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I have a question about the propagation of sound waves. We have two TV's in our house that are almost right on top of each other. One is located on the first floor and the other one is located on the ground floor. The TV's are two identical Samsung TV's. If I select the same channel and the same volume level on both, $TV_{1^{st}}$ sounds louder on the ground floor than $TV_{ground}$ on the $1^{st}$ floor. Can this be true?

I am also thinking about doing an experiment to check if my subjective experience is actually true. I am not sure if I can trust the sound-meter app on my iPhone though.

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    $\begingroup$ At least one effect that could explain this is that the acoustics are different on the two floors (due, e.g. to different carpets, walls in different places, etc) $\endgroup$
    – lemon
    Commented Jan 25, 2015 at 10:14
  • $\begingroup$ Did you try to exchange the TVs? I.e. place the TVs of the upper floor on the ground and vice-versa? I suggest you first to make sure that the TVs are indeed identical, because you hear stronger the TV near which you stay. $\endgroup$
    – Sofia
    Commented Jan 25, 2015 at 10:32
  • $\begingroup$ The TV's are too heavy to exchange. However, I used a decibel app and there is no significant difference between them. But as I mentioned in my opening post, I don't know how accurate these apps are. $\endgroup$
    – qmd
    Commented Jan 25, 2015 at 11:10
  • $\begingroup$ @Rzeta Just a minute: in fact, you don't have to exchange the apparatuses. Before you come to one room, switch off the TV in the other room. So, you'll know for certain if the two TVs are identical. $\endgroup$
    – Sofia
    Commented Jan 25, 2015 at 11:33
  • $\begingroup$ I am not really sure what you mean. I don't have them running at the same time. When I go upstairs to check $\endgroup$
    – qmd
    Commented Jan 25, 2015 at 12:10

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I think the simplest explanation is that the distance is smaller when you are listening to first floor television from the ground floor, because...

1. In most homes for most people, your head is closer to the ceiling than the floor.

2. Usually televisions are closer to the floor than the ceiling.

Also, television in the first floor is physically on top of an object that is on top of the floor. Ground floor television does not have such a direct connection.

If the app reports decibels, the scale is logarithmic.

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A sound being heard "louder" can be because of ..

  1. The source is emitting at a higher Db level (doh).
  2. There are multiple sources that superimpose, causing a 'louder' composite signal.
  3. (added it later) The same single could bounce of surfaces and end up adding up to itself through superposition.

Also take in account that sound waves do propagate at different speeds through different materials (carpets and foamy stuff "delay" the sound, while metals and masonry "speed" it up).

(added it later) Also one more thing is that although the sound you're hearing from both TVs may SEEM the same, their actual waveforms; when seen through a digital sampler; WILL be completely different.

So a room with all its carpets, walls, masonry, re-enforcement, ... etc (as lemon pointed out) is really a chaotic environment to try and accurately determine what and how much of what is contributing to the result you're experiencing.

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This sounds like ;) it's because the object that the TV is resting on is acting as a sound conductor. Solids conduct sound better then gasses.

On the 1st floor, sound goes from the TV, through the platform, through to the floor and to the ceiling of the floor below without losing much energy before being transmitted through the air.

On the ground floor, sound goes from the TV, through the air, to the ceiling and through the floor and through the air again. You would lose more sound energy going through the air.

You might be able to verify this by placing your TV on a sound insulating material. Foam may work. Or you could try suspending it on wires. ;) :D

Oh, and then there is also the total internal reflection effect. A wave moving from a slower medium to a faster one will, at the critical angle, reflect back into the slower medium. This will further reduce the amount of sound going from your ground floor to your 1st floor.

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