First off, what is acoustics? Acoustics is the study of sound and how different things (like temperature) affect it. There are, of course, a million and one things that affect the way sound waves travel, but let's just cover the basics, which is essentially how sound waves work, temperature, and environment.
The amplitude (size) of a sound wave is described by the amount of energy it carries. A high amplitude wave carries a lot of energy which makes things seem loud, while a low amplitude wave is the opposite. The average amount of energy in the amplitude is the intensity (amount of power). So, to make things short and sweet, the louder you yell, the more intense and decibels there are.
There are two types ways that sound waves moves:
Longitudinal: which is where the sound waves all move together in the same direction.
Transverse: which is where the sound waves move up and down (right angles).
Sound travels faster through higher density and pressure areas than it does in lower pressure and density areas. What does this mean? Sound travels faster through a liquid than a solid; it just seems the opposite because we hear sound so much better in air than in water
Sound waves also diminish with distance. Pretend you and another person are standing on opposite sides of large room. The father away they are, they louder they have to yell because the sound waves get smaller the farther they have to travel and because the objects around will also absorb some of the sound as it moves.
There are also sounds we can't hear, of course. These sound waves that are too low for us to hear are called infrasound while sound waves that are too high are called ultrasound. There is also one more type of sound wave, that is virtually impossible to create because of the density of the environment required for it to exist; this type is called hypersound.
-Hot & Cold-
Sound will travel faster in warm air than in cold air. Why? Warm air is usually denser than cold air, thus, allowing sound to travel faster. Also, cool objects tend to absorb sound waves much better than heat. The trick with cold though, is that eventually the temperature, density, and if you're dealing with water, salinity will equalize and sound will begin to travel faster. Just think of ice and how well sound travels over that compared to snow.
We all know that sound can be absorbed and reflected, but how exactly does an object affect the sound waves? Reflection is when the sound wave bounces back off of an object and creates an echo. Of course, the harder the object is that the sound waves are bouncing off of, the better the echo. The sound wave is also affected by the size of the object(s) as well. If an object is smaller than the sound wave, it will simply travel around it, causing it to scatter.
The less dense the objects around, the more the sound is absorbed. Imagine for instance, a large snowfield. The snow will absorb sound waves much better than warm grass because it is less compact than the grass itself. So, if you're looking to sound-proof a room, or lessen any reflection, just look for items that aren't dense, like foam, fabrics, etc.
The shape of a room will also change how sound is reflected. Normally, if a room has a high arched ceiling, this will help create a "megaphone" effect (just think of all those cathedrals with gigantic ceilings). Domed ceilings can also cause the same effect surprisingly; they cause the sound waves to become focused which means more reflections as well. However, the best way for sounds to echo is normally in a square or rectangular room as it provides the best angles for the sound waves to reflect off of. So, the best way to create less reflection in a room is no domed or cathedral style ceilings with rounded walls, and filled with plenty of "plush" objects.
Hopefully you're not horribly confused or asleep by now, and have a good idea on how acoustics works. Of course, there are many other types of acoustical studies, such as the acoustics of music, architect acoustics, and many, many more. Incase you need more information on the basics or something a little more advanced, here are two good websites to help you go into greater detail if you need to: