Properties of low-frequencey sound

1. Nov 3, 2005

locke

I would like to know the physics behind the low attenuation and low absorption of low frequencey sound. Every book/resource I've consulted so far presents these two propeties as empirical facts without really seting them in any kind of theoretical framework. Can anyone here point me in the right direction?

2. Nov 3, 2005

mathman

Rough idea: Low frequency is equivalent to long wavelength. If the wavelength is long compared to a typical dimension of the object (for example, wall thickness), the sound wave doesn't interact much with the object.

3. Nov 4, 2005

locke

Seems sensible. But why is it that low-frequencey sound is absorbed less by the ground (to take an example of an object with larger dimensions) than higher frequencies?

Thought: When a pressure wave impacts the surface of a wall, I would imagine this sets up a vibration that travels though it in every direction (though probably with more intensity in the direction of the wave). Crudely modelling the wall as a highly-dampened, driven oscillator: It would seem reasonable to expect that the closer the frequencey of the driving vibration is to the natural frequencey of the system, the easier it will be for the driving vibration to propogate through the system. Since solid objects tend to have low resonant frequencies, is this why low f sound waves are not highly absorbed by solid objects?

Would this also explain why low f sound waves are so dispersive? Since a large portion of anything solid the wave interacts with will be set vibrating and thus emitting sound in many directions? Or am I just spouting gibberish?

Last edited: Nov 4, 2005
4. Nov 4, 2005

thephy

It is due to the crystal model of solid. When the frequency of the sound is too low to cause resonance vibration with the atoms of the solid, sound waves are not highly absorbed by solid objects. And on the other hand, it is the fact that when the frequency of the sound is too high, sound waves are also not highly absorbed by solid objects. The difference is that low frequency sound will penetrate the solid without much absorption but high frequency sound will be reflected by the solid.

Last edited: Nov 4, 2005
5. Nov 4, 2005

locke

I always assumed that we could detect transmitted sound on the far side of a barrier because the incident sound waves vibrate the barrier. In light of what you have told me, I'm obviously mistaken.

My next question then is how are we able to hear something on the far side of a wall? Can sound travelling in air only be transmitted through a slightly pourous medium?

6. Nov 7, 2005

locke

Sorry to keep posting questions here, but I'm a little confused. Surely if a vibration is closer to the natural frequency of a solid, the greater the sound transmitted through the solid (rather than absorbed by it).

These questions are getting disorganised, to summarise:
-is sound of frequency f aborbed more, or less by a barrier with natural freq f?
-is sound heard on the far side of a barrier because it vibrates it, or because it somehow passes through without interacting with it?

Last edited: Nov 7, 2005
7. Nov 8, 2005

thephy

Sound is heard on the far side of a barrier because it vibrates the lattice of the solid. And only when the f of sound is similar to that of the solid, the resonance vibration comes out and the sound is absorbed more because its energy is transmitted to the solid. That's my opinion.

8. Nov 8, 2005

Jay-qu

The disspersive nature of low frequency sounds is due to diffraction. The same thing happens for light. As for it passing through solids easier I dont know if its correct to say resonance makes it pass through easier but that if the sound is at the objects natural frequency than it will resonate and amplify the wave.