[jfengel]

I still don't understand why audio dB are negative. That's relative to what? What happens at 0dB?

[eru]

Well, the brightness of celestial objects is also sometimes negative:

> The apparent magnitude of known objects can range from −26.832 for our Sun to about +31.5 for objects in deep space imaged by the Hubble Space Telescope.[3]

See https://en.wikipedia.org/wiki/Apparent_magnitude

[deepspace]

0db is usually defined as the loudest sound that the audio system can produce. Hence, everything else must be negative.

[rdbl27]

More specifically, 0 dB is the loudest sound the audio system is rated to produce without distortion. It's common to be able to actually drive systems harder than their specified engineering limits, which is why meters have a short positive dB section marked in red.

[mitthrowaway2]

Of course, typical of the wonderful ambiguity of decibels, 0 dB is also usually defined as the quietest sound that the human ear can perceive.

https://en.wikipedia.org/wiki/Absolute_threshold_of_hearing

[ianburrell]

That's why important to give the scale. dBfs is full scale level, and db SPL is sound pressure level.

[kevin_thibedeau]

That is dB full scale where 0 is an absolute ceiling and you can deduct from there.