 # The Scoop On SPL: Getting A Handle On Decibels And Distance

Looking at the way sound pressure levels (SPL) drop in decibels (dB) depending on the listener's distance from the sound source.

This is a basic primer on sound levels and how they work, especially when a sound system is outdoors. It’s not an exhaustive study on acoustics or how much sound level is dangerous. Rather, we’re going to explore how sound pressure levels (SPL) drop in decibels (dB) depending on the listener’s distance from the sound source.

We’ve all been there when playing or running sound for a band outside, or maybe even doing a few DJ gigs for an outside wedding party. There are a few houses down the street complaining about the loudness of the music. Or maybe there’s a noise ordinance stating no more than XX dB after XX pm. Here’s how to get a handle on what all of this decibel brouhaha is about.

## The Backstory

Who is this SPL you speak of, and what’s a dB? Glad you asked. A dB is a decibel, which is a combination of the terms deci and Bel. The deci means 1/10th, as in decimal place, or like a dime is 1/10th of a dollar. So a decibel is 1/10th of a Bel.

But what is a Bel? It’s a ratio of power measurement named for the famous inventor Alexander Graham Bell, the father of the telephone. We typically use the term dB (decibel) for most measurements since that gives us a much more usable number than a Bel, which is huge.

Technically, dB SPL is a unit used to express the relative pressure of a sound wave, equal to 20 times the common logarithm of the ratio of the pressure produced by the sound wave to a reference pressure, usually 0.0002 microbar (more on that a bit later).

I won’t bore you with all of the calculations but know that a decibel (dB) is simply a ratio of power, and that +3 dB equals a doubling of power, +6 dB equals 4 times the power, and +10 dB equals 10 times the power. Subtracting decibels by using a minus sign works the same way: -3 dB is 1/2 the power, -6 dB is 1/4 of the power, and -10 dB is 1/10 of the power.

Figure 1 makes the dB relationship clear. This is called a logarithmic relationship, just like the Richter Scale, the numerical scale for expressing the magnitude of an earthquake. And log ratios are handy since they make it easy to compare power levels with simple addition and subtraction.