What’s in a Decibel?

What’s in a decibel? A lot, apparently. I’m no sound engineer, you can be assured of that. I’ve done lots of stuff about physics and sound propagation and all of that, and I can describe to you how decibels work and even how to do some calculations based upon distances and densities and things. Great. Top stuff.

Doesn’t really help when it comes to the raw practicalities of getting Voice Over work done right for audiobooks, now. Well, that’s not totally true. It helps a bit. You see, when it comes to submitting an audiobook for ACX, there are some strict guidelines issued that, if breached, will result in rejection of the audio file (not audiophile, that’s something else).

As you can imagine, there’s a desire to have quality, consistent recordings sent up for the listeners to enjoy, and one way to at least ensure you won’t bust the eardrums or have the audience screaming “What? What did he just say?”, is to keep the recording between some ‘loudness’ values.

Peak? -3dB. Floor? -60dB.

Easy peasy. Just make sure you’ve cleaned out the noise from the signal, quieten out any top-end values to be under -3dB and you’re good to go.

Wait, there’s one more thing: The RMS must be between -18 and -23 dB. RMS? RMS. Remember back to your signal processing days? Root Mean Square. Take the root of the mean of the square of the signal, and you end up with an ‘average’ signal, without the need to discard or negate the negative side of the signal (since squaring will make it positive).

OK, so what’s that all about? Yeah, good question. It means the average loudness of your voice needs to fit between these two values. Not too loud, not too soft.

Well, that’s me done because I’m a naturally soft speaker. All my files fell below the -23dB cutoff. Better give it up and go play ping-pong, yeah? Nah. Because with Audacity, I can amplify the signal and boost it up to something more in line with the requirements. Thing is, amplification of a signal also amplifies the noise, so that needs to be removed as well (otherwise the clip fails the -60dB floor test).

To help out, there’s a tool for audacity called ‘ACX Check’ that can be downloaded. Get it from : https://wiki.audacityteam.org/wiki/Nyquist_Analyze_Plug-ins#ACX_Check

That helps out a lot, letting you know if you’ve managed to get into the realm of happiness, if you’ve got some bad noise somewhere, or a pip that blasts past the -3dB threshold. The only thing I’ve found is that the RMS calculation takes into account all sound blocks, including breaks and pauses, meaning that the RMS figure is artificially low for a sentence with a pause in it.

Now, if I take the value and ramp up, then check it with the ACX Check, it may pass the RMS check, but then fail the noise check. So I then take a sample of the voice along with the silence, and the RMS check fails, but the noise floor passes. Where that leaves me, I don’t know.

I’ve resubmitted the sound files after going through them all and tweaking the amplification, so I’ll let you know how it goes.

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