Over the next few articles I will explain some of the different meanings of the word “decibel” as it relates to sound. “Decibel,” by itself, is not a unit of measurement of loudness. Decibels are a way of counting large numbers, very similar to the Richter scale. For people to be talking apples to apples about noise, they need to be clear about what flavor of decibels they’re using.
In this article I will talk about the different kinds of L Values. In the next article I will talk about weighted values and sound spectra. You need both of these pieces to determine what “decibel” means.
Sound level measurements are taken over time. If you have an inexpensive sound level meter, it probably just has a screen with a single number that bounces up and down on the screen depending on what it’s measuring right at that moment. While this is useful for getting a general idea of what sound levels you’re measuring, it’s not very useful for making numerical comparisons. The type of sound level meter used by acoustical engineers makes a complete measurement with a beginning and an end; basically a short recording.
This chart shows a basic sound level measurement. The curve is the Sound Pressure Level (SPL), which is the sound level at a specific moment. SPL fluctuates with time. If you were making this measurement with an inexpensive meter, you would see the value on the meter move up and down in time in a way that resembled the curve below.
The problem is a sound that lasts over time never has a single SPL value. Since it fluctuates over time you have to find a way of describing the entire curve with a single number. It turns out there are several ways of doing just that.
Very frequently, the metrics Leq, Lmax, and Lmin are used. These refer to the Equivalent Level, the Maximum Level, and the Minimum Level. Lmax and Lmin are the easiest to understand, they’re simply the highest and lowest values the sound level meter saw during the measurement:
Leq is trickier to understand. Technically the following charts aren’t a correct representation, because Leq depends on the actual numerical value for sound pressure level, not the decibel value. But understanding that is not necessary for understanding the idea behind Leq.
Leq is the Time Weighted Equivalent Level. That’s kind of a mouthful but it’s not too complicated. Weighting is a way of averaging. Here’s how it works out:
The below chart shows everything below the curve highlighted in red. The size of the red area is, essentially, the SPL multiplied by the amount of time of the measurement. In order to multiply something by a curvy line, you have to use calculus, which is actually what an “integrating sound level meter” is doing.
If you take the entire red area and rearrange it so that it makes a nice square shape, you’ll have something like this:
This red square has the same area as the red area under the curve in the previous figure.
The Equivalent Level, Leq, is the sound level that would result in a square with the same area as the curve.
Leq and Lmax are used quite frequently. Lmin is not used as often, but is usually recorded alongside Leq and Lmax anyway.
Another method of deriving a single number from a sound level measurement is with a Statistical Analysis, or “Ln values.” “Ln” by itself describes the methodology and isn’t an actual level. The actual levels are written with a number in place of the letter n. Typical Ln metrics are L10, L50, and L90. You can have any Ln value you want, L37.5, for example.
What the number refers to is the “percentile” of the value. Specifically, the amount of time the sound level was above the Ln value. L10 is the level, in decibels, that the sound level exceeded for 10% of the time. L50 is the value that the sound level was above for 50% of the time and can be considered the median value. L90 is the value that the sound level was above 90% of the time.
Graphically speaking, Ln values are calculated by adjusting a line up and down until exactly the correct percentage of the line is below the curve. The L10 line is adjusted until 10% of it is below the curve, the L50 line is adjusted until exactly half of it is below the curve, and the L90 line is adjusted until 90% of it is below the curve. Once those lines are adjusted to the proper height, you read the value from the left axis of the graph to determine your Ln values.
So on the following chart, if our Lmax was 80 dB and our Lmin was 40 dB, L10 would be about 78 dB, L50 would be about 65 dB, and L90 would be about 45 dB.
The chart below shows the lines without showing the SPL curve. The sections in red are what would appear below the curve. For L10, the section below the curve (red) is 10% of the total length of the line. For L50, the red portions make up half of the length of the line. For L90, the red portions make up 90% of the length of the line. The length of the lines left to right is equal to the amount of time of the measurement.
Ln values are very useful for long term noise measurements, such as what you would use for an environmental noise study. Such studies often have measurements that last for several days. L90 is commonly used to determine the ambient, background level. If your L90 value is 45 dB, that is the same as saying “the sound level was 45 dB or higher 90% of the time.”
In the next article I will write about the different ways of combing all the different frequencies of a given sound into a single number. This includes A-weighted decibels, or dBA, which are the most commonly used single-number flavor of decibels, and what the great majority of noise ordinances refer to.
Is something still unclear? Did I make a mistake? If so, please ask any questions or share any comments in the comments section of this article. I will attempt to clarify anything I didn’t explain well or correct any mistakes I may have made.