Introduction: The Concept of Sound Pressure (SPL)

Have you ever been told to turn the volume down on your music?  Have you ever wondered why when something is loud it is said to have more volume?  In this instructable I hope to clear up some major misconceptions about sound, and hopefully help you understand how sound works.

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: https://www.instructables.com/id/How-Sound-Works/

Step 1: What Is Sound Pressure?

Sound Pressure Level is normally abbreviated SPL.  SPL is calculated in decibels or db.  Please note that db is a relative term.  It can refer to almost anything, however, the most well known is SPL.  Most of the time when someone is talking about db, they are probably referring to SPL.

Step 2: The Difference Between Sound Pressure and Volume

The human ear hears pressure, not volume.  The difference between sound pressure and volume is that the closer that to the source of the sound, the more pressure you will hear.  The farther away you get from the source of the sound, the pressure will be less, but the volume will be the same.  A good analogy would be a faucet.  At the top of the faucet where the water comes out, there is more pressure than at the bottom, but the volume of water is the same.  (See picture)

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.

Step 3: Why Is It Important?

SPL can also damage the ear.  The human ear starts to degenerate at an SPL of about 85-90 db.  You can buy SPL meters at almost any store that sells professional audio equipment, or radio shack.  Because a standard rock concert has an approximate SPL of 100-130 db, if you are mixing for any event, you should keep a SPL meter handy and try to keep the SPL at or below 80 db.  If you are attending a rock concert, you should make sure to have hearing protection with you.  You can buy discrete hearing protection at almost any department store or store that sells professional audio equipment.

Step 4: The Inverse Square Law

One thing to remember when you are mixing, is that in normal conditions, when you double you distance from the source of the sound, you reduce the sound by 6 db (4X).  So if you are in the back of the room and your speakers are at the front, the pressure at the front of the room is much more at the front than at the back.

Step 5: Conclusion

If you only take one thing away from this instructable, it should be a warning not to ruin anyone else's hearing, and to protect your own.  If our generation continues to attend loud rock concerts, then in 20 years, 10 year old kids will be using hearing aids.