Close Mic / Phase Cancellation / Comb Filtering Confusion

A

Ashton Lamont

Active member
I'm struggling to understand how these factors may be relevant in the specific situation in which most of my event shooting is done.


That situation is weddings in which the priest wears a wireless lav to the church's inhouse system.


Ideally they would be using a Sennheiser G3 setup with the same frequency band as I have.Then I could simply tune my receiver to the frequency they are using.However that is seldom possible – there is all sorts of old equipment in use completely incompatible with the G3's.


Next would be to get a feed from the church's deck. But again that is seldom possible as the decks are often in inaccessible places and are in sealed racks (to prevent theft) which prevent access to the various sockets normally at the rear.


So sometimes I look to persuade the priest to wear one of my Sennheiser lavs to my transmitter in their pocket.


This involves attaching my lav within a few inches of the church lav. There is seldom if ever the opportunity for a sound check because the priest will want to put on the lavs themselves when they robe up a few minutes before the start of the ceremony. At that stage I am elsewhere buzzing around shooting everything else happening at this busy point.


I would welcome any guidance you can offer on how having my lav so close to the other lav may adversely affect quality. Would the same issues apply regardless of whether the church is also using similar gear to my G3's (I would be on a different frequency), or whether theirs is a completely different system?


Again some lecterns and pulpits have a wired microphone. Some of my colleagues attach a lav mic to that mic then going either to a wireless transmitter or to a small audio recorder. Is having the two mics so close together not asking for trouble?


Are there any circumstances in which me using my Senny G3 system could cause interference / loss of quality / drop-outs to the church's system?


I had a wedding where the audio quality through the church's PA speakers was terrible most of the time; to the point where my usual backup of having a standalone audio recorder close to one of the PA speakers was unusable. But my mic'ing of the priest and the groom using my G3's was lovely :- ) I switched off all my stuff momentarily – bar the transmitters on the groom and priest which I couldn't reach – but that made no difference. I asked the staff afterwards and they said the PA is always bad so I felt less guilty :- ) I've shot many other times where there are two lavs and no issues at all.


I'm sure this must all be basic stuff but for non-audio specialists it does get very confusing very quickly. Thanks!


Pete
 
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Ashton Lamont said:
Again some lecterns and pulpits have a wired microphone. Some of my colleagues attach a lav mic to that mic then going either to a wireless transmitter or to a small audio recorder. Is having the two mics so close together not asking for trouble?
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Two mics together won't be an issue at all if they are each feeding a completely different system. With one mic going to the live system, and the other going directly to a camera or recorder, they have no way to interfere with each other. Ever see a press conference with 15 mics clamped to the podium in one, massive pile?

I have clipped a lav to a podium/lectern/pulpit gooseneck many times in a pinch and it works just fine.

Even if both mics were feeding the PA system, it may or may not be problematic (there is a 2-mic technique often used on podiums, but that's beyond your situation or needs).

Ashton Lamont said:
I would welcome any guidance you can offer on how having my lav so close to the other lav may adversely affect quality. Would the same issues apply regardless of whether the church is also using similar gear to my G3's (I would be on a different frequency), or whether theirs is a completely different system?

Are there any circumstances in which me using my Senny G3 system could cause interference / loss of quality / drop-outs to the church's system?
Click to expand...

As long as your system is on a separate frequency, and if running the same block using a frequency that is compatible with theirs, you shouldn't have any issues. If they are running G3 systems in the same block and you simply tune your receiver to their transmitter you'll pose no risk to the system. However, if you try to run two transmitters on the same frequency, you'll have problems and render both transmitted signals useless.
 
Two lav mics in close proximity, should not present any problems.. providing they're not being fed to the same audio track, in that case.. you very well could have an salvageable phasing nightmare.
Note: If placing the body pack transmitters next to each other, RF spill could be an issue.. exhibiting noise and transmission issues.. same goes for cell phone type devices.
 
Thanks for the explanations Alex and Rick.

Rick, the scenario in which two lavs using two different transmitters / receivers and frequencies could well arise where I plug the receivers into the XLR sockets of a Zoom H4n. That recorder has the 1st XLR go to the left channel and the 2nd XLR go to the right channel of the same track. Likewise with my AC90 cam. This is easy to sort in my NLE which is Vegas Pro. But is there a rule of thumb about how close the lavs can be without risking phasing? n.b. there are moments when the groom and priest are no more than 2 feet apart. Also, is RF spill more a theoretical possibility or a real risk. My understanding is that the Sennheiser G3 units are well-shielded but could they readily cause noise and transmission issues if right next to an older or more modest transmitter. It can be hard to get a celebrant to wear a lav let alone asking them to find two separate pockets or belt positions for two systems. Thanks.

Pete
 
Ashton Lamont said:
... the scenario in which two lavs using two different transmitters / receivers and frequencies could well arise where I plug the receivers into the XLR sockets of a Zoom H4n. That recorder has the 1st XLR go to the left channel and the 2nd XLR go to the right channel of the same track. Likewise with my AC90 cam. This is easy to sort in my NLE which is Vegas Pro. But is there a rule of thumb about how close the lavs can be without risking phasing? n.b. there are moments when the groom and priest are no more than 2 feet apart.
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Just to make sure I understand, as this seems to be a bit different of a question from your original post: you're using two wireless systems (one on the groom, one on the clergy), recording to two separate tacks on your recorder or camera, and you want to know how to deal with or avoid phase cancellation between the two. Is this correct?

Because they are recorded separately, your control over phase cancellation comes in post. Very simply, split the 2-track file (linked stereo pair) into separate, mono audio tracks. Pan each to center. Use your mix automation to mix between the two. Whomever is speaking, bring up that mic and lower the other by just a few dB. That will help you avoid phase cancellation between the two.
 
You'll have check any inter-modulation frequency issues. If that's ok, two or more lavs can be dealt with in post.. if they're on separate tracks.
Unfortunately we can't always adhere to the 3:1 rule.
 
Rick R said:
Unfortunately we can't always adhere to the 3:1 rule.
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For the OP's benefit: two mics running simultaneously should be three times the distance from each other as to their sound sources. So, if a mic is 1' from the source, the second mic should be at least 3' away to avoid phase cancellation. This is crucial when micing sources like choirs and orchestras. With lavalieres on moving subjects, it is impossible to maintain. Again, it requires recording them separately and then mixing in post.
 
The 3:1 rule is a very handy thing for positioning microphones.
However, it would not appear to apply to the original question here.
If there are two microphones attached to the subject (the officiant), it makes no difference what distance apart they are because they will never be used together.
If one of the mics is for the venue reinforcement system, then the presence (or not) of another mic for video/recording makes no difference at all (acoustically).
And a microphone which is put there for recording/video has NO EFFECT on the other system.
We are talking about ACOUSTIC issues here ("Phase Cancellation / Comb Filtering")
By "used together" I mean that even if we are recording the feed from the venue system ("Mic A") along with the other mic ("Mic B"), we are using one or the other, and never mixing them together, so there is never any opportunity for "phase cancellation or comb filtering")
It makes no difference whether we are talking about wired or wireless mics here.

Now, if we are talking about RF issues, there is a small possibility of interference between the two transmitters.
It is not a very big possibility, but I would certainly test the combination before the processional starts.
 
And another point on this for the OP and others. The phase problems are ONLY if you mix the two mics down to mono. This was a hot topic back when pretty much all music was going to get played at some point on AM radio which was mono, or farther back when the main release format was mono.

So depending on what you are going to do with the recording it may not factor in at all. Today most music is broadcast on FM (stereo) or on the internet (stereo) and AM radio is largely relegated to talk shows and sports. Pretty much nobody releases music in mono so it's not even a lot of an issue there.

About the only time it is still a real issue is when you are recording production sound with multiple lavs picking up a lot of cross talk, and you are not doing a real dialog edit. Mostly ENG or nobudget filmmakers who don't do any real sound work. If you edit out the cross talk you again don't generally have much of an issue. For most people working today "mono compatibility" is never going to come up.

As Richard and others pointed out, from your post it's not an issue with what you describe as going on.
 
Gosh, thanks everyone for your input. Head spinning a little. Alex, yes my post was aimed at discovering what elephant traps might lye in wait for me during production in my particular set of circumstances, but also by implication what delights may rear up in post. In ceremonies generally I use just one wireless set on the groom as getting priests to agree can be a time-consuming load of grief here. And I set up Zoom H1's and H4n's on lecterns etc. Sometimes I use three wireless sets in the wedding speeches - on the three usual speechmakers - but they are always several feet apart from each other. Thanks again.

Pete
 
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