Old 05-14-2021, 02:56 PM   #1
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Default 94dB Calibration

Can someone help me with calibration? I have a 94dB calibrator attached to a BeyerDynamic MM1 microphone going to Chan 1 of a 4-channel Behringer mixer. When I record the sound file in mono, the playback "true peak" measured by YouLean Loudness Meter 2 measures at -12.3dB. My question is why do I not see 94dB for the recorded file?

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Old 05-14-2021, 05:21 PM   #2
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Different 0dB references (and a couple of other complications).

The digital reference of 0dBFS (zero decibels full scale) is the maximum count with a given number of bits (or .0 in floating-point point). So digital levels are usually negative.

The acoustic reference of 0dB SPL (sound pressure level) is approximately the quietest sound that can be heard so SPL levels are positive.

There is no standard calibration. Different mics have different sensitivity, preamps have gain controls, amplifiers have volume controls and some amplifiers and speakers are bigger than others, etc. (Movie theaters are calibrated, but of course the exact loudness depends on where you're sitting.)

However, there is a direct correlation and you can make your own calibration. For example, if you lower the acoustic loudness by 3dB to +91dB) your digital level will also drop by 3dB (to -15.3dB).

The complicating factors are A-weighting and short-term averaging (not peak) in SPL meters, and if you're "measuring" with a regular mic, the frequency response of the mic can affect the readings.
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Old 05-15-2021, 01:49 AM   #3
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A calibrator like that is used to calibrate the SPL level of the measurement mic, not for the entire chain...

And, as Doug already stated, there's no standard for the rest of the levels once you're in the digital world.

Of course, once your mic is calibrated, you could use measurements inside the DAW as absolute levels. But that only matters for measurements, not for regular audio. Inside your DAW everything is referenced to O dB fs. The max. the ADC can do.

And even that level is slightly different from one ADC to another.
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Old 05-26-2021, 12:03 AM   #4
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Your SPL meter should show 94 dB SPL(sound pressure level), not the digital readout of the levels in your DAW.

Standardized audio files are used when calibrating mixing rooms. You play the audio over single speakers one by one and adjust the gain either in your interface digitally which is precise and reproducible, or you adjust the gain on your monitors.

For a set of files and a good methodology, try this page.

I calibrate my 5.1 setup with Dolby noise or the files from the page above for each individual speaker to 79 dB SPL A-weighted on an SPL meter, slow integration time. My room is fairly small, else I‘d be calibrating to higher values. I mainly mix dialogue for tv/film/streaming, so my monitoring is often at its most useful at 79dB SPL. Surround speakers are calibrated to less because our hearing is more sensitive to noises behind us. That‘s why in EBU R128 loudness measurements of 5.1 mixes the rear speakers are weighted 1.5 dB less as well.

You might want to put on ear protection if you‘re calibrating to 94 dB.
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