact with materials in an interior space (see Figure 1).

Aspects of interior acoustics that need to be considered for most commercial spaces include:

• the reverberation time

• background noise

• sound transmission through walls, partitions and floors.

 Figure 1: How sound interacts with materials.

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What is reverberation time?

Reverberation time is the time taken in seconds for a sound signal to decay by 60 decibels (dB) once the source stops sounding. These times can vary greatly from almost no time to 6 seconds or more in incredibly reflective spaces. Reverberation is due to continued multiple reflections in a space.

When a sound wave strikes a surface such as a floor, wall or ceiling, the direction of travel is changed by the reflection. Reflection of sound waves follows the same physical law as light – the angle of incidence equals the angle of reflection.

The denser a material is, the more sound energy is bounced back into the space. A more porous material will reflect less, reducing the amount of energy in the space and shortening the reverberation time.

Noise reduction coefficient

The amount of energy reflected back into the space at the point of impact is known as the NRC (noise reduction coefficient).

This is a number between 0 and 1, with higher values representing higher levels of sound absorption. It is the mean value of the sound absorption in four frequency bands ranging from 250 hertz (Hz) to 2000 Hz (see Figure 2).

An NRC of 0.85 indicates that the material absorbs roughly 85% of the sound energy that reaches it – it only reflects 15% back into the room. Materials behave very differently across different frequencies, and an NRC will not define how much bass or high frequencies are absorbed.

Figure 2: Acoustic frequency ranges.

Optimise reverberation times

Reflected sound is one of the defining features of interior acoustics and affects the feeling of a room. Sound energy that lingers is detrimental to speech clarity and intelligibility.

Reverberation time needs to be controlled – the optimal time depends on the use of the room (see Figure 3).

Figure 3: Reverberation design times.

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A little background noise can be fine

Background noise is the level of sound energy in a space. It is measured in decibels, with 1 dB the threshold of hearing and 130 dB the threshold of pain.

This background noise level could be made up of environmental noises such as wind and rain, traffic noise, alarms, people talking and noise from birds or other animals. It can also be mechanical noise from devices such as computers, refrigerators or air conditioning, power supplies or motors.

For a lot of people, a little background noise is helpful to calm down and focus. In some cases, it also boosts productivity. However, it becomes detrimental when it is too loud or people can hear unwanted noise such as speech across partitions.

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Increasing the sound attenuation

Building partitions such as ceilings, walls and floors allow some sound energy to be transmitted. When occupants are able to hear unwanted noise through partitions, the amount of sound transmitted can be reduced by increasing the sound attenuation.

The amount of energy attenuated by a structure is known as the sound transmission class (STC) and is a measure of the energy lost through the system. A wall with a 40 dB STC will reduce a 100 dB sound down to 60 dB once it has passed through the structure.

The higher the STC, the less one can hear. STC is determined by the mass of a system and by the ease of sound paths.

When the mass of a barrier is doubled, the STC rating increases by approximately 5 or 6 dB, which is clearly noticeable. Adding insulation within a wall or floor/ceiling cavity will improve the STC rating by about 4–6 dB.

Sound energy will find the weakest structural elements, often doors, windows, ceilings with an open plenum and electrical outlets. It is important for all elements of a system to have the same amount of STC for efficient attenuation.

Knowing what material properties to select and where to use them will greatly enhance an interior space (see Figure 4).

Figure 4: Sound absorption and attenuation properties of various building materials.

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Create the right environment

A fundamental concern for comfort, productivity and health and safety is at the heart of interior acoustics.

Accounting for acoustics in commercial, hospitality and educational spaces can greatly increase users’ overall comfort and aural health. This leads to increased productivity and better concentration, while poor acoustics provide less than ideal working and learning environments.

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Figure 4: Sound absorption and attenuation properties of various building materials.
 Figure 1: How sound interacts with materials.
Figure 2: Acoustic frequency ranges.
Figure 3: Reverberation design times.

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