An essential sound is one made based off of purely the material properties of the object itself, and that any objects involved constitute a musical instrument. This philosophy is present in many lowercase works and often bends the definition of a musical instrument and the ways that musical interaction is thought of. This concept is sometimes a key feature of sound sculpture.
Some examples of essential sounds are the crackling of hot coals as they burn, or the sound of the rain falling onto various materials. These sounds have the property of being purely based of the material interactions between the parts of these instruments. Almost never are essential sound instruments directly interacted with by a player. Most often the player simply sets the material properties that produce an essential sound into motion (such as lighting the coals but not interacting with the sound any further).
Another example is the Iannis Xenakis piece Concret PH 1) This piece is a recording of hot coals being burned in a specific acoustic space. Though the tape recording has been manipulated the coals, and to an extent the room they were recorded in, can be considered a essential sound instrument.
The concept of the rain falling onto an object being a musical instrument is a very broad definition. What needs to be taken into consideration is that not all rain, or any force making sound at any time is considered under the definition. What is considered is specific examples with intentions attached to them. Hot coals recorded as a conceptual piece of music or performed live on stage is different then any instance of coals burning without the intention to burn them as a work of music.
This concept needs some considerable fleshing out. Overall its difficult to discern what makes it different from any kind of interaction done to make sound, and the word essential is a bit misleading.
I wrote a piece for the cars driving on the freeway, in which there is no interaction with the freeway directly, the interaction is simply for the audience to be standing on an overpass in order to hear it. The freeway in this case is an instrument that is indiscrete, self contained, and stochastic. The cars themselves are fungible but the freeway itself, and the location the listener stands are not fungible. This work is interesting because it questions the boundaries and definitions of a musical instrument (is the whole freeway now a huge instrument?) and the boundaries of a performance, was the freeway being performed before my composition and continuing to now? Is it just a piece where the people stand or is it also performing along its whole boundaries?