explain in terms of energy transfer?

Question

explain in terms of energy transfer (dissipation) why sound becomes fainter further away

Answer 1

Sound requires a medium for the waves to exist in,
XPL: air, water, wood,etc.
Sound cannot not exist in a vacuum. Some of the energy of the sound wave is lost by passing through a medium, like erosion of a mountain via rain. Eventually, far enough away from the source, the sound wave looses all of its energy to the medium. Louder sounds have more energy, and travel further...

Answer 2

Sound becomes fainter the further away the listener is from the source for two reasons, of which I have to say that the second one is generally much more important. But first let me deal with the issue of energy dissipation:

1.) Since sound requires a medium for its propagation, some energy is given to the molecular constituents of the medium it passes through; however, that net energy transfer is generally VERY small.

In the usual idealized mathematical treatment, none of that energy is retained by the medium. That treatment is often justified, for example, by the fact that the velocity of sound is certainly measured to be close to the adiabatic value given by v^2 = gamma P/rho.

[Newton himself missed the factor of gamma, assuming in effect that the microscopic and transitory pressure and density changes were isothermal. He used a small pendulum, adjusting its length to give swings in time with the echo-length in a long, flagstone covered walkway at Trinity College, Cambridge, to determine the speed of sound. The discrepancy from his expectations worried him greatly, and he returned to the problem again and again, in successive editions of the Principia, with unsuccessful attempts to reconcile the discrepancies.]

The isothermal process that Newton contemplated would in fact imply SUBSTANTIAL and IMMEDIATE microscopic energy transfer as the wave passed by. (The understanding of gas behaviour was rudimentary at the time; the concept of adiabatic changes had yet to be discovered.)

At SOME level, the assumption of adiabaticity must be inaccurate. However, the departure from adiabaticity would involve some power of the ratio of the timescale for an element of disturbed gas to come to equilibrium relative to the sound crossing time. That ratio is incredibly small!

2.) Most people would agree that where sound is concerned, by far the largest effect making sound fainter the farther it travels out in the three-dimensional world is the spreading out of the waves over larger and larger areas. The total energy emitted in some given time has to be spread out over an area that increases proportional to (distance)^2. Thus the sound naturally becomes fainter as 1/(distance)^2.

In other geometries one would have different dilution factors. A suspended (and VERY LARGE) circularly symmetric disk (large to avoid reflection back from its circumference), struck at its centre, would propagate sound waves through itself that diminished in strength as 1/(distance).

By the time that one consides a long cylindrical rod struck at ITS end, no direct diminution with distance alone would be expected. That effect is perhaps confirmed by the surprisingly large distances over which people claim that light tapping on pipes or railway lines can be detected (or even sound propagating down the confined region of a straight river with high banks). However, in this case there can be substantial interference effects dependent upon the cross-sectional geometry of the cylinder; its parts can be left vibrating to some degree. In addition to that, there is some energy transfer to the surrounding medium itself (air, for example). Such effects will ultimately dissipate the sound energy passing through the cylinder (or along the river).

Live long and prosper.

Answer 3

Because the sound waves are absorbed by the air. Sound produces waves which vibrate the air particles. The more particles ie passes through the fainter it is. This is why you can hear anything in a vacuum.