Civil Engineering Reference
In-Depth Information
Fig. 6 Spectral normal-hemispherical transmittance and reflectance in the infrared spectral
range of a 10-mm layer of granular aerogel (Cabot Nanogel TDL 302) (Bavarian center for
applied energy research
2010
)
3.3 Acoustic Properties
The acoustic properties of silica aerogels are very interesting: the sound speed in
the material is lower than that in air, down to about 40 m/s through monolithic
aerogels and to about 100 m/s through granular ones. The sound propagation
depends on different parameters, such as the interstitial gas nature and pressure, the
density, and the texture (Forest et al.
1998
). In order to characterize the acoustic
properties of silica aerogels, the acoustic attenuation was measured by means of
the two-thickness method (Forest et al.
2001
), and it showed that the large granules
of aerogels (mean granules size 3 mm) have an attenuation (about 1 dB/cm) lower
than the one of a conventional insulating material, such as glass wool; neverthe-
less, in the small granules (mean granules size about 80 lm), the attenuation
values are in the 0-12 dB/cm range, depending on the frequency range: above
500 Hz, the values are higher than the glass wool ones (about 3 dB/cm).
The efficiency of granular silica aerogels as sound insulators in the
100-2,500 Hz frequency range was also demonstrated, when used in multilayer (a
fist layer (1 cm) made of large granules and the second one (3 cm) made of small
granules): in the 30-1,700 Hz range, the sound transmitted through the aerogel is
15 dB lower than the one transmitted through glass wool of the same thickness.
For these reasons, translucent insulting material, when used in interspace of
glazings, can improve sound insulation of the building envelope, if compared to
systems with air. In Buratti and Moretti (
2012b
), a prototype of window was
realized and tested: it was composed of an aluminium frame with two float glasses
(4 mm thickness) and granular aerogel in 15-mm interspace. The benefits of
aerogels in sound insulation were evaluated by means of the sound reduction index
