Physics Forums (http://www.physicsforums.com/index.php)
-   Classical Physics (http://www.physicsforums.com/forumdisplay.php?f=61)
-   -   Sound diffraction (http://www.physicsforums.com/showthread.php?t=656909)

 einstein1921 Dec5-12 01:53 AM

Sound diffraction

Hi, my friends. Today, I come up with a question suddenly. It is about Sound diffraction.
why can we hear someone's voice even when he is in a closed office. Many books says that it is because of Sound diffraction. But I can't understand. how does it happen? would you be so kind to explain it in detail? Thank you!!

 Bobbywhy Dec5-12 04:08 AM

Re: Sound diffraction

Diffraction of sound: the bending of waves around small* obstacles and the spreading out of waves beyond small* openings.
http://hyperphysics.phy-astr.gsu.edu...d/diffrac.html

Also see the images: https://www.google.com/search?q=soun...w=1350&bih=647

Cheers,
Bobbywhy

 Philip Wood Dec5-12 06:37 AM

Re: Sound diffraction

But the office is closed!

Don't forget that sound can travel through the thickness of doors, and even through some sorts of wall. The sound is attenuated (weakened) but here something else comes into play...

The ear's sensitivity is 'logarithmic'. This implies that it is disproportionately sensitive to very weak sounds. So although the sound intensity might fall by a factor of 1000 as the sound passes through the material of the door, it may still be audible.

 einstein1921 Dec5-12 06:40 AM

Re: Sound diffraction

Quote:
 Quote by Philip Wood (Post 4185090) But the office is closed! Don't forget that sound can travel through the thickness of doors, and even through some sorts of wall. The sound is attenuated (weakened) but here something else comes into play... The ear's sensitivity is 'logarithmic'. This implies that it is disproportionately sensitive to very weak sounds. So although the sound intensity might fall by a factor of 1000 as the sound passes through the material of the door, it may still be audible.
why sound can travel through the thickness of doors? please explain it ! Thank you very much!!!

 andrien Dec5-12 08:30 AM

Re: Sound diffraction

or you confused sound with light,which can not go after a wall.Sound can,right!

 phinds Dec5-12 08:40 AM

Re: Sound diffraction

Quote:
 Quote by einstein1921 (Post 4185093) why sound can travel through the thickness of doors? please explain it ! Thank you very much!!!
Sound is a mechanical vibration in a medium. It can be transfered to another medium. So ... inside air to wall to outside air. It gets attenuated each time, which is why you don't hear the sound as clearly outside the office as you do inside the office.

EDIT: it can also be reflected off of objects (mostly hard, smooth ones), thus can "go around corners"

 Philip Wood Dec5-12 08:54 AM

Re: Sound diffraction

Sound travels through solids and liquids as well as gases such as air. In a liquid or solid, the vibrations which make up the sound are passed from one particle to another via the forces which hold the particles together. In many materials (such as steel) the wave energy propagates very efficiently in this way. In softer materials, or materials containing large particles or fibres, much of the energy is scattered or dissipated (making the material slightly hotter), so the sound propagation is inefficient. That's just what you want in the material of a wall or door.

 davidm1732 Dec5-12 02:36 PM

Re: Sound diffraction

As an extension to what phinds said...
The sound is the vibration of the air. The vibrating air is in contact with the wall, and transfers some of that energy to the wall. Now the wall is vibrating. The vibrating wall transfers some energy to the air inside the office. Now the air inside the office is vibrating, allowing you to hear the noise, even if the office is sealed. If the door is very thin, you hear more through it, because it's easier for the air to vibrate the door. If you make the door very heavy and stiff, it's harder to get the door to vibrate, so less energy is transfered through it. If you make the door infinitely stiff and heavy, then no sound energy passes through it.

 All times are GMT -5. The time now is 11:18 AM.