The area of sound, or acoustics, can be very interesting. I'm certain you will learn a lot with your project.
One of the first things you need to do is some basic research. The questions you asked can be answered easily with some basic research. Here on the Science Buddies Ask An Expert forum, we focus on helping students with specific problems.
I think, perhaps, you should start with the basic research and then we can help you solve some of the problems you might encounter along the way.
You can start right here on the Science Buddies web site. Here are some project descriptions that will give you a place to start, complete with referenced information:http://www.sciencebuddies.org/science-f ... ml#summaryhttp://www.sciencebuddies.org/science-f ... p034.shtmlhttp://www.sciencebuddies.org/science-f ... p017.shtmlhttp://www.sciencebuddies.org/science-f ... p029.shtml
In your research, you will find other units used to describe sound levels. The unit of dB is most common in the technical/scientific community. There is also a "sone", which is the measure of percieved loudness of a particular frequency. Broadcast engineers and technicians us "VU" units to measure the level at which they transmit sounds and music.
Go do some basic research and then dig into this fascinating realm of science and have FUN!c