This is a research project of a psychoacoustic bass enhancer based on following papers:

Oo, N., Gan, W. S., & Lim, W. T. (2010). Generalized harmonic analysis of Arc-Tangent Square Root (ATSR) nonlinear device for virtual bass system. In 2010 IEEE International Conference on Acoustics, Speech and Signal Processing (pp. 301–304). https://doi.org/10.1109/ICASSP.2010.5495913

Gan, W. S., Kuo, S. M., & Toh, C. W. (2001). Virtual bass for home entertainment, multimedia PC, game station and portable audio systems. IEEE Transactions on Consumer Electronics, 47(4), 787–794. https://doi.org/10.1109/30.982790

Bai, M. R., & Lin, W. (2006). Synthesis and Implementation of Virtual Bass System with a Phase-Vocoder Approach. Audio Eng. Soc, 54(11), 1077–1091.

Arora, M., Moon, H., & Jang, S. (2006). Low complexity virtual bass enhancement algorithm for portable multimedia device. In AES 29th International Conference (pp. 2–5).

Shi, C., Mu, H., & Gan, W. (2013). A psychoacoustical preprocessing technique for virtual bass enhancement of the parametric loudspeaker. In 2013 IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP) (pp. 31–35).

Basically, this algorithm adds some distortion to the low-frequency part of the spectrum in order to simulate missing low frequencies. TBH, if the distortion level is low then the bass enhancing effect is too subtle and if the distortion level too high then this is perceived as a distorted signal.