The acoustic properties of the silica aerogel (SA) granules of various sizes from 0.50 to 3.35 mm, distributed into six groups of nominal sizes and measured via a two-microphone impedance tube, are presented. The absorption coefficients of the SA granules were evaluated at ultra- to super-low frequency range from 50⁻1600 Hz. It was observed that nominal SA granules with sizes of 1.2 mm (AG2) and 1.7 mm (AG3) displayed the best absorption coefficients. When tested with granules filled at 5 cm depth, AG2 and AG3 absorption coefficients peaked at 980 Hz with values of 0.86 and 0.81, respectively. A novel approach to measure transmission loss (TL) by using "inferential" principle is presented. This novel method, named "Inferential Transmission Loss Method" (InTLM), revealed that the average TL, TLavg for both AG2 and AG3 SA granules was 14.83 dB and 15.35 dB, respectively. Gelatin silica aerogels doped with sodium dodecyl sulfate (GSA⁻SDS) composites comprising of 1.2 mm (GSA⁻AG2) and 1.7 mm (GSA⁻AG3) granules of various configurations were fabricated and evaluated for absorption coefficients and TL with known traditional acoustic panels. The results showed that GSA⁻AG3 had a better absorption coefficient over other configurations for the same corresponding thickness reaching the peak of 0.6 from 1300 to 1450 Hz with TLavg between 10.7 and 20.3 decibels. The four-layered GSA⁻AG2 and GSA⁻AG3 composites showed exceptionally high absorption from 500 to 800 Hz suitable for narrow band applications. Lastly, the "InTLM" was matched with the sound meter measurements, with high accuracy between 0.3 and 3.2 dB for low-frequency testing (50⁻1600 Hz).
Keywords: absorption coefficient; acoustics; composites; inferential transmission loss method; silica aerogel.