EQ to remove background fan noise?

I'm a little late to this thread because I had difficulty getting a sample of your audio.

It turns out that the AC noise in the background notches out very nicely. I used the Linear Phase EQ (available in FCPX) and did this simple setting:

I took the control "dot" at about the 200Hz position and dragged it all the way down at about the 260Hz position, then adjusted the "Q" (the green range) by grabbing one of the lines and dragging it toward the horizontal dot position. I increased the overall gain (far right control) to +6dB (this is a "to taste" adjustment — I used the FCPX audio meters to try to average out at -6dB - that may be too loud for your preference.)

You may also consider adding an Expander and use the preset: More Punch. (This will reconstitute a little more "fullness"). You may need to readjust the Gain/volume of the clip after adding the Expander. Decreasing the Volume will only further help reduce any perceptible (residual) noise. (Make sure there are no unexpected "pops" in the audio...)

This application of audio effects does not seem to alter the overall timbre of the voice... always an important consideration.

HTH

That looks right.

You're not going to get rid of all the noise, but if you compare it to what it was, it's greatly improved. Also, you can probably back off the +6 gain and use Volume control instead. [The basic idea of noise reduction is to "lower the noise floor" then pull up what's left.]

What concerns me somewhat is that you have several points in your audio where there is clipping (when you tap the marker on the whiteboard, slap the cover back on the marker, and a few other places where the audio "pops" — it's "hard" and loud). [At 9:31:29, it looks like something hits your camera! At 12:17:28, you snap the cap on the marker and it clips at that point even with a Limiter.]

I'm going to alter my original recommendation: set the output dB of the Linear Phase EQ to 0. Set the Expander to More Punch (like before) and add an Adaptive Limiter. The Input (Gain) side can be boosted to +6dB, the Out Ceiling should be set to about -1.5dB and set the Lookahead to rather high (about 190). This should keep the pops from clipping your audio. So far, that seems to keep your audio from crossing 0dB. [BTW - the *order* of effects is important!]

Here's a sample of the effects I applied on my system:

http://fcpxtemplates.com/ngtopics/irobotstock-audio.zip

If they're not the same result for yours, let me know.

Select the audio clip and open the audio inspector. See if any settings there are helpful, especially the Equalization setting and the Noise Removal setting.

If there were a visual way to see the audio spectrum that would be a big help. If it showed a narrow-enough peak for the noise, you try to could EQ it away. I don't think FCP can do show you the audio spectrum, but maybe GarageBand or Logic Pro (or WTFIC) can.

Failing that, open the Effects Browser (Cmd-5) and use the EQ or Channel EQ or one of the other EQ effects. Maybe one of them will be helpful.

You could also roll off quite a lot of the bass using the bass rolloff control far left. Change the frequency that currently says 20.0 hz up to 150 or even higher and see if that helps. Most people listen to dialogue on iphones and other midrangy devices and you don't need to sound like a broadcaster/voice over person IMO. That might help the additional notch at 260 hz that fox_m suggested. Ultimately you probably won't get rid of it.

Also this tool (Izotope's RX series) I have found to be indispensable for dialogue clean up but it costs some money but not that much. It works great. But nothing will fully replace clean audio input, but it's understandable that you can't completely control your environment. https://www.izotope.com/en/shop/rx-elements.html

Good luck.

Just adjust the master volume (it's the end of the chain) so that the "average" of peaks are around -10db and/or the loudest "level" is around -6dB.

In general: put on a good pair of headphones, set your Mac's output at 50% and try to determine "what is comfortable" to listen to. You have to let the end user determine their actual preference for loudness.

If you're an audio engineer: any clipping at all will get you fired. If you're not an audio engineer: just do the best you can. Sometimes you just have to let something pass (or you can try to edit it out if it's isolated enough).

Also fox_m's advice on the Adaptive Limiter is quite helpful. And his advice to do the best you can is priceless as well. You learn as you go.

I am afraid that background hum will be very difficult to remove...

That's the order, however, set the Look Ahead on the Adaptive Limiter to about 190ms. Also, try turning off the DC Offset (Adaptive Limiter).