When I close a door, it’s harder to hear sounds from a room on the other side, I’ve always wondered why.
In: Physics
Because the speaker isn’t put on the door itself.
The waves have to travel through the air and find a blockade so they can not travel much further. If you’d put the speaker on the door, the sound would really travel through the solid and it would vibrate and sound louder than before.
Additionally, the sound the gets transferred into the door also gets transferred into the door frame and walls.
Because the difference in the conduction of sound beaten the two mediums causes the sound to reflect off mostly. Different materials with far different conduction will do this. This why why the put gel on a woman’s belly when scanning her baby.
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The sound is reflecting off the door, not travelling through it, you would have to mount the speaker to the door, if you have music playing from your phone, shut the door and walk out the room, the music will be quieter, now do the same thing but have a friend press the phone onto the door, the door will act like an amplifier and you will be able to hear it.
Any sound that is ‘harder to hear’ is due to a reduction in its intensity by some means.
One of the scenario when this intensity reduction happens is when sound encounters a change in medium, like say when traveling from air into a wooden door.
Since the door has some thickness, there are two air-wood interfaces, at each of which, the sound intensity is split into sound that is reflected back, sound that is absorbed and sound that goes through.
Now,sound absorption happens even without medium change, and is more in gases than liquids or solids. Which is why sound travels farther in solids and liquids.
But the reflection phenomena at an interface reduces the amount of sound that passes through, and that happens twice over for a door.
Hence less sound is heard on the other side of a door.
Waves travel through homogenous materials relatively unimpeded – they loose some strength over time, buy for small distances like this its practically irrelevant.
However, when a wave travels from one material into an other one, the following happens:
– only a part of the wave travels on into the second material
– another part of the wave is reflected back at the boundary
– the more “different” the two materials are, the more of the wave is reflected. I.e. a wave encountering two slightly different layers of air will mostly be transmitted. A wave travelling from air into solid wood will mostly be reflected. (The property that has to be different is called (acoustic) “Impedance” if you want to read further.
In short, what happens is that part of the wave is reflected at the air-door boundary, and then again at the door-air boundary.
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