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, lowfrequency sound absorption. Herein, a composite aerogel based on singlewalled 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 highmodulus SWCNT-CMC composite nanofibers. A sample with an areal density of 20 mg cm À2 (bulk density: 5 mg cm À3 , thickness: 39 mm) achieves a soundabsorption 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 soundabsorption 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.

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