posted 25 April 2005 18:26            

Given an increase of amplitude, does an individual molecule of air in the acoustic medium actually traverse more SPACE, or does it simply have more FORCE in the form of pressure or something? In other words, does a molecule, perhaps, NOT actually traverse more space, but rather simply have more energy in the form of SPEED, say?) relative to a nearby molecule? Does it matter? Is there a difference?

What does AMPLITUDE really represent, in other words... energy in the form of speed, or is it spatial displacement, of the air molecules?

What are we actually hearing occur in a LOUDER sound in terms of the behavior of individual molecules of air? Is there any way to know this? Am I ignorant of some basic physical facts?

Is this a dumb question?

posted 26 April 2005 06:06            

The answer (as I believe it) is both speed and pressure will increase, as the two are directly related. A louder volume will make the speed, the distance traveled and the pressure of the molecules all greater. The speed of a molecule determines how far it can travel into its near by molecules and how much sucsess it will have in squashing them together. The amount of squash of these molecules determine how fast they can launch other molecules. The energy keeps getting transformed from kynetic (speed)energy into potantial (pressure) energy and back again. Its difficult to measure as it is happening in a streem and many different energy transformations are happening at the same time, but all the pressure energy was created by speed energy and all the speed energy was created by presure energy (apart from the speaker movement that created all the energy in the first place). So apart from frictional losses and after the speaker has stopped generating additional energy the total speed and pressure energy added together will remain the same.

You can show this idea using wave forms in kyma and viewing them on the osciliscope . If we assume that a molecule is moving back and forth as a sine wave, then the greater pressure energy will be at the two peeks. To show it's energy we need to multiply it by it's self to get the square of the pressure. The speed of the same molecule will be the original positional sine wave shifted by 90 degs. If we take that 90 deg shifted sinewave and multiply it by it's self, we get a waveform that represents the kynetic energy. If we add this to the other squared wave form we get an output that is a constant DC. The energy may be transforming but there is no gain or loss.

Hope this makes sense

posted 26 April 2005 14:35            

(I really like Pete's description of how the amplitude and the velocity are periodically trading off with each other--like the difference between potential and kinetic energy. An air molecule moves until it bumps into another air molecule, transferring its energy to the neighbor, which then moves until it runs into *its* neighbor, etc.)

posted 27 April 2005 00:31            

When you turn up your amplifier, your speaker cone moves further, thus displacing the air molecules more.

Of course for a given frequency, the time it takes for the cone to go from one position to another is constant. Thus, the cone is moving faster (linearly) at higher amplitude. So the two phenomena are related.