Abstract:
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.
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.
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).
Sound and noise are described using a metric called the decibel.
The decibel scale is logarithmic, similar to the Richter scale used to describe
seismic events, and translates a wide range of sound pressure levels that affect
the human ear to a logarithmic scale. The range of decibels most commonly encountered
in acoustics extends from 0 to 140 dB. Figure 2.2 correlates the sound pressure
levels of common sound sources to the logarithmic decibel scale.
Sound Absorption
All materials have some sound-absorbing properties. Sound energy that is not absorbed must be reflected
or transmitted. A material’s sound-absorbing
property is typically described as a sound absorption coefficient at a particular
frequency range. Sound absorbing materials
used in buildings are rated using the Noise Reduction Coefficient (NRC), which is
basically a type of average of sound absorption coefficients from 250 Hz to 2 kHz,
the primary speech frequency range. The NRC theoretically can range from perfectly
absorptive (NRC = 1.0) to perfectly reflective (NRC = 0.0).
Adding sound-absorbing materials to a space usually becomes an
interior design issue in the library. Many options are possible to provide
sound absorption on walls and ceilings, which are attractive and maintainable. Absorptive
materials are often covered with acoustically transparent surfaces such as fabric,
perforated metal and spaced wood slats. These surfaces allow the sound energy to
pass through and be absorbed by the material located behind. Figure 3.1 shows
the
Sound Insulation
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.
Sound Insulation Construction
example of a wood slat panel treatment that effectively screens
the acoustic blanket and creates a handsome ceiling in a public area.
Perforated metal panels, as shown in Figure 3.2, are commonly used
to create a certain finish appearance. For best results, the material should be
as thin as possible, with the smallest hole diameter and the greatest open area
(the greatest number of holes).
Some absorptive materials are attractively designed to be exposed to
view, such as normal suspended ceiling tiles. Generally, thicker porous materials
provide better sound absorption. 5/8- inch thick ceiling tiles have an NRC of 0.50
when mounted in a lay-in grid ceiling. A 1-inch thick glass fiber ceiling tile can
have an NRC rating of 0.80 or greater. Figure 3.3 illustrates the appearance of
a suspended acoustical tile ceiling. Another approach to adding acoustic absorption
to the space is to suspend acoustic baffles as shown in Figure 3.4
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