Most commercial music services rely on collaborative filtering to recommend artists and songs. While this method is effective for popular artists with large fanbases, it can present difficulties for recommending novel, lesser known artists due to a relative lack of user preference data. In this paper, we therefore seek to understand how content-based approaches can be used to more effectively recommend songs from these lesser known artists. Specifically, we conduct a user study to answer three questions. Firstly, do most users agree which songs are most acoustically similar? Secondly, is acoustic similarity a good proxy for how an individual might construct a playlist or recommend music to a friend? Thirdly, if so, can we find acoustic features that are related to human judgments of acoustic similarity? To answer these questions, our study asked 117 test subjects to compare two unknown candidate songs relative to a third known reference song. Our findings show that 1) judgments about acoustic similarity are fairly consistent, 2) acoustic similarity is highly correlated with playlist selection and recommendation, but not necessarily personal preference, and 3) we identify a subset of acoustic features from the Spotify Web API that is particularly predictive of human similarity judgments.