Sound is a constructed experience

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Chapter 7 endnote 1, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

A sound, therefore, is not an event that is detected in the world. It is an experience constructed when the world interacts with a body that detects changes in air pressure, and a brain that can make those changes meaningful. [...] Some people believe that these vibrations are the essence of the sound because a sound cannot be heard without them. But this explanation misses the point. Vibrations are not sufficient for a sound to occur. Sounds do not have simple, single causes.

Vibrations are not sufficient for a sound to occur. If humans had no ears (with all the machinery inside), we wouldn’t even have a concept of sound. It is possible to experience those same “vibrations” as something other than sound. Go to a loud concert and experience the physical thud of the bass drum in your chest.

In principle, these vibrations could even be “seen” as visual images rather than “heard” as sounds. This is known as “sensory substitution.” For example, “seeing” can be transformed into “touching.” A blind person can wear a sensor, for example, a small camera on her forehead, that translates visual images, pixel by pixel, into something that will stimulate a part of the body that receives touch sensation, for example, patterns of pressure across the tongue. The brain quickly learns to “see” with the tongue. Sensory substitution was originally discovered by the neuroscientist Paul Bach-y-Rita to treat patients who suffered from damage to one sensory system.[1][2] Many devices have now been invented to allow people to “see” with sound. Currently, vibrations on the fingers (i.e., touches) can be “heard” as sound.[3] These sensory substitution effects occur because all neurons, in effect, are multi-modal. The brain only appears to have different systems dedicated to processing different sensory modalities.[4]

Vibrations are also not necessary for a sound to occur. Just ask anyone with tinnitus, a permanent ringing in the ears.[5]

Taken together, this means that the tree falling in the forest does not make a sound, unless they are accompanied by a working human ear and brain that can construct a concept of a "Tree" to simulate what a tree sounds like when it falls. The falling tree only makes vibrations. Those vibrations can be detected and experienced in any number of ways. If you simulate the sound of a tree falling in the forest, you can hear it inside your head via the same mechanisms that plague you when you have a song stuck in your head (i.e., an "earworm").

Notes on the Notes

  1. Bach-y-Rita, Paul, Carter C. Collins, Frank A. Saunders, Benjamin White, and Lawrence Scadden. 1969. "Vision substitution by tactile image projection." Nature 221 (5184): 963-964.
  2. Bach-y-Rita, Paul. 2004. "Tactile sensory substitution studies." Annals-New York Academy Of Sciences 1013: 83-91.
  3. Schürmann, Martin, Gina Caetano, Yevhen Hlushchuk, Veikko Jousmäki, and Riitta Hari. 2006. "Touch activates human auditory cortex." Neuroimage 30 (4): 1325-1331.
  4. For a broader discussion, see Pascual-Leone, Alvaro, and Roy Hamilton. 2001. "The metamodal organization of the brain." Progress in Brain Research 134: 427-445.
  5. Tinnitus is a problem with processing auditory prediction error. See Sedley, William, Karl J. Friston, Phillip E. Gander, Sukhbinder Kumar, and Timothy D. Griffiths. 2016. "An Integrative Tinnitus Model Based on Sensory Precision." Trends in Neurosciences 39 (12): 799-812.