AudioBook Mastering—Subaudible Noise

I’m re-writing my AudioBook Mastering paper and I got stuck.

How do you tell when you have sub-audible noise or even audible rumble enough to know when to use LF-Rolloff?

You’re not allowed to use Analyze > Plot Spectrum. Nobody understands that.

Spectrogram display turns to trash down there.


Here’s an idea from a low blood-sugar brain on the way to lunch.

ACX Check has three little-used readings below the three primary ACX Compliance values. Make one of those the energy below 20Hz. Could be something to add to ACX-Test. There is no condition where energy down there is valuable. Add DC offset value?


Unless you have a really deep voice, just use it. If you use the default “Size” setting, it’s more gentle. Increase the “Size” setting (for a stronger effect) if you know you have a problem with rumble.

Unless you have a really deep voice, just use it.

No question. But I’m talking about those microphones that have intense and rising noise…at 10Hz.

“I’ve done everything I can think of, the voice presentation sounds perfectly clean on a good sound system, but I still can’t pass noise.”

We’ve all seen those microphones or systems that create their own earthquakes starting at, say, 40Hz and rising steadily to 3Hz or 4Hz. Nobody can hear that, and it can destroy ACX noise readings.

Conspiracy has it microphones get made with that noise because they’re cheap to build, they’re going to inexperienced users, you can’t hear it and who’s going to know?

That happens enough that I want to test for it.

– Do you inspect noise below 20Hz?

– Do you inspect for noise below 100Hz (LF-Rolloff cutoff)?

– Do you inspect the difference?

Where is the cutoff for Too Much Information?

Initially, I had a special, purpose-built tool that could inspect portions of the audio band, but we already have a sound quality tool. Build this analysis into that.

Which analysis? This is complicated by the fact that LF-Rolloff can theatrically affect deep men’s voices. This can make mastering instructions really amusing.


Nobody wrote on a rock we can’t have a generic 20Hz Hi Pass Filter and not test for anything.

100Hz HPF is an industry standard because it’s the most reliable way to shoot voices in the field when the voice are being paid millions and you have to come back with something.


Unless you have a really deep voice, just use it.

I’m just now catching up. You mean just use LF-Rolloff all the time.

I wish it was appropriate all the time, but it can do a number on someone with a “broadcasting” voice. It presents with the contradiction of the better voice, the more likely to be damaged.


There’s your answer: Use it, and if destroys the rich deep timbre of your voice, undo and don’t use it. Somewhere along the line, someone has to make a judgement call - it’s better, or it’s worse. If you’re listening on laptop speakers or dime store ear buds… don’t.

That’s a re-write, isn’t it?

Much of the conditional branching goes away if I do that…

AudioBook Mastering (Rev4).


Low rolloff for speech

Low-rolloff is an equalization setting for Audacity intended to remove low pitched tones, thunder, rumble, church organ pedals and earthquake sounds.

Select the work to be corrected or a whole performance by clicking just above MUTE at the left of the timeline.

Effect > Equalization > Select Curve: Low rolloff for speech, Length of Filter: about 5000 > OK.

Low rolloff mimics the 100Hz rumble audio filter used in Outside Broadcast, Field Recording and Location Movie Shoots.

It’s featured in the Shure A15HP microphone adapter
and it’s a standard button on many sound mixers.
It’s the most low pitch sound you can remove from a performance without most people noticing something wrong.

Some male announcers with low-pitch, rumbly, broadcast voices may notice their voice tighten slightly and some of the broadcast rumble will vanish. Those performances may need to be corrected by hand instead of pre-baked Low-rolloff.

Low-rolloff (and the earlier, custom LF rolloff for speech) rose to prominence in AudioBook recording because of a collision. ACX AudioBooks measures noise using Z protocol which includes rumble and tones too low pitched to hear. At the same time USB microphones became popular and many of them produce very low pitch rumble and noises by an accident of manufacture. So it’s possible to fail ACX Conformance with noises neither you nor anyone else can hear.

Low-rolloff removes those noises.

LF_rolloff_for_speech.xml (299 Bytes)

No longer necessary. The “Low rolloff for speech” curve is now shipped with Audacity as standard.

In 2.1.3?

… That’s why I couldn’t find it.
Screen Shot 2017-06-11 at 10.14.16 AM.png

I’ll clean that up more later. I’m juggling ACX Mastering—trying to keep it from turning into the three-volume set.

The basic document is three paragraphs…until you screw up.


I split it in two.


I think flynwill volunteered to turn that into a wiki entry.

Now to patch Low Rolloff for real.