Acoustic signals are used for species recognition, mate choice, and resource defense, but the frequencies used are restricted by body-size, phylogeny, habitat structure, and biotic and abiotic sounds in the habitat. Acoustic space is a limited resource, and studies have suggested that species avoid competition by partitioning this space in time and frequency. Alternatively, other studies have reported temporal synchrony and spectral overlap among species. These studies have focused on individual taxonomic groups (e.g. fish, bird, insects, and anurans), and no study has assessed patterns of acoustic activity for all species in a community across a gradient of species richness. Here we compare the use of acoustic space (i.e. soundscape) in nine tropical forest sites, and show a highly significant positive relationship between the percent of acoustic space used and total bird, amphibian, and mammal species richness. If community richness is relatively low, species may avoid overlapping and maintain signals within the optimal frequency range of a given habitat, but in species-rich communities, species will encounter greater signal overlap and will be forced to use other frequencies. Our acoustic species richness hypothesis unifies the acoustic partitioning and network hypotheses by demonstrating how variation in species richness across sites affects the patterns of activity within the acoustic space. This novel approach for analysing soundscapes contributes to our understanding of ecological community dynamics (e.g. niche theory) and provides useful tools for monitoring species in the context of restoration ecology, climate change, and conservation biology.