Sound particle
In the context of particle displacement and velocity, a sound particle is an imaginary infinitesimal volume of a medium that shares the movement of the medium in response to the presence of sound at a specified point or in a specified region. Sound particles are not molecules in the physical or chemical sense; they do not have defined physical or chemical properties or the temperature-dependent kinetic behavior of ordinary molecules. Sound particles are, then, indefinitely small (small compared to the wavelength of sound) so that their movement truly represents the movement of the medium in their locality. They exist in the mind’s eye to enable this movement to be visualized and described quantitatively. Assuming the medium as a whole to be at rest, sound particles are imagined to vibrate about fixed points.
A more rigorous scientific aproach recognises the existence of small "parcels" of air or whatever medium carries the sound wave. Such infinitesimal volumes are taken significantly smaller than the sound wavelength. However the parcels will have all the typical physical properties representative of the medium. For solids and liquids the parcels would need to be significantly larger than the interatomic distances to be considered as bulk solid or liquid. For gases the parcels need to be significantly larger than the mean free path of the random brownian motion, again so as to be representative of the gas in bulk. such parcels in say, gas, would obey the relevant physics. For example parcels exert pressure on neighbours and can exchange heat by conduction depending on particular cicumstances. Sound waves inherently have wavelength scale temperature changes from compression and expansion within the wave. Often heat conduction is slow and then adiabatic gas laws better approximate the physics within parcels.
Such parcels of material are semi macroscopic and comprise myriad molecules or atoms. These real atomic and or molecular particles have violent thermal brownian motions. The averaged motions correspond to the parcel movement and is the displacement of the macroscopic sound wave. Often times in context of sound waves or indeed water surface waves we ignore the random thermal motions and simply imagine the actual molecules smoothly moving in unison with the wave.
The terminology "sound particle" in the context here should be regarded as highly non-scientific and avoided in favour of recognising medium parcels and displacement or simply recognising thermally averaged molecular locations to extract sound wave physics. In terms of waves on stretched strings for example, it is clearly sufficient to acknowledge the idea of a small localised piece of the string or indeed the location of string molecules (ignoring irrelevant thermal averaged out motions) as "particles" for our dynamical string. It would be overbearing and confusing to talk about string wave particles. The particles pertain to the string and in no sense do such classical particles belong to the wave. Indeed in the absence of any sound would a "sound particle" still exist? Clearly an absurd and misleading terminology.
See also
References
 | This article includes a list of general references, but it lacks sufficient corresponding inline citations. (May 2015) |
- Haughton, P. M. (2002). Acoustics for Audiologists. Academic Press.
- Unnikrishnan, C. S. (10 April 2005). "On the gravitational deflection of light and particles" (PDF). Current Science. 88 (7).
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