http://forum.audacityteam.org/viewtopic.php?p=327047#p327047

Title
Purpose
Install
Use
Comments and Notes
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LF Rolloff for Speech

PUPOSE:
LF-Rolloff is a custom equalization setting for Audacity intended to remove low pitched tones, thunder, rumble, church organ pedal and earthquake sounds.

INSTALL:
Download LF_rolloff_for_speech.xml.

Open some work in Audacity or generate a short test clip. Content/quality is not important.

Effect > Equalization > Save/Manage Curves > Import > Locate and click LF_rolloff_for_speech.xml. > OK > OK.


USE:
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: LF-Rolloff for Speech, Length of Filter: about 5000 > OK.


COMMENTS and NOTES:

LF 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 LF-Rolloff.

LF-Rolloff rose to prominence in AudioBook recording because of a collision.  ACX AudioBooks measures noise using Z protocol which includes rumble and tones too low 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.

LF-Rolloff removes those noises.