The acoustical design issues for buildings involve the principal issues like site noise considerations, including the control of noise transfer to a project’s neighbours, particularly if they are residential, establishing noise standards for each use space, including limitation of excessive ventilation noise, room acoustics considerations, sound isolation between various use spaces, vibration control for mechanical equipment, audio/visual system considerations.
Building planners should develop objective acoustical standards for library projects as an important component of the project program. The information contained in this article about library acoustics is intended as a source for these standards.
As the architectural and engineering design of the project evolves, the design should be reviewed in light of the agreed upon acoustical programmatic requirements for the building project. Since acoustics is typically not a code requirement, a city or state building official cannot be expected to comment on the correctness of the acoustical design in the contract documents.
Therefore, it is the responsibility of the facility’s planners, user groups, architects, engineers, and others involved with the project to assure that the project acoustical needs are delineated and that there is follow-through, particularly for verification testing after the ventilation system has been installed and balanced.
Sound and Noise
Sound waves in air result from a physical disturbance of air molecules, such as when a truck drives by a building or when guitar strings are plucked. Sound waves combine and reach a listener via numerous direct and indirect pathways. The listener’s inner ear contains organs that vibrate in response to these molecular disturbances, converting the vibrations into changing electrical potentials that are sensed by the brain, allowing hearing to occur.
Acoustical analysis involves not only the sound source but also the listener and everything in between on the path of the sound. The perception of the receiver can be influenced by the treatment of either the path or the source. Some source sound is desirable, for example a lecturer’s voice, and some source sound is undesirable, such as the sound output from an idling truck outside a window. Undesirable sound is usually called noise.
Unless it is a pure tone, a sound wave is typically made up of vibrations at different frequencies. Like the impact of a stone in a lake, ripples in the water are created that are analogous to sound in the air. The frequency is basically the number of waves that pass a single point in one second, moving at the speed of sound in air. One wave per second is a frequency of one hertz (Hz). A frequency of 1,000 hertz is a kilohertz (kHz).
Everyone has experienced unwanted sound intrusion – a television in the next room, a loud neighbour walking on the floor above, or a jet flying over. Measures are often required to reduce intrusive noise. One of the most essential techniques in acoustics is reducing the transmission of sound through solid barriers in buildings. This form of sound reduction is referred to as Sound Insulation.
Principles of Sound Insulation
The reduction of sound energy from one building area to another by absorbing it or reflecting it with an intervening solid panel of material is called sound transmission loss (TL). Typically, building materials attenuate more high frequency noise than low frequency noise. The higher the mass or weight of a wall, the more force is required to make it vibrate. For this reason a massive wall has higher TL at all frequencies than a lighter panel.
Another way to increase the transmission loss of a panel or construction, such as a wall, is by increasing its thickness and isolating one side of the construction from the other. This is commonly done by using two panels separated by an air cavity, and is known as a dual panel partition. Doubling the air space width increases the TL by about 5 dB. Usually, the dual panel approach is more effective and lower cost than increasing wall mass.
These sound reducing partitions are needed between spaces with different acoustic requirements or spaces that require acoustic privacy. They are also necessary in some cases as part of the exterior building envelope, if environmental noise at a site is a particular concern. Walls, floors and ceilings enclosing spaces where unwanted noise is generated, such as mechanical rooms, normally require a high standard of sound reduction.