What the heck is a decibel
If you are a musician or audio engineer, it is very important to know what decibels are. Unfortunately, decibels are extremely confusing. For one thing, there are so many different kinds of decibels! You only care about two of them: the decibels you see on a noise meter, and the decibels you see on a mixer. The decibel scale is meant to reflect the subjective experience of your hearing. The idea here is that a change of one decibel is the just noticeable difference: if you make something one decibel louder, that is just enough for the listener to notice that it's louder. Makes sense, right? Unfortunately, decibels are logarithmic, which makes it hard to develop an intuition for the actual sound pressure levels that they represent. Let's dig in.
The decibels you see on a noise meter
The first thing to understand is that zero decibels does not mean silence. Instead, it refers to the quietest sound that humans can hear, which is roughly equivalent to the sound of a mosquito from ten feet away. All other decibel levels are multiples of this reference intensity.
The next thing to understand about decibels is that adding them means multiplying the actual sound power levels. Playing your instrument one decibel louder means that you are putting out one and a quarter times as much sound power as you were before. Playing your instrument one decibel quieter means that you are putting out about four fifths as much sound power as you were before.
20 dB louder = a hundred times more sound power
10 dB louder = ten times more sound power
6 dB louder = four times more sound power
3 dB louder = twice as much sound power
1 dB louder = 1.259 times as much sound power
0 dB louder = the same sound power
1 dB quieter = 0.794 times as much sound power
3 dB quieter = half as much sound power
6 dB quieter = a quarter as much sound power
10 dB quieter = a tenth as much sound power
20 dB quieter = one one-hundredth as much sound power
The confounding thing is that your subjective experience of loudness is on a completely different scale from the actual power level of the sound. As a rule of thumb, it takes a tenfold increase in sound power (ten more decibels) for you to experience a doubling of subjective loudness. It takes a twentyfold increase in sound power (twenty more dB) for you to experience a quadrupling of subjective loudness. Your ear can detect vast ranges of air pressure shifts, and it perceives these shifts on a logarithmic scale. Just remember:
Playing your instrument 10 dB louder feels twice as loud, but is actually ten times as much sound power.
Playing your instrument 20 dB louder feels four times as loud, but is actually a hundred times as much sound power.
Playing your instrument 10 dB quieter feels half is loud, but is actually one tenth as much sound power.
Playing your instrument 20 dB quieter feels a quarter as loud, but is actually one hundredth as much sound power.
Another super important loudness concept: distance matters. Doubling the distance from the sound source reduces its apparent loudness by six decibels (more or less; it depends on many factors). The frequency content of the sound matters for subjective loudness too, because your ears are more sensitive at some frequencies than others. However, all those complexities are far beyond the scope of this blog post. Consult Hyperphysics for some good technical explanation.
So, how do you measure decibels? There are lots of free loudness meter apps you can get for your phone. I use the Audio Tool app, it's fine. You can also try this online loudness meter - click the gear icon and change the standard to Volume for decibel readings.
Below I list some representative loudness levels drawn from the internet. Remember that zero dB is not silence, it's the quietest sound you can hear, equivalent to a mosquito ten feet away. Anechoic chambers can get to -10 or -20 dB. That doesn't mean that they are quieter than silence; it means that their ambient sound pressure levels are a tenth or a hundredth what a mosquito produces from ten feet away. That's quiet! Some more points of reference:
Light leaf rustling, calm breathing: 10 dB
My bedroom, with a fish tank and light traffic outside: 30 dB
Normal conversation three feet away: 40–60 dB
Traffic on a busy roadway thirty feet away: 80–90 dB
Symphony orchestra from the middle of the audience: up to about 90 dB
Hearing damage: 85 dB at your ear over a sustained period of time
Lawnmower: 90 dB
Rock concert: 90-120 dB
Jackhammer three feet away: 100 dB
Threshold of pain 120–140 dB
Trumpet (two feet away): up to 130 dB but usually you are not sitting this close
Party balloon popped with a pin right at your ear: 155 dB
Jet engine: 150 dB right next to it; 110-120 dB or so at 500 feet away
Rifle being fired three feet away: 170 dB
Wear those earplugs! Your high-frequency hearing is going to get worse with age anyway; being exposed to loud noise will just make that happen faster. I am already one of those middle-aged guys who has to be looking right at you to understand what you're saying, and I watch movies with the closed captions on. Don't be like me. Wear your plugs.
The decibels you see on a mixer
Mixers (physical ones or the ones in audio software) do not show you the absolute decibel levels that they are putting out. They show you decibels relative to whatever you are sending into them. In a program like Ableton Live, the default setting on each fader is zero dB. Once again: that does not mean silence. It means that the mixer is putting out ten to the zeroth power as much sound, which is one times as much. As the audio engineers say, zero dB is unity gain: whatever sound is going in, whether it's from a microphone, an existing recording, or a software instrument, it will come out of the mixer at that same loudness. The faders control how much louder or quieter the sound is going to be, not its actual loudness coming out of your speakers. The actual loudness depends on your speakers!
Live's faders show three different decibel levels: the instantaneous level (dark green), the root mean square (RMS) (bright green), and the peak (the little line with the yellow number next to it.)
In the image above, the fader is set to 0 dB. The root mean square (average loudness over a short time span) is about -38 dB, while the loudness at this instant is about -16 dB. The line at the top says that there was a recent peak at +5.48 dB, close to clipping out.
So what does this have to do with the markings on your volume knobs? Nothing. Those markings are not very informative. They represent increments between zero sound and the maximum sound level that the device can put out. And what is that maximum amount? Who knows. All you can do is get out your decibel meter on your phone and measure. And your measurements will depend on the frequency content of the sound, the room, your location in the room, the speaker's location in the room, and so many other factors. It's enough to turn a person into Nigel Tufnel.
https://www.youtube.com/watch?v=hW008FcKr3Q
Last thing: let's talk about noise pollution. Like all other kinds of human-produced pollution, noise pollution is omnipresent and gets worse with every passing decade. It is especially acute in the oceans, from shipping, naval sonar, and seismic surveying. Here's a horrifying statistic: a US navy sonar blast can be as loud as 235 decibels, and it travels hundreds of miles underwater. There is good reason to think that whales beach themselves (commit whale suicide) en masse because of noise.
Noise pollution is also completely out of hand in cities. It isn't just annoying! It causes physical stress, loss of sleep, and lots of cumulative mental and physical health problems. It is worse for poor people than for rich people, since poor people have to live near airports, highways, heavy industry, power plants, and so on. But it's a problem for anyone who lives within earshot of a garbage truck or a leaf blower, which at this point, is all of us.