Adding Gage's Normalized Difference Soundscape Index.
From: Kasten, Eric P., Stuart H. Gage, Jordan Fox, and Wooyeong Joo. 2012. The Remote Environmental Assessment Laboratory’s Acoustic Library: An Archive for Studying Soundscape Ecology. Ecological Informatics 12: 50–67. doi:10.1016/j.ecoinf.2012.08.001.