Why can you hear sounds outside, but not inside a building?

[Jormungandr]

This came up in a dinner conversation and my mind is drawing a blank on the explanation. (Too much food and too little blood to the brain ) Let's say, you're in a building or house near a window. Why can you hear sounds outside of the building be it closing a trunk, someone dropping tool or even a conversation, but the people outside can't hear what's going on inside where you are? Any help would be appreciated.

[Johnny L.A.]

IME noises outside tend to be louder than noises inside. Examples of inside noises that are often heard outside are televisions and stereos that are playing at high volume, or arguments.

[Francis Vaughan]

A critical issue is the signal to noise ratio. Inside a building is generally quieter, so even a muffled sound from outside is loud relative to a lower background noise. Whereas outside, even exactly the same noise source, if created inside, and then muffled to the same degree by the building structure would be competing with the general road noise and environmental noise to be heard, which has not been muffled by the building.

For example. You stand by a window that drops the sound level by 20dB. (Numbers pulled from thin air.) The sound (say dropping a tool) you are interested in is 75 dB SPL. The road noise is 65 dB SPL. So, inside the sound of the dropping tool is 55 db SPL, and the road noise is 45 db SPL. But outside, if you listen for the sound of the same dropping tool happening inside, you get: sound of tool 55dB SPL against a road noise of 65 db SPL. So, when you are inside the sound of the tool dropping outside is 10db louder than background, but when outside the sound of the tool inside is 10 dB softer.

[CookingWithGas]

In addition to how load the sound it, frequency is also a factor. Shutting a car trunk yields a fairly low-frequency sound, which is less directional and penetrates walls more easily than higher-frequency sounds. If you were playing your stereo inside the house and cranking the bass, a guy walking down the sidewalk will hear it.

You have a bit of confirmation bias going on here as well. You only notice the sounds from outside that you can hear. There are zillions of sounds that you don't hear but you never think, "You hardly ever hear sounds from outside...."

[AWB]

A good example of background noises outside is the sound of insects. This morning, I heard crickets chirping from outside our townhouse. Then I got up to take our dogs for a walk. I didn't hear the crickets, until I stopped and concentrated. I then realized that I was also hearing traffic noise from 1/2 mile away, airport traffic, birds, dogs, and wind through the trees.

I think if it were ever truly silent outside, it would freak the bejeezus out of me.

[terentii]

The walls and windows reflect a great deal of the acoustic energy back into the environment, creating what's essentially a pocket of dead air inside the house. Open a door or window, and the sound is free to enter your house; you'll hear a lot more of it, though interior walls, furnishings, etc., will act as baffles and mufflers, so it will never be as noisy as out on the street.