Why when i measure the RMS or peak of a audio file it comes in negative decibels?

Im trying to find the average RMS and the peak in an audio file, but everytime I try to do it, the results are in negative decibels. Any way to turn it into decibels that isnt negative?

Decibels are relative -

With digital levels the the 0dB reference is the “digital maximum” of 0dBFS (decibels full scale) so digital dB levels are usually negative.

Numerically, 16-bit files can hold values between 32,768 to +32,767, representing the positive & negative halves of the waveform. 16-bit samples that hit those negative or positive values have 0dB peaks.

A series of samples with a numerical value of zero is pure digital silence and minus infinity dB.

24-bit files have bigger numbers but when you play the audio, everything is scaled to match your (DAC) digital-to-analog converter so it’s not “louder” than an 8 or 16-bit file.

DACs (playback), ADCs (recording), regular WAV files, and CDs are all limited to 0dB maximum.

If you “try” to go over 0dB (positive) with integer formats you get clipping (distorted flat-top and flat-bottom waves).

Audacity uses floating point internally. With floating point a numerical value of 1.0 is 0dB. With floating point there is essentially no upper or lower limit so it can go way over 0dB.

Sound in the air is measured in dB SPL (sound pressure level). 0dB SPL is approximately the quietest sound you can hear so dB SPL levels are positive.

There other dB references for electrical signals, and with audio these dB values are also usually negative.

There is no calibration between digital levels and acoustic loudness. Loudness depends on the gain and power of your amplifier and how close you are to the speakers, etc. But, there is a direct correlation. If you lower the digital level by 6dB (more negative) the acoustic loudness also drops by 6dB.

Wait so how do we measure it then? Is there any way to convert it or calculate it to something like normal decibels?

Depends on where. If you measure sound in free air, that’s Sound Pressure Levels and that is measured in positive dBs. 0dB is where you can barely hear it working up in positive dB to explosions and thunder.

Inside electronics, zero is maximum volume before the sound becomes damaged and it gets quiet from there in negative numbers.

And yes, that does present some math problems. Quick, subtract 5dB from -20dB.

It’s -25dB.

5dB quieter than -20dB is -25dB.

I try to stay away from “Higher” and “Lower.” That causes no end of problems. I use “Louder” and “Quieter.” See diagram, above.

Koz

Are you reading for an audiobook? We made two tools that can make that a little easier. Audiobook Mastering will force your Peak and RMS (Loudness) to meet ACX standards. Then all you have to worry about is background noise…And Lip Smacks.

Koz

With an SPL Meter.

You can make a calibration by comparing the digital level to the SPL meter. (You should use RMS or LUFS (1) rather than peak and it’s best to use pink noise for calibration.)

For example, let’s say your RMS level is -20dBFS and the SPL reads 80dB SPL. If RMS drops to -30dB, the acoustic loudness is now 70 dB SPL. (2)

Of course, that’s only valid for you listening/measurement position and it’s only valid until you change the volume control.

Movie theaters are calibrated, and there are calibration standards for home theaters but very few people “set and forget” the volume on their home theater setups.

One more complication is that SPL measurements are usually A-weighted to account for our ears being most sensitive at mid-frequencies and these frequencies sound louder than high or low frequencies.

LUFS uses the same concept, with a different curve with an added factor that our ears exaggerate the differences in bass… When you turn-down the volume it sounds like you’ve turned-down the bass even more.

(1) Audacity’s Loudness Normalization can use LUFS (perceived loudness) so you can set an LUFS Loudness but Audacity doesn’t have a built-in way of simply measuring LUFS.

(2) I like to do down in these examples, because if you go up digitally you can get digital clipping or you can clip your amplifier and then the digital and acoustic loudness changes no longer “track” together.

:smiley: Like when someone says “Turn down the air conditioner”. Do you want the temperature turned down lower, or do you want less cooling? :smiley:


I bought that before they made the digital one. I can measure pulsing or drifting sounds easier than trying to interpret flipping numbers.

Koz