The Sound of Running Water
Auditory textures such as the sound of running water, rain, fire, wind, or insects are readily recognized by our auditory system. We study running water and showed its sound has many interesting physical features, such as scale invariance. We use these features to create artificially synthesized versions of such sounds. We study human listeners' perception of these sounds, and what features in the sound lead to its categorization as a "natural" sound.
Publication (open access): Geffen MN, Gervain J, Werker JF and Magnasco MO (2011). Auditory perception of self-similarity in water sounds.. Front. Integr. Neurosci. 5:15. doi: 10.3389/fnint.2011.00015
Sounds that repeat steadily and continuously, such as the pattering of rain or a quiet fire, are called auditory textures. Many such textures relate to matters that are of great ecological importance to living beings, such as fire or water, and evolution put pressure on the brains of organisms to readily discriminate them. Indeed, sounds of water and fire are easily, effortlessly recognized by our auditory system, and therefore present a unique opportunity to study how the brain recognizes sounds.
In the above study, we concentrated on the sound of running water, typified by the sound made by a stream. We show that such a sound has interesting physical properties, such as scale invariance: if we play back the tape at different speed, it still sounds like running water. Scale invariance means that what happens at different frequency ranges within the sound (i.e. in the treble or the bass regions) are related.
Based on the properties of the sound of running water, we created a method to synthesize it. We created fairly realistic synthetic versions of this sound very simply.
We played these artificial versions, as well as the originals and a number of altered versions, to a number of human subjects, and studied which particular features of the sound lead to its perception as "natural". We show that chief among these properties is retaining scale invariance.
This suggests that our auditory system performs comparisons across broad frequency ranges to recognize auditory textures.