Have you ever observed a stream of water flowing where there were no ripples, no splashing, and no bubbles? Did you notice that there was also no sound, or nearly no sound? Here’s my theory: Trapped pockets of air collapsing create the sounds we associate with water.
This idea first occurred to me when I was working on the film “The Revenant.” Alejandro Iñárritu, the Director, wanted the ground around the Leonardo DiCaprio character and the bear to sound wet, very wet. In fact, that was almost as important to him as the vocalizations the bear made. Several attempts were tried in post production using foley and wild effects recording in the field to make footsteps and other movement sounds that would feel wet. All those attempts had failed, and I eventually realized why: The surfaces were wet, which would sometimes create a “slappy” sound during a footstep, but the slappy sound did not translate to the ear as “wet.”
“Squish” is the epiphany I finally had. What makes a surface feel wet (other than a splash, which I will get to soon) is the sound of squishing. What is the sound of squishing? It’s the fizzy sound of bubbles moving and collapsing. When a porous material is soaked with a liquid like water, but also has lots of little air bubbles within, and when that material is pressed and/or twisted, the water moves and the bubbles collapse, creating the squishy/fizzy sound we associate with walking on a soaked lawn, or with tightly twisting a soaked towel.
The solution to the problem on “The Revenant” was to make sure that not only was the ground wet, but also that it and the grass-leaves-pine needles covering it were porous, that there were air pockets which would move and collapse when pressed. The other part of the solution was to create small splash sounds, even though there are no splashes visible in any of the shots. It was a successful sound cheat.
What is a splash? In terms of sound, I would argue that it is the collapse of an air pocket, or more precisely, a sequence of air pockets collapsing. The first air pocket collapse is the initial impact (e.g. a footstep in a puddle of water), then there are then multiple secondary impacts as separate droplets or other volumes of water set in motion by the initial impact are caused to impact nearby. Most of the sounds we associate with water moving in a stream, ocean surf, swimming, etc. are caused by splashing, not merely by water “moving.”
So, what is the sound design implication? Wet surfaces often don’t sound wet. The way to make them sound wet is to create a squish and/or a splash. Wet grass is a notoriously difficult sound to get. “Squish” is at least one solution that will work in many cases.