What is the difference between 0 dB and 0 dBFS?

If you use a digital audio workstation, then you will come across the concepts of 0 dB and 0 dBFS a hundred times a day. Clearly you need to understand what they mean...

There are a number of ways of measuring signal or sound levels in decibels. In this article I will describe the difference between dB and dBFS.

By the way, when I say 'sound' I mean an audible sound traveling in air. A signal is a representation of that sound in either electronic or digital form.

Differences in signal or sound levels are measured in decibels (dB). So a measurement of 10 dB means that one signal or sound is 10 decibels louder than another, or that a signal or sound has been made 10 decibels louder than it was before.

0 dB

It is important to note that these sounds could be quiet, medium or loud. Decibels, in their dB form, do not describe the absolute level of a signal or sound, only any comparison or change in level.

0 dB therefore means no difference in level, or no change in level.

A common error

It is a common error to say that, for instance, the sound level of a helicopter from a distance of 50 meters is 100 dB. What is usually meant is that the sound is 100 dB louder than the quietest sound the average human ear can hear but the person quoting the measurement doesn't understand this important detail.

The level of the helicopter is 100 dB SPL (standing for Sound Pressure Level), where 0 dB SPL is the reference level. You can think of 0 dB SPL as the sound made by a dead leaf hitting the ground in the fall. You can just hear it, but anything quieter would be imperceptible.

dB SPL relates only to actual sound, not to signals.

0 dBFS

'FS' stands for 'Full Scale' and 0 dBFS is the highest signal level achievable in a digital audio WAV file. Higher levels are possible inside digital audio workstation software, but in the files that are recorded on disk, 0 dBFS is the highest level.

All other levels can be measured and described with respect to 0 dBFS. So for example a signal that is 10 decibels lower than the maximum possible level is -10 dBFS. A signal inside the digital audio workstation could be +10 dBFS, but it would need to be lowered in level for output as a WAV file, otherwise it would be clipped, meaning that the tips of the waveform would be squared off at 0 dBFS.

Summary

Decibels are used to describe differences or changes in level. 0 dB means 'no change'.

Values in dBFS are used to describe signal levels in comparison with the highest level a WAV file can handle.