Practical audio theory – Decibels

I have worked in all kinds of places in my audio carreer. At radio stations, at audio post production houses, in music studios and in dubbing studios. No matter where you go, one thing always causes confusion amongst colleagues: dB’s. You see the same confusion in audio magazines and at audio schools. So this is a very tricky subject you could fill a book with.

And yet I’m going to try to write down the basics in a few sentences. Wish me luck. 😉 Oh, and please let me know if you still have any questions after reading this part: gijs@audiokickstart.com.

Ok, so here we go. When I talk about a temperature of 35 degrees it can mean different things. Depending on if I am talking about degrees Celsius, Fahrenheit or Kelvin. They all have a different reference point. At 35 degrees Celsius it is pretty warm outside and many people will go to the beach and cool off in the sea. But at 35 degrees Kelvin that same sea is covered with a thick layer of ice and it’s so cold that there might not even be life.

The reference point is different. With Celsius it is the freezing point, but with Kelvin it is the absolute zero below which temperatures do not exist.

Why are we suddenly talking about the weather? Because it’s the same with measuring sound. We measure in decibels, but here too there are different reference points. And that’s the basis of all the confusion: If you set the volume of a recording to ’30 dB’, it doesn’t say anything about the loudness. You have to mention what reference point you use. For example dBFS or dBm. ‘FS’ and ‘m’ indicate the reference point. Below are the four reference points that you will come across most when mixing.

dB SPL:
SPL stands for Sound Pressure Level. The reference point is the absolute limit of our hearing. So 0dB = no sound. 100dB = a lot of sound. 130dB = too much sound, it hurts your ears.


dBFS:

FS stands for ‘Full Scale’, the scale used in digital audio equipment. You will find it on most plugins and in your DAW. The reference point here is the loudest sound you can record in digital equipment before it will overload. If you try to record something that is louder, it will distort right away. As a result, this scale uses negative values, like -10dBFS, or -40 dBFS. You will never see +40dBFS, because that would be louder than the loudest sound you can record.


dBm:

Many years ago a few people decided that all audio equipment should be able to be connected to each other. Without overloading the equipment. Great decision! So a reference point was needed. It makes most sense to agree that 0 dB is a certain amount of electricity in the equipment. The well-known VU meter you find on analog mixing consoles (see image above) shows this: if it points to 0 dB, the signal has the optimal level for that device. That’s all you need to know to understand the principle, but if you want to dive a little deeper, read on! 🙂 At that time 0 dB = 1 milliwatt at a resistance of 600 Ohm was chosen. After that a few more flavors were added, such as dBu and dBV. All with a different zero point, but all with zero points related to electricity.


dBu:
This one is a tiny bit more technical to explain. If you don’t want that, just remember that dBu uses yet another reference point related to a certain amount of electricity within audio equipment. So just like dBm. And for the people that want to know more: That one milliwatt at a resistance of 600 Ohm (dBm standard) produces a voltage of 0.775 Volt. In practice, 600 Ohm turned out to be a resistance that’s not very practical in audio equipment. So soon the reference point was changed to that 0.775 Volt, independent of the resistance. This is dBu. Nowadays dBu is still used  in professional audio equipment. We use a slightly higher voltage these days by the way: 1.228 volts. That is 4dBu more than 0.775 volts. That is why these days we use +4dBu as our reference point (0 VU) on professional audio equipment.