convert negative db scaling to positive

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103033249
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convert negative db scaling to positive

Post by 103033249 » Thu Mar 15, 2018 6:21 am

I'm running Audacity 2.2.2 version with Windows 10

I'm doing some sound measurement experiment with Audacity. However, the db scaling shown on the software is negative. Is there a way that I can convert the negative db scaling to an equivalent positive db scaling?

Thanks,

steve
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Re: convert negative db scaling to positive

Post by steve » Thu Mar 15, 2018 9:00 am

As is common practice for "signal" levels, Audacity uses dB relative to "full scale" (full track height / full scale on the meters / maximum integer sample value).
To get positive dB values, pick a lower reference level. For example, the minimum non-zero level for 16-bit audio is around -90 dB relative to full scale, so if you take that as your reference, then all signals above that level become positive (just add 90 to all of your measurements).

You "could" use any arbitrary level as the reference level, but the convention used universally for signal measurement, is "full scale" (sometimes written as "db FS"), which give you negative dB values for all normal signal levels.

Of course, although a low enough reference will give you positive dB values, these values are still have no direct correlation to the "sound pressure level" from your speakers (as measured by a "SPL meter"), which still depends on how loud you play the sound.
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DVDdoug
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Re: convert negative db scaling to positive

Post by DVDdoug » Thu Mar 15, 2018 2:45 pm

I'm doing some sound measurement experiment with Audacity.
Do you have an SPL meter? Different microphones, preamps, soundcards, and different volume-settings will give you different readings so you need to an SPL meter to calibrate your set-up before you can even get started.

The "easy part" is a simple calibration. For example, if your SPL meter reads 80dB SPL and Audacity reads -20dB FS then you simply add 100 to Audacity's reading to get SPL. (You'll have to do that manually... There's no way to change Audacity's calibration.)

The "hard part" is the frequency weighting and (short-term) averaging. Because Audacity does not do these two things, your Audacity readings & SPL meter readings will only match with pure-tones at the calibrated frequency and will be different with real-world sounds.

kozikowski
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Re: convert negative db scaling to positive

Post by kozikowski » Thu Mar 15, 2018 4:31 pm

http://kozco.com/pix/SoundLevelMeter.jpg

It doesn’t have to be crazy complicated. That’s my old Radio Shack Sound Level Meter. It’s set for 90dB C-Weighing at zero on the meter.

You can use Slow Response to average the reading over several seconds.

And no, you can’t do it directly in Audacity.

Koz

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