And yes, 120 dB SPL would be 1,000,000 times the pressure of 0 dB SPL. This means that +6 dB SPL would be 2 times the pressure, +12 dB SPL would be 4 times the pressure, +20 dB SPL would be 10 times, +40 dB SPL would be 100 times, +60 dB would be 1,000 times and +80 dB SPL would be 10,000 times. Pressure in acoustics is like voltage in electronics in that it has a squared relationship to power, so the calculation becomes 20 log (ratio). So 0 dB SPL is 20 micro-pascals of modulation of the atmospheric pressure, which is the quietest sound that a human can hear under perfect conditions. SPL does indeed have a reference level which is 20 micro-pascals of sound pressure. Many times you might see just: “110 dB” but that’s nonsensical because 110 dB is a ratio without any reference level to compare against. We often see the term SPL combined with dB for loudspeaker ratings, as in 110 dB SPL at 1 meter. Figure 1: The dB for power relationship.īut before we get to that, we have to define SPL, which again, stands for sound pressure level. And log ratios are handy since they make it easy to compare power levels with simple addition and subtraction. This is called a logarithmic relationship, just like the Richter Scale, the numerical scale for expressing the magnitude of an earthquake. 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.įigure 1 makes the dB relationship clear. 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. 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). 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.
So a decibel is 1/10th of a Bel.īut what is a Bel? It’s a ratio of power measurement named for the famous inventor Alexander Graham Bell, the father of the telephone. The deci means 1/10th, as in decimal place, or like a dime is 1/10th of a dollar. A dB is a decibel, which is a combination of the terms deci and Bel. Who is this SPL you speak of, and what’s a dB? Glad you asked. Here’s how to get a handle on what all of this decibel brouhaha is about. Or maybe there’s a noise ordinance stating no more than XX dB after XX pm. There are a few houses down the street complaining about the loudness of the music. 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. 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. It’s not an exhaustive study on acoustics or how much sound level is dangerous.
This is a basic primer on sound levels and how they work, especially when a sound system is outdoors.