This instructable is more of a how it works as opposed to a how to. Once you understand how it works, it is much easier to learn the how to.
Please note that I am constructing this from a sound engineer's point of view. Most of the content is theory, so please keep that in mind when you read this.
If you haven't read my first instructable "How Sound Works" yet, I suggest you read it first. You can find it here: http://www.instructables.com/id/How-Sound-Works/
Step 1: What is sound pressure?
Step 2: The difference between Sound Pressure and Volume
One thing to remember with SPL is that as you turn the pressure up, you generally also increase the volume. Volume does affect sound, even though it does not directly affect how we perceive how loud something is. One of the main characteristics of a professional speaker is that it can produce more pressure with less volume than a consumer speaker can.