Re: Does sound only exist in the human brain?

Hi Tracey

OK, probably more of a semantics/philosophy question than science, but 
here goes...

The Oxford English Dictionary defines sound as a mass noun 
meaning "vibrations that travel through the air or another medium and can 
be heard when they reach a person's or animal's ear" in which case sound 
exists regardless of whether anyone or anything is there to hear it.

However, Webster's (a US dictionary) defines it as a noun (not a mass 
noun) meaning "the sensation experienced when the brain interprets 
vibrations within the structure of the ear caused by rapid variations of 
air pressure".

This second definition implies that sound DOESN'T exist outside the brain, 
but this is not true - the Websters definition is simply incomplete, and 
in afct some dictionaries give both definitions. The explanation for this 
is as follows;

First, very low frequencies can be felt by animals not equipped to hear 
them, and so, although an elephant may hear a loud sound with a frequency 
of 5 hertz, a human could feel it as a rumble. So, it exists as a sound 
within the elephant's brain, but also externally. A loud sound could also 
affect something unequipped to experience it (e.g. a drum would vibrate as 
seen if beads are placed on it and a sound played). Similarly, very high 
pitched sounds can be heard by bats, but not humans, and if loud enough 
could cause damage/pain even though unheard. So, again this could exist 
both internally and externally.

Second, the ear is simply a transducer i.e. a system that converts energy 
from one form (vibrations) to another (electrical impulses) therefore, 
sound exists both within the brain as sensation AND externally as 
vibrations in a medium - it is just the energetic form it takes that 
changes.

Third, the vibrations and the experience of sound are different but 
fundamentally related entities - for example, if someone was mad 
and 'heard voices', these would not need air vibrations to generate them, 
but would probably also be a different sensation from true hearing as they 
would be created at a deeper level than the auditory pathway and would not 
pass from the ear at any point. Thus, they are separate from hearing as 
normally perceived and although they exist only in the brain, are not 
strictly speaking 'sound'. This is also relevant for the sounds heard due 
to tinnitus which are perceived even when in silent surroundings - it is 
simply damage to auditory apparatus being perceived by the brain via a 
transducer i.e the ear which can ONLY create sound, even when affected by 
a different stimulus. This is why your ears ring if you are hit on the 
head - it is a solid physical stimulus rather than vibration, but your 
ears are only able to respond one way.

So, in summary I would say that no, sound does not only exist in the brain 
but that the sensation we experience is in a different form from the 
vibration that causes it, as the latter could still pass through the 
physical substance of the brain AT THE SAME TIME as the electrical 
impulses that generate the sensation/experience it causes. 

Anyhow, I hope that helps clarify (and not confuse) matters,

Yours,
Dr David Hubble, Hampshire, UK