I'm not sure why but when recording human speech, I have always turn on the lo-cut filter on my Rode NTG-1 mics. And, when I use my Shure FP-31 or FP-33 field mixer with a wireless lav, I use the mixer's lo-cut filter.

Is it true that it filters low frequencies outside the range of the human voice or should I consider being more precise with the filter's usage?

When do you use it and not use it?

TIA

Yes it removes the low frequencies that you may not wish to record outside of the human voice range and can also be useful in cutting down low frequency rumble due to wind or distant traffic rumble on location.

Most filters or attenuators tend to be in circuit in between the mic capsule and op amp so can also prevent low frequency overload or distortion, some mixers have a two stage cut and one is normally OK for general use but the second stage tends to be more severe and for use in very noisy environments, you can also apply low cut in the audio dubbing stage but the ones on mic's or mixers tend to be more useful for cutting unwanted location noise.

They can also be useful to reduce the proximity effect that raises the low frequency content on some microphones if they are used closer to the person speaking.

+1 Best to turn on the mics low cut for all spoken voice recording .. anything in speech below 80cycles is likely to cause trouble down the line, especially when mixing in backgnd music and Fx.

Some mics don't don't have low cuts so engage the next low cut in the circuit .. at the mixer. In the studio use a separate pop filter that clips to the rig .. to keep the talent in place back from the mic. and helps to keep deep male voices from swamping the soundtrack.

If the talent complains later he can't hear his dulcet lows don't turn up the low end on the console .. you're just bringing up the mud. That's probably going to be worse.

So that's one time you could say 'we'll fix it in the mix' and get away with it. :)

Cheers.

The trouble is, the FP-31, 32, 32a had steep HP filters around 150Hz, to the point that it would remove any 'body', thinning out the voice to excess. I cuss and spit on the ground recalling those dogs.

That actually explains why I thought it was a bit thin going thru my FP31. Do you know if it was improved in the FP33?

FP-33: "Down 6 dB at 150Hz. The roll-off slope is 6 dB per octave."
FP-32: Graphic from manual: Looks the same.