Fluorescent Hummmmmm?

[Sam Roberts]

I just shot some scenes in the classrooms of a school, out on their playing field and I also did some interior interviews lit with my own lights.

All the footage from the classrooms has a low level hum throughout. It's not there in the exteriors or in the interviews. Could it be the Flourescent lights in the classrooms?

Also any ideas how to minimize it in post using Soundtrack Pro?

 

[David Rasberry]

The hum could be ballast noise, or EMI interference on your microphone from the ballasts. I used to have a shotgun mic that was very sensitive to EMI from high voltage sources.

There is a trick I used to use recording interviews in noisy industrial environments that may work if the hum is constant. If you recorded room tone before you shot, or if you can isolate a sample of the hum to a different track, you can invert the phase of the noise track and mix it back in with your voice track to cancel the noise out. How well this works depends on the nature of the hum and getting the track timing right so waveform peaks on the soundtrack correspond with out of phase dips on the noise track.
You may also be able to notch filter the fundamental frequency with a parametric EQ set to very narrow bandwidth, like 1/10th octave. Set filter depth to around -10dB to start and slowly sweep the frequency range until you hear a change in the noise level. Find the center frequency that works best, then refine the bandwidth and filter depth to reduce the noise as much as possible without hurting overall sound quality.

Good luck

 

[Charles Angus]

You can also noise print/noise reduce in Soundtrack Pro - look it up in the manual, I think there's a pretty good tutorial. It often works ok, especially on hum.

 

[Sam Roberts]

Thanks guys...I appreciate your suggestions on fixing it in post. I'll try both.

After I posted I listened more carefully and the Flourescent theory bothered me because Flourescents buzz, they don't hum. This was like a 20hz electrical ground problem type sound. So I just spent the last two hours doing tests. And I figured it out. It's pretty bizarre.

My sound guy had to leave the shoot at noon and I spent the afternoon shooting B roll with a Sennheiser mounted to the top of the camera. Since it was background sound only I just used some crappy ear buds to monitor and I guess they didn't go that low- not enough for me to notice. But back in the post suite, when I played the footage back on my studio monitors it sounded pretty nasty.

Anyway, it turned out to be the vibration of the Red Drive traveling down the rails, into the body of the camera and up to the shock mount of the mic. An extra piece of foam in the mic cradle fixed the problem.

 

[James Press]

Good to know--good tip for young players.

Another very quick solution in STP is to open the audio file and view it using the frequency view (rather than waveform). Any consistent frequency (eg a hum or buzz) will appear as a horizontal line, present throughout the length of the clip. You can then select it and delete it.

 

[Kris Anderson]

If it's only 20hz you'll be able to filter it out using a simple high pass filter without affecting the rest of your sound.

 

[Sam Roberts]

That's exactly what I did...worked like a charm.