Effect Of Area
The larger the window area, the more acoustic energy
is able to pass through it. However, because of
the way in which sounds add together, this is not
a dominant effect. If the opaque parts of a façade
(walls) are considered to have a sound insulation
at least 10 dB higher than the window, then each
halving of window area increases the effective sound
insulation by 3 dB. If, for example, a 100% (fully)
glazed façade had a sound insulation of 30
dB, changing to 50% and 25% glazed façades
would secure corresponding aggregate sound insulations
of 33 dB and 36 dB, respectively.
Effect Of Distance
With increasing distance
from a noise source, there is a corresponding decrease
in noise level. The decay rate is dependent upon
whether the source originates at a point or over
an area, which, in turn, depends on its shape and
the distance at which the sound is heard.
Noise from an aircraft
or a single vehicle may be considered to be a point
source, and the decay rate approximates to 6 dB per
doubling of distance. Noise from a line of traffic
or railway train does not diminish as rapidly; 3 dB
per doubling of distance is a typical decay rate.
For example, if
a roadside noise measurement, 5 metres from the vehicles,
indicates a level of 78 dBA, this would reduce correspondingly
to 75 dBA at 10 metres, to 72 dBA at 20 metres and
to 69 dBA at 40 metres. This 9 dBA drop in level from
the initial measurement is almost equivalent to a
halving in its perceived loudness at the kerbside.
Effect
Of Height
With increasing building height, there should be an
associated decrease in received street noise level,
but the acoustic horizon of the upper floors is correspondingly
increased. These are opposing trends, and the result
is that the vertical noise field is almost uniform.
Barriers
Sound waves have long wavelengths, as mentioned earlier.
Therefore diffraction effects are significant; that
is, bending around obstacles is relatively easy. Fences,
earth mounds and screens are therefore relatively
ineffective, unless positioned either close to the
source or close to the reception point. Window furniture,
like blinds, has no influence on the noise transmission
through windows.
Other
Glazing Types
Apart from lamination, other glass types do not exhibit
any departures from the sound insulation of ordinary
clear float glass. Thickness for thickness, toughened,
wired, coated and tinted glass products have exactly
the same acoustic performance. Patterned glass products,
which have surface indentations, behave identically
to ordinary clear float glass of the same average
thickness.
Fixing
Effects
Better sound insulation is achieved by installing
glass in resilient gaskets instead of tightly clamping
it with beads. Any structural vibrations are then
less able to be transmitted to the glass, thus maintaining
its acoustic integrity. |
