When Jill speaks on the other side of the doorway, Jack can hear what she says because of the way the sound waves spread out as they travel.
If you remember from physics, sound waves are longitudinal waves, meaning that the displacement of the medium is in the same direction as the motion of the wave. In other words, the sound carried by the wave moves in the direction that the wave travels.
Sound waves spread out as they travel, covering a larger and larger span until the sound has dissipated beyond our capacity of hearing. The fact that they spread out as they move is what lets you hear someone talking without seeing them, like in the diagram with Jack and Jill. The waves propagate through the doorway and reach Jack’s ears even though he isn’t directly in line with the door.
This sonar scan shows a sunken ship.
Echolocation, which is what bats use to see, works by reflecting sound waves off objects. At its most basic, echolocation uses the time it takes for the sound waves to return to figure out how far away the object is. Sonar, which is used by submarines to navigate as well as detect/communicate with other vessels, is basically an advanced form of echolocation. Sonar can also be used to create an image of objects on the seabed that otherwise couldn’t be seen unless up close, for example in a deep-submergence vehicle (DSV).
A cone shape is better at projecting sound than a cylinder, which is why megaphones and wind instruments have curved outlets that are larger than their inlets.
Be it water, air, or something else, sound waves need some sort of medium. Inside a vacuum, such as outer space, there is no medium for sound to travel through, so everything is silent. Different media can affect how well the wave travels and how long it takes for the sound to dissipate. Air is actually not the best medium. You can test this by playing a music box while holding it up in the air, and then playing it again while holding it against a tabletop. It sounds louder when it’s on the table because the vibrations travelling through the solid surface are more amplified than those through the air. This phenomenon also accounts for the old adage, “Sound travels faster over water.” If you’ve ever stood on a shore and heard people out in the middle of the lake or down the shore talking, you might have noticed that they sound closer than they are. This is because the water’s surface can serve as a better medium for carrying sound than air.
It’s easy to hear the effects of different media- notice how much louder a knock on wood sounds when your ear is pressed against the wood itself, or how rustling your pillow is amplified when you’re laying with your ear against the pillow. This might seem like common sense, but if air were a more efficient medium, more of the depth and volume of these sounds would be transferred to your ear, much like if you had your ear pressed against the object.
You can demonstrate this idea by tying dental floss to two corners of a lightweight metal object (cookie cooling racks work really well for this). Hold the free ends of the floss and swing the rack so that it hits something and vibrates- the noise won’t be very loud. Then press the free ends of the floss to the skin just in front of your ears (aka right in front of your tragus) and swing the rack again (you might have to also swing your head). The dental floss will convey the sound to your ears faster than air does. If you’re doing it right, the sound will be amplified and resemble the ringing of big bells.
All images courtesy of Google Images.
This video from NPR explains how light works, then translates that idea to sound.