Mysterious hum in latest voice over

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Peter Stedman
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Mysterious hum in latest voice over

Post by Peter Stedman »

When making my voice overs I always do it the same way. I use my Sennheiser ME66 mic into the Sony HXR-NX3 camera. Results always please me but I’m no audio expert. When making videos for You Tube, I always send the link to a colleague for proof viewing/listening. Yesterday he phoned to say that my latest effort had a distinct hum coming in just prior to my VO clips and ending just after.

I viewed the video on YT via two PCs and my laptop and heard no hum. I double-checked the project on the PC. No problem at all. I made a DVD of the video plus another but without the background music. I played these on my various players and still no hum. Today I visited my colleague to hear for myself this mysterious hum and there it was clear on his Apple machine. He played my DVD and that also produced the hum. Then I called on another friend and he listened to the DVD and the VT video. No hum can be detected.

Well there’s the mystery. Just so that you can hear for yourself here is the YT link. It’s unlisted so only those with the link can view. I’m not asking for comments on the actual video, but can any of you hear this mysterious hum that comes in just prior to me speaking and ands after the end of the clip? Any answer to the mystery?
https://www.youtube.com/watch?v=gBZt3HpTCN8

Thanks for your interest. Pete.
Roy1
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Re: Mysterious hum in latest voice over

Post by Roy1 »

HI Pete, Can't hear any hum and I turned the volume full up. Maybe it's your friends equipment.
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TimStannard
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Re: Mysterious hum in latest voice over

Post by TimStannard »

Peter, I can hear it clearly on both headphones and PC speakers.
I'm guessing it comes in before and finishes after your speaking and this precisely matches your edits (ie you don't edit your voiceover track exactly exactly where you speak, but leave half a second or so either side).This suggests that the problem is with the recording of the voiceover.

Analysing it in Izotope RX I can very clearly see the hum centering on 120Hz. This is exactly double mains frequency of 60Hz and I suspect you have therefore picked up something related to mains. This may be a noise that you didn't hear being picked up by the mic or it may be introduced by the way the equipment is all connected up.(Earth loops were often a problem with band equipment, for example)

Interestingly there is no sign of any hum at the fundamental frequency (60Hz). If the source was external, rather than the equipment, I'd venture to suggest that you have the "lo cut" set on your ME66, which cuts the low frequencies.

If the sound was generated within the equipment, then some other filter may have cut it out.

Of course I may be entirely wrong about it being mains related.
Tim
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TimStannard
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Re: Mysterious hum in latest voice over

Post by TimStannard »

Update:
Peter, here it is in Izotope RX where you can actually see the hum (I highlight it in pink)
https://youtu.be/KDe9c2hgQAs
Tim
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Peter Stedman
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Re: Mysterious hum in latest voice over

Post by Peter Stedman »

Thank you Roy, as you say very odd,but see Tims technical answer.

Thank you Tim for you great technical answer and for going to all that trouble. I understand some of what you write but certainly not all. Re (Earth loops were often a problem with band equipment, for example) I understand this as way back in the 1970s I had a radio mic when working cabaret, and this certainly produced a hum until I was told of the cure for this. You will have noted that Roy didn't get the hum.

I looked at the lo-cut switch on the mic module and I 'think' this is on but I'm not really sure which way is On/Off. I've never touched this in all the years I've had the mic. We certainly don't have the answer to why the hum only is heard on my friend Apple computer and not on PCs. Naturally I want to be sure on the sound quality so I will record again but set my gear up in another room away from my editing room because I agree with you that it appears to be something to do with other equipment near by although it's never happened before. Just as well I got my friend to proof view/listen as always.

Thanks again for all that tech. gen.

Cheers. Pete
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John Roberts
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Re: Mysterious hum in latest voice over

Post by John Roberts »

Oooh Tim, it's not like you to make a schoolboy error :wink: UK mains frequency never has been nor ever will be 60Hz, but 50Hz.

I've had a look at the waveform as well, and there is a definite peak at 120Hz as well as lesser peaks at 60Hz and multiples thereof, therefore I would hazard a guess that it is not mains borne interference causing the hum.

My most likely guess is a cooling fan, possibly contained within the editing equipment tower (although I don't know your set up Pete, so can't really comment for sure). I've filtered out just the 60Hz and 120Hz frequencies and the remaining noise sounds like a small cooling fan - certainly something mechanical as opposed to electrical. Possibly the microphone and/or stand were not effectively isolated from the surface they were mounted on, which in turn might be transmitting the vibrations from the PC cooling fan to the microphone element. I always stick my table top microphone stand on a pillow if recording like this - works wonders!

I've uploaded a 'before and after' video to YouTube, but I'm not sure if the link is working or not because it doesn't appear in the forum preview!

Hope this helps :)

https://youtu.be/niIdpMMeTTU
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Peter Stedman
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Re: Mysterious hum in latest voice over

Post by Peter Stedman »

Thanks John. I looked and listened to your YT clip and the hum was really clear and then disappeared . . . Wow all this technical gen. Phew. I have solved the problem by recording the VO again with the camera/mic in another room away from all my editing gear. Put the new video version on YT and let my colleague have the link and he says it is fine now. Not a hum in sight. (Or should that be 'within hearing?')

I don't know why the problem hasn't cropped up before, perhaps this time the camera was nearer to my editing gear than previously. Certainly I will now always do my VO recording in another room and yes I always put the camera on a padded mat as you have mentioned.

Thanks both again for all your efforts even if I didn't understand a lot of it. I have learnt something new (one does every day of course) and that's what it's all about. I will now remove the dodgy video from YT. Why it could only be heard on a Mac and not on PCs is a mystery still

Did I say 'thank you' ? Well, I will say it again. Thank you!

Cheers.

Pete.
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John Roberts
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Re: Mysterious hum in latest voice over

Post by John Roberts »

Hi Peter,

My pleasure - we're here to help each other as I'm sure Tim will agree :D

I'm glad you have sorted the issue - my money is on mechanical vibrations from some device being picked up by the microphone or stand. These issues can come and go depending on the device causing the vibrations (a cooling fan might operate more often in warm weather, for example) and also be dependent on a piece of equipment being moved as little as a fraction of an inch, bringing its case into contact with a cupboard side and causing it, and everything else mechanically connected to it, to vibrate in sympathy.

Don't be concerned about issues apparently only being evident on a Mac and not a PC, in this case it is simply not true. The frequency involved is fairly quiet and low down in the frequency spectrum, therefore if a viewer's audio system (with apologies to Roy) is not capable of reproducing those frequencies they will not be heard, regardless of the computer system used to play it. Most TV speakers are incapable of reproducing these frequencies adequately, for example.


I would comment however, that if those frequencies cannot be heard on the system being used to monitor audio when compiling a film's edit, then that system is inadequate. The frequency in question here - 120Hz - is not termed as low bass at all. A normally tuned bass guitar or contrabass will reach down to 40Hz, many 5-string examples are tuned even lower. To not be able to hear those registers, and everything else around that area that concerns us as filmmakers such as traffic rumble and wind noise, means an audio upgrade is in serious order!

For £300, one can buy a pair of JBL LSR305 2-way digital bi-amped professional studio monitors, famed for a full range frequency response curve flatter than a hedgehog on the M1, PLUS a pair of Sony MDR-7506 professional closed-back headphones. This equipment is the best available at the price and will last years. How much camcorder/camera/lens/computer can we buy for £300? :wink:

John
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TimStannard
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Re: Mysterious hum in latest voice over

Post by TimStannard »

John Roberts wrote: My pleasure - we're here to help each other as I'm sure Tim will agree :D
Indeed. Yet my schoolboy error is now there for posterity! Thanks for correcting me. I located the hum, thought 120Hz doesn't ring a bell so halved it and for some reason got totally fixated on 60Hz being UK mains. Duh!
Anyway, thanks to your corrections, John. We eliminate the problem being generated by the technology. Interesting you filtered out 60Hz - I don;t think there is anything there (not trusting ears, but looking at the spectral representation within Izotope).
Most imporantly, Peter has eliminated the problem by re-recording. Lucky it wasn't an on site piece to camera.

No longer relevant, but for information, Peter. Lo-cut is "on" with the switch moved to the position with the "bent" line (to the right if you have the business end of the mic pointed away from you.) Often lo-cut on the mic is used to reduce wind noise and I used to consider this "cheating" and always assumed I can reduce low frequencies in post (correctly). However, a retired BBC engineer explained to me that if you eliminate the low frequencies when recording, there is more bandwidth available for the rest of the signal or, to put it another way, you are not taking up so much bandwidth with lo-cut on, so there are positive reasons to use it. Lo cut is now my default setting if I am recording the human voice (as the human voice doesn't get that low)
Tim
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John Roberts
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Re: Mysterious hum in latest voice over

Post by John Roberts »

Hi Tim :D

My software picked up the peak at 60Hz, as well as 120Hz, 240Hz etc so they would in all likeliness be connected to the same thing. As an experiment I boosted the 60Hz frequency and dropped the 120Hz, and the audible result is definitely mechanical.

I've uploaded another version of the hum tests to YT, if you're interested :lol:

https://youtu.be/cyhOZPUN8FY
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TimStannard
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Re: Mysterious hum in latest voice over

Post by TimStannard »

With apologies to the rest of the forum.
John, I don't think the 60Hz is present in the voiceover recording. Are you getting this 60Hz when Peter is speaking?

The 120Hz is very definitely strongly present during the voiceover. I can both see and hear the 60Hz hum (along with many others in the music only sections, but the 120Hz seems to cut in just before the V/O and drop out just after.

Was the sample in your video under V/O?

If you can be bothered, look between 3:45 and 3:46. There are several dominant frequencies at 3:45 from the music but half a second later there is a very clear start of a hum at 120Hz & 240Hz - yet I see no particular increase at 60Hz (indeed this frequency seems to drop in amplitude,consistent with ducking the background music). The 120Hz (and drops out again suddenly at 3:51 about a second after the voice cuts out.

I don't know if it's possible my download (using keepvid) has removed different frequencies to whatever you used, but I am definitely not getting 60Hz under the v/o.

EDIT: Incidentally I did wonder whether the clip you posted was a Tangerine Dream album ;)
Tim
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John Roberts
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Re: Mysterious hum in latest voice over

Post by John Roberts »

Hi Tim :D

Yes, the 60Hz is present under the V/O recording, there are a couple of seconds where Peter stops talking but leaves the channel 'open' and this is the bit I worked on, to eliminate as much as possible the frequencies from the music etc.

Of course, the amount of boost I applied to the 60Hz frequency is pretty extreme, mainly because I wanted to isolate it as I was curious as to what was causing the peak. It's also distinctly possible that the 60Hz is the result of something else, possibly Peter is a closet Tangerine Dream fan! It certainly sounds mechanical but as to the exact source, I'm not sure.

It's great that Peter has resolved the issue, though :D

John
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TimStannard
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Re: Mysterious hum in latest voice over

Post by TimStannard »

John Roberts wrote: It's great that Peter has resolved the issue, though :D
Indeed. The rest is purely academic (although forums are great for that as well ;) )
Tim
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