Wood is a complex material with a vast number of attributes, including acoustic properties. This blog explains the physical mechanisms behind the acoustic performance of wood. It will help answer questions about why wood is so versatile, as well as how to achieve optimal acoustic performance for your projects. It includes information about how wood sounds in different environments, such as outdoor and indoor spaces, and it also provides practical suggestions for making your furniture sound better acoustically.
Wood is a composite material with density determined not only by the wood species, but also by the particular part of the tree from which it was cut. For example, wood from the tree’s outer layer is lighter than wood from its heartwood. Accurate determination of density is therefore dependant on where on the tree you cut. Density affects sound propagation and for this reason resonances are weaker in materials with a higher density.
Addition of wood ceiling tiles and wood floor tiles in the ceiling effectively reduces the sound absorption of concrete floors, improving acoustics. In some cases, a combination of wood floor tiles and concrete floor tiles are used to give even better sound absorption. Wooden walls such as those used in older houses increase reverberation time, resulting in longer reverberation characteristic times. Wood can be used in construction to reduce acoustics, because of its low density compared with other materials such as concrete and glass. Using wood for walls and ceilings has the effect of making rooms appear quieter. For example, most concert halls are made with heavy wood beams supporting the ceiling to allow for longer reverberation times of musical notes before the sound decays.
Does wood reflect sound, or does wood absorb sound?
Sound waves are reflected by wooden acoustic panel. Different wood species have different reflection coefficients, with the highest values being found in softwoods. This means that the sound of a sharp tap is reflected by hardwood, while it is absorbed by softwood. The sound energy is converted into heat and therefore it is not heard. The lower the frequency, the more sound gets reflected. The highest reflection coefficients for hardwood are found in the frequency range of 16 to 50 Hz. Softwood exhibits high reflection behaviour throughout its whole frequency range. The high absorption coefficient in softwood up to 20 kHz is caused mainly by resonances between air and wood membranes. Softwood high-frequency absorption (above 20 kHz) has little dependence on resonance effects and is caused by contact between wood pores and air.
When designing a room, the main concern is to reduce acoustics to the minimum. This can be achieved by using materials that absorb unwanted reflections and sound absorption is used as an important design element in all kinds of rooms, even in offices where many people work. As an example, the walls and ceilings of auditoriums are designed so that they effectively act as a sound barrier. Without this acoustic treatment, realistic music does not sound good because of reflected sounds and echoes.
What properties control the quality of sound reflected by wood?
The quality of sound reflected by wood depends on the frequency. Softwood gives better high-frequency absorption than hardwood. The lower the frequency, the more sound energy gets reflected by both hardwood and softwood. However, softwood absorbs more low frequency sound than hardwood does at high frequencies. But as a room’s size increases, this effect diminishes and it becomes less relevant for practical purposes. For this reason, acousticians do not recommend reducing low-frequency sound absorption as this can actually increase the noise level of an entire room.
Do sound absorption treatments work well on wood surfaces?
Wood surfaces are excellent absorbers. The effectiveness of a sound insulation material depends on how much it absorbs. The higher the absorption coefficient, the more reflected natural sounds get reduced. The best wood-based acoustic treatment is a well-tiled ceiling with treated boards and walls. Sound treatment that is not properly installed may in fact have the opposite effect it may allow sound to be trapped within a room, resulting in an increased noise level. Good acoustical treatments do not act like a barrier against sound, but make it more diffuse by reflecting it in many directions.
The sound absorption of light-frame ceilings using OSB and plywood is poor because of their low density. The noise from the ceiling often creates a problem for rooms with the light frame structure because the sound energy gets reflected, amplifying it and creating an echo-like effect. An alternative is to use layers of hardwood boards instead of oriented strand board. Adding a layer of fiberglass below hardwoods also increases absorption, reducing echoes even more.
Wood is a purely natural material that has a large variety of characteristics. It can be designed in a range of wood selections with diverse physical properties, as well as various uses and finishes. Wood is available in every region with climate conditions, textures and species that vary between different parts of the world. The size of the available choice makes wood a versatile material for both construction and acoustic design.
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