Sound is a back and forth motion of air. A wave.
If there is a wall, the air can’t move back and forth, because it just hits the wall. If the wall is a little bit flexible, then the air can actually move it back and forth, and the wall will now push back and forth on the air past it. This is how sound can pass through walls.
Let’s forget about walls for a moment. If you are pushing somebody on a swing, what happens if you switch between push and pull too fast? The swing won’t actually move very far, and instead you just end up moving yourself quite a lot.
If you push too *slowly*, the swing just kind of moves with your hands. It doesn’t swing, it just gets dragged around by you.
To move the swing the most, you have to go at just the right frequency of pushing and pulling.
A sound shaking the wall works just the same. Too high-pitched and it bounces right off, too low-pitched and it will also bounce off. It has to be a frequency in “the middle”.
Different objects have a different “middle”. Walls’ is very low.
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