My parents are listening to 70s music downstairs and I can hear the drums and basslines but not anything else unless I open the door.
In: Biology
Sound is a pushing and pulling wave. Walls are heavy, so you have to push for a long time to get them to move. High pitch sounds push and pull much faster, so the wall doesn’t shake as much. This means that walls shake much more with lower frequencies, and so they pass on the sound to the air on the other side. Edit: To add to this, higher pitched noises bounce around instead of going through things for this reason.
As you know solids (Ex: A Wall) are just a bunch of atoms vibrating very slowly. We can think of them as obstacles. When a lower sound is produced, the sound wave is “shallower” than a higher sound. Think of it like valleys vs mountains. This means that if it attempts to bypass the solid wall, there is less of a chance it will collide with atoms, meaning more of the sound wave gets to your ear.
Lower sound frequencies lose energy slower than higher sound frequencies. Imagine you’re walking two dogs, one of them is very hyper (high frequency sound) and constantly running back and forth to smell everything it can, while the other is calmer (low frequency sound) and only occasionally goes out of its way to smell or chase something. The hyper dog is going to get tired much faster and need to stop much sooner than the calmer dog. Sound works basically the same way, high frequency sounds lose energy as they travel through their medium much faster than low frequency sounds.
Lots of things going on here, but here’s a major reason why this is the case.
Speaker sound is electrical energy converted to air pressure waves that pulse. These pulses hit your ear drum. Your ear drum moves as the result of the air pulsing against it. Your brain converts those pulses into electrical energy and you hear the sound.
As these pulses hit a wall, the wall shakes back and forth just like your ear drum. A 50hz bass frequency wave has a length of about 22.5 feet, much longer than the thickness of the wall, so the wall has plenty of time to physically move back and forth to reproduce that sound on the other side (that you hear from upstairs). But because there’s a lot of mass in the wall that absorbs some of the energy, the sound isn’t reproduced with much fidelity.
A high frequency wave, on the other hand, can have a much shorter length than the thickness of a 4 inch wall. And when it hits the wall, the wall isn’t able to cycle back and forth before the next wave hits. So the waves lose their energy against the wall before they can go through it.
As I said, lots of other things going on here, but this is the main reason why thicker, heavier walls don’t allow as much sound to propagate through them as thinner, lighter ones. And why in all cases higher frequencies will attenuate more than than lower ones.
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