Waveform's "frequency" isn't centered... Hard to explain, but what does this mean?

I have some audio clips recorded by certain equipment where the waveform of the audio looks strangely off-balance to me. Here’s a screenshot of what this looks like in Audacity:

The thing that specifically strikes me is that the waveform isn’t perfectly centered around the “0” horizontal line. It’s slightly above the line. This is most noticeable between the two big sections, and again as the 2nd section trails off.

So, first, I was wondering what this even mean? Does it indicate an issue in recording the signal? Is it actually fine and not something to be concerned with? (The clip doesn’t really sound like it has a problem, though I do feel like I hear occasionally loud popping even when nothing’s really happening in the clip.)

If this does indicate a problem, is it something that can be corrected? Or would it need to be re-recorded?


High-pass filter at 20Hz /48dB octave …

High-pass 20Hz 48db

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That worked. I can’t really hear any difference, but it does “center” things. I’ll look more into what this was actually removing from the signal. I appreciate the very clear (and easy) fix.


That’s extremely informative. Thanks again.

What’s the Certain Equipment? It’s really unlikely this is a digital problem. More like a bad power supply or bad Microphone Preamplifier. Also a bad Analog to Digital converter. Sound Mixer failing?

Rumbling? Thumping? This is where having a good pair of headphones or a good music speaker system comes in handy. You know in a thunder storm sometimes your windows rattle for no good reason? That’s super low pitch sound generated by the lightning. Same with earthquakes. There’s a standing movie joke that you can’t photograph an earthquake, but you can photograph the wine rippling in a wine glass. That’s the musical pitch we’re talking about here.

There is a presentation company (Nebula??) whose brief opening theme song has an earthquake rumble in it. On my wide-range sound system it’s enough to make me look around to see the big truck driving by, but I bet most people never hear it.

So describe the sound system before the digital service.


Unfortunately, I don’t really have the specifics. The audio I was referring to came to me from a voice actor, and I don’t know his particular setup. But I didn’t see the same kind of thing in audio from other voice actors, so I assumed it was something unique to that one person’s setup.

There are two similar characteristics and two simple words. :wink:

Offset (or “DC offset” which technically describes an electrical signal that has DC combined with the AC audio, and this is what you’d see on an oscilloscope).

With an offset the whole waveform, including silence (silence is a straight-flat line), is shifted up or down from zero. Although you can’t hear DC (zero Hz) and your speaker can’t reproduce it, you can get a “click” or “pop” at the beginning where the speaker suddenly pops-out or in, and at the end when the speaker returns to its normal resting position.

An asymmetrical waveform is bigger (higher in amplitude) on the positive (upward) side than the negative side (downward), or vice-versa.

An asymmetrical waveform usually sounds normal unless it’s caused by some crazy kind of distortion.

Oh… sometimes with a “cheap soundcard” you can get asymmetrical clipping. Normally you should avoid clipping (distortion) altogether but when you hit the “digital maximum”, digital clipping should happen equally negative & positive at exactly 0dB (+1 and -1 = 100% on the Audacity scale).

If you remove the offset or asymmetry you can amplify (or normalized/maximize) louder because the positive & negative peaks can both be “maximized”.

I made a graphic for that.

Fair warning you should apply filters before you do any editing. Once you get that little tick or pop, it’s a lot harder to get rid of.

Select the whole performance > Effect > Filter Curve EQ > Presets & Settings > Factory Presets > Low Rolloff for Speech > OK.

That’s the filter built into the Audiobook Mastering Macro to get rid of rumble, thunder, thumps, and some mouth pops.


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