Acoustic Properties of Monolithic Silica Aerogels

Silica aerogels offer advantages in building applications as they are thermally and acoustically insulating and can be translucent. To date, the acoustic performance of monolithic silica aerogel in terms of both absorption and transmission loss has not been examined in detail. In this work we have investigated the acoustic properties of monolithic silica aerogel samples fabricated using a rapid supercritical extraction method [1-3]. The acoustic absorption coefficient (α) and the transmission loss (TL) of monoliths were measured at normal incidence in an impedance tube. Good acoustic performance was achieved: 12.7-mm-thick cylindrical monoliths have peak α of 0.88 at ~1500 Hz; when the thickness increases, α decreases (to 0.78 and 0.54 for 19-mm and 25.4-mm samples, respectively), with peaks at lower frequencies (1300 and 1100 Hz). The TL increases with aerogel thickness with values as high as 10-15 dB in the 100-1600 Hz range. When compared to granular aerogels [4] the monoliths have TLs that are 5-7 dB larger in the 100-1600 Hz range. To compare performance, glazing packages were fabricated from glass panels with air, granular, and monolithic aerogel in the interspace. The TL was evaluated and found to be in the 35 – 45 dB range for all samples. The glazing system with monolithic aerogel had the highest TL, particularly in the 200 – 1000 Hz range. We estimated a 3-dB increase in the sound insulation index (Rw) for the monolith when compared to the double-pane glazing system with air, which is 1-2 dB better than the performance of the granular aerogel glazing. This study demonstrates that the use of transparent monolithic silica aerogel in the interspace of conventional glazing systems would result in significant improvement in noise insulation.