This must all be analogous to somelthing...
I personally dislike analogies and prefer to attack problems directly. But if I had to choose an analogy from the choices discussed here, I would choose the slinky.
I don't mind using analogies...so long as there are no misconceptions about what the analogy shows. To simply say that the Newton Cradle thing is a good analogy for sound propagation is bad practice. To note that it's a good model for showing how energy can be added to a system and then have that energy propagated through the system at a speed that is unrelated to that of the disturbing force is a pretty good use. It's also pretty good to use that analogy for expaining the fundamentals of how impedance mismatches relate to reflections and provide the mechanism by which the energy can be extracted from the system.
The Slinky model is a good analogy for something like the water wave and can be used to visually demonstrate both traverse and longitudinal waves and how they move through some medium. The trouble with the Slinky is that it demonstrates
none of the facors related to
sound propagation. When introduced into a discussion about sound propagation it mostly just muddies up the waters.
The propagation of sound in a gas has its own unique features that need to be understood. Once those are understood you can go back and make the connection with the analogs whether Newton's Cradle, steel rod and ball, or slinky. They all transmit energy and so possess certain common features. But that's really turning the problem on its head. Its better to attack the problem of propagation of sound straight on.
I can't disagrree with that.
In Friday's post where Crashsite talked about compression cycles, and rarifaction, and adding energy I thought that Crashsite was finally moving in the right direction. He was missing one major concept but it seemed possible that he might discover it soon.
Pray tell me the major missing concept!
There's a maxim, in the training world, that technical training is
not an
Agatha Christy mystery. You don't hold back information to heighten suspense. Of necessity, the instructor does hold back information to ensure that it's presented at the correct point in the presentation but, if it's needed to understand the subject, there's no point to tease it while keeping it a mystery.
However, with yesterday's post about vector summation, it seems things are back at square zero. The post was more like numerology than physics and the so called vector summation wasn't.
At this point I feel fairly comfortable with the vector analysis. I've been touting the vector angle (no pun intended) for most of this thread.
There is one fact about sound propagation that definitively tells me that it
must be directly and pretty much exclusively related to molecular speed.
When you realize that the speed of sound is essentially unaffected by pressure, that must mean that
sound travels just as fast when air molecules are further apart or closer together. Failing some sort of "tunnelling mechanism", sorcery or other supernatural phenomena, the speed of sound must be as fast as a molecule can carry it before striking another molecule (also moving at a similar speed) and in that manner propagating the sound.
My graph analysis assumes the molecules to be moving randomly at a nominal speed of 1100 mph (due to heat). The molecules are moving in
all different directions. The disturbing force (as it's being handed off from molecule to molecule)
is moving radially outward (propagating) from the point of disturbance only. But, we can be selective as to which direction we sense that propagation (the direction to our detector).
But, just because we can be selective about what direction our detector is, we
cannot control the amount of sound energy that's being propagated in the direction of our detector by molecular interactions along axes other than the one between the disturber and our detector. We are receiving the
vector sum of the molecular speeds (with the energy from the disturber being carried along with them).
I know that's a pretty involved concept but, envision what the total speed of the molecules in any given direction is when the vector amplitudes and directions are averaged.
