Ultralight Single‐Walled Carbon Nanotube Aerogels for Low‐Frequency Sound Absorption

Absorbing low‐frequency sound below 1000 Hz with an ultralight material is a critical challenge. Nanofibrous sound‐absorbing materials have many interfaces for air‐borne sound loss. In addition, nanofibers with a high elastic modulus can increase the loss of sound propagating in the solid, enabling good, low‐frequency sound absorption. Herein, a composite aerogel based on single‐walled carbon nanotubes (SWCNTs) and carboxymethyl cellulose (CMC) as an ultralight material that effectively absorbs low‐frequency sound is reported. This ultralight aerogel has a hierarchical porous structure composed of high‐modulus SWCNT–CMC composite nanofibers. A sample with an areal density of 20 mg cm −2 (bulk density: 5 mg cm −3 , thickness: 39 mm) achieves a sound‐absorption coefficient of 0.44 at 500 Hz. The sound‐absorption behavior is sufficiently described by the Biot–Johnson–Champoux–Allard model, indicating the significant effect of the high elastic modulus of the nanofibers on the sound‐absorption performance. The article presents a new design principle for ultralight materials with low‐frequency sound absorption that takes into consideration both the porous structure and the elastic modulus of the material framework.