The acoustic benefits of timber

Timber’s sound performance makes it music to the ears

The first three words that come to mind when defining the acoustics of timber materials are resonant, stiff, and lightweight. This makes timber an ideal material for musical instruments and vibrant spaces such as concert halls and cultural spaces such as wharenui and fale. The visual and aural experience provided by timber adds a sense of comfort and warmth that isn’t imparted by other materials. 

Timber is a versatile and easy-to-work-with product that can be designed to be a sound reflector, absorber, or diffuser. This, combined with its pleasing aesthetic, makes it a commonly used surface treatment that can achieve various acoustic outcomes.

The stiff and lightweight nature of timber, unfortunately, makes it a poor sound insulator. However, this can be overcome with a range of strategies, such as structural isolation and using reconstituted wood products with more mass and damping.

The most effective strategies for improving the sound insulation performance are using air cavities and structural disconnections. A classic example of this is the double stud wall construction or acoustic floating floor (see Figure 13 – NZ Wood Guide, Chapter 13.5).

Reconstituted wood products such as Triboard™ and Strandboard® improve the acoustics of natural timber products by increasing the mass and damping. These are often utilised in wall and floor systems, enabling high levels of sound insulation to be achieved with timber.

For apartment design, the code minimum performance is STC 55, however, higher-quality developments often target ratings of up to STC 65+. 

Light timber frame construction is a common and well-known construction technique in New Zealand. The acoustic design of light timber frame buildings has progressed significantly in the past 10 years. It is now common to see systems and projects achieving performances of up to STC 65.

Mass timber is starting to make significant inroads into the market. Like light timber frame, it is common to see systems and projects achieving performances of up to STC 65. Mass timber can achieve even higher performances due to its increased weight (see Figure 42 – NZ Wood Guide, Chapter 13.5).

Uses of timber waste products such as sawdust have been shown to have a use in improving sound insulation performance. Research undertaken by the University of Auckland and Scion (Chung, H., Emms, G., & Fox, C. (2014). Vibration reduction in lightweight floor/ceiling systems with a sand-sawdust damping layer. Acta Acustica United with Acustica, 100(4). https://doi.org/10.3813/AAA.918742) has shown that a floor system with the upper cavity filled with a 60/40 sand/sawdust mix can achieve STC 65+ (see Figure 20 - NZ Wood Guide, Chapter 13.5).

A key consideration for timber buildings is the control of flanking sound. This is critical in apartments and other buildings with high sound insulation performance requirements. Flanking sound travels around the wall or floor by some other path, which can significantly degrade the onsite performance (see Figure 6 - NZ Wood Guide, Chapter 13.5). For the example in Figure 6, the wall may be rated STC 65, but the floor flanking path would reduce the onsite performance to approximately STC 45. 

Designers should keep the following in mind to minimise flanking sound:

• Wall and floor linings shouldn’t be continuous between acoustically separate spaces.
• Minimise the use of timber blocking.
• Avoid connecting double stud walls together.
• Small air cavities (<45mm) should be avoided.