Looking for a good lavalier mic that is impervious to GSM / cell phone interference

[Greg Miller]

Luke,

If you check the "sponsor listings" at the bottom of the thread, you'll see that there's a rental house in Illinois. I don't know your geography out there, but perhaps they're conveniently close to you, and perhaps they can even rent you an SM11 for a test.

If your previous report is correct -- the interference was NOT picked up by the balanced cable between your present mic's power pack and your mixer -- then I strongly believe that this mic will solve your particular problem.

And thanks for the courtesy of following up with a report! I'll be waiting to hear how it works out for you.

[Luke Arndt]

Just to update, I ordered a Shure SM11. :-) Now I eagerly await its arrival so I can give it a go.

[Luke Arndt]

My Shure SM11 arrived today.

Unfortunately... the GSM buzz is still there! It may be slightly reduced, but the mic is definitely picking it up.

These MX150s (http://www.bhphotovideo.com/c/product/842433-REG/Shure_MX150B_C_XLR_Microflex_Cardioid_Subminiature_Lavalier.html) sound intriguing with their "CommShield RFI Rejection," but man... they're expensive!

I've also toyed around with the idea of getting some goose-neck microphones and just placing them on the table in front of speakers. Or I guess there's a remote chance something like the Shure A15RF (Shure A15RF RF Filter, Problem Solver | Full Compass) could fix it, but it sounds unlikely.

Hoping this thread will someday have a happy ending. :-)

[Greg Miller]

Luke,

Sorry to hear that report. But something here does not compute.

Back in post #6 you stated:

Originally Posted by Luke Arndt

I should add that it appears the interference/noise is coming in on the thin wire between the actual microphone itself and the power pack. Everything seems okay from the XLR cable on, so it seems the vulnerability is with that thin microphone cable.

At the time, you felt that there was no interference getting into the balanced XLR cable (and hence into the mixer). Now you have a completely balanced system, yet the interference is getting into the system. These two situations are contradictory.

(By the way, whenever I recommended the SM11, I always said, "If that report [about no interference into the XLR cable] is true, then the SM11 should solve the problem.")

Anyway, let's reason this through. The first question: has anything else changed? Source of power for the mixer? Extension or jumper cables being used? Etc., etc., etc.? You might want to re-read Pete's comments in post #41.

If nothing has changed, then apparently some interference is getting into the mixer. In that case, the first thing I'd do is check to be sure the phantom power to that mic jack is turned off. Otherwise, RF might be following the phantom components back into the mixer's power supply.

The Shure filter you found might also work well, as it claims to keep RF out of a balanced audio line (which is what you now have... completely balanced). There's no schematic shown, so I'm not sure exactly what's inside; presumably at least a pair of bypass capacitors, and if it's really good a pair of series inductors as well.

An alternative to the Shure filter (or the next thing to try if the filter isn't a complete cure) are the ferrite beads that Richard suggested in post #32. I'd get the snap-on type, and wind three or four loops of wire through one of them, as he suggested. Locate this bead as close as possible to the XLR connector, just before it plugs into the mixer. (If you can't fit multiple turns of wire through one bead, then get several beads and place them end to end just ahead of the XLR.) That should keep any RF on the shield from entering into the mixer enclosure.

While you're waiting for the beads to arrive, you might also take a look at your mixer's schematic, and see whether the inputs are transformer coupled, or just active balanced. Transformer coupled would be preferable, IMHO, as the balance would probably be better, and a good transformer shouldn't pass any RF.

One other thing: Are you sure the interference is getting into the record audio path, and not just the headphones? Does it show up on the meters? Any wire connected to the mixer might serve as an antenna. The headphone wire might pick up the RF and let it enter the case (through the headphone jack) where it could get only into the headphone amp, or it might bleed into all the circuitry. As you now see, if the mixer itself is RF-susceptible, the problem becomes much more complex.

Please give us some more feedback about all these details, and we will solve this thing for you!

[John Willett]

Originally Posted by Luke Arndt

My Shure SM11 arrived today.

Unfortunately... the GSM buzz is still there! It may be slightly reduced, but the mic is definitely picking it up.

These MX150s (Shure MX150 Subminiature Lavalier Microphone MX150B/C-XLR B&H) sound intriguing with their "CommShield RFI Rejection," but man... they're expensive!

I've also toyed around with the idea of getting some goose-neck microphones and just placing them on the table in front of speakers. Or I guess there's a remote chance something like the Shure A15RF (Shure A15RF RF Filter, Problem Solver | Full Compass) could fix it, but it sounds unlikely.

Hoping this thread will someday have a happy ending. :-)

Do a test - connect a balanced cable but no microphone at all - do you get RF interference like this?

If so, it's not the mic.

[Greg Miller]

Excellent suggestion, John. I'm Sorry I overlooked that.

[Steve House]

Why use lavs at all? A PZM boundary layer "conference table" mic or two on the table itself might work better as the mic itself would be farther away from the phones in the participants' pockets than would be the case when they're wearing lavs. The inverse square law can be your friend.

I assume you're feeding the mics via the mixer directly into the camera (my understanding of the requirements of deposition work is that courts get suspicious of edited tapes and double system sound, preferring unaltered originals). Are you also feeding a backup audio recorder? How about outlining your entire recording chain for us from start to finish. I'm wondering if the interference might be finding its way in AFTER the mixer, either in the connection between the mixer and the camera or perhaps even in the camera or recorder itself. Are you sending balanced or unbalanced audio from the mixer to the camera, ie, are the camera audio inputs XLR or are you using a miniplug 'external mic' input on the camera?

[Tom Gresham]

Thanks to this thread, I bought three Tascam PR-10 recorders. I'll keep them handy for those times when I have more people on camera than I have wireless mics, or when I need the talent far enough away that the Sennheiser wireless setup won't work (seems to be 40 to 50 yards).

Thanks for the tip!

[Luke Arndt]

Okay, bit of an update!

I'm pleased (and somewhat embarrassed!) to report that I was wrong! The Shure SM11 mic does not pick up the GSM interference. I believe the reason for my mistake was I didn't allow enough distance between the Shure and the Audio Technica mic. After I separated them farther from each other, I could tell for sure that the SM11 IS impervious to the noise. My apologies for the false alarm...

Anyway, a couple of thoughts on the SM11:

* This thing is surprisingly large. Not a deal-breaker, but it is big. For video depos, it shouldn't matter, as I don't need to hide them.
* The sound quality on it is okay, but not great. It's not as rich or full as the AT803B mics that I normally use, but I think it would get the job done.

For what it's worth, I DID get a slight bit of interference when I placed the phone right on the head of the SM11 microphone. It wasn't a huge amount of interference, and obviously that's not going to be a problem in the real world.

For those who asked, here's my setup: Microphones (Audio Technica AT803B or Shure SM11) via XLR to the SignVideo ENG-44 mixer. That outputs via XLR to the camera, Sony PD-150.

In my testing, I did also try bypassing the mixer, and connected the mics directly to the camera. No real difference in interference. I did notice, though, if I placed the phone directly on top of the mixer, I would get a little buzz. Aside from that, it doesn't look like the mixer affects things at all, positively or negatively.

As an aside: When my phone was on a call and I brought it within a foot or two of the camera, the camera itself started shaking/vibrating a little bit! Kind of amazing the signal could be that powerful.

Steve: I have actually been thinking about that myself, using a few boundary mics, shotgun mics, or gooseneck mics. I wonder how the audio quality would compare if I used something like that. My AT803Bs sound great by and large, but maybe I can match their quality by using a few boundaries or something.

This is huge progress, and I want to thank everybody for all of their input up to this point! I now know that I can get rid of the interference if I use these SM11s. This is exciting!

[Greg Miller]

Luke,

That's great news! Based on your previous reports, I was pretty sure the SM11 would solve the problem... I'm glad it did so.

As to the size... the specs clearly state the dimensions... it's roughly twice as long as the AT803B. Really, it's about the size of one joint of your middle finger, right? The problem is perception: you're accustomed to pea-size electrets and even Cheerios are big by comparison. Nobody is going to cancel a deposition job because the SM11 is too big. But you might get some complaints about GSM interference. C'mon, be a man, deal with it. ;-)

Two drastically different mics, like the AT803B and the SM11, will certainly sound different. You may be able to somewhat mediate that using some EQ.

For example, I note that the SM11 has about a 4dB peak centered around 9 kHz and extending down to around 7 kHz. The A-T also has a HF peak, but it extends down lower, so that mic might sound as if it has a little more presence. More notably the SM11 low end rolls off notably, starting around 500 Hz; it's down around -9dB at 100 Hz; the A-T is relatively flat down to around 50 Hz, so it will have a lot more balls. You could try applying an inverse curve to the SM11, to make the sound "theoretically flat." However, I'd be careful with that much bass boost, as it will increase room noise and also "thumps" when the talent moves around.

If you wanted to get really involved, you could do a free air test of the two mics, side by side, recording pink noise played through a speaker that's 3 or 4 feet away. Then compare the curves, and apply EQ to the SM11 so that its final response is an approximate match for the AT803B.

Of course too much EQ can cause various problems. But you might consider the above as a starting point if you are really concerned about the sound quality.

Boundary mics can sound quite good for the right application. But of course they will be more susceptible to room noise, paper shuffling, coughs, HVAC, plumbing noise... all things that can detract from the clarity and intelligibility of the deposition. What's your goal?

But just think... now that the GSM problem is solved, you have lots of free time to devote to these other experiments. And you won't need to confiscate phones or hold a gun to anyone's head. ;-)

Happy Trails!

[Luke Arndt]

THANKS again for all of the great info in regards to the microphones.

If I'm understanding correctly, the reason the SM11 is "special" is because it's a balanced dynamic mic as opposed to an electret? I was sifting through this thread, and I couldn't see any mention of any other mics like the SM11, but I was wondering if such a thing exists. For instance, Pete mentioned he uses the same mixer I use but with an Audio-Technica AT899 microphone, and he has no problems.

I guess I'm just wondering if the SM11 is a one-of-a-kind, or if there is a collection of such mics that I could choose from. A quick search on B&H for "dynamic lavalier" returned just the SM11 and some $464 Electro-Voice lavalier (http://www.bhphotovideo.com/c/product/745189-REG/Electro_Voice_F01U118092_649B_Dynamic_Lavalier_Microphone.html).

Anyway, now that I know it's possible to find a lav that is balanced, it's now a matter of finding the best one. The SM11 looks like it will suffice; I'm just wondering what other options there are before I buy anything else.

Also, the idea of the ferrite beads intrigues me too. I might order a few and try them with my existing mics, to see if there's any improvement. Is there any specific type of ferrite bead I should be looking for? Richard mentioned a ferrite torroid, but was saying I would need a large one that would allow the microphone's head to pass through it.

Thanks again, gentlemen! Your expertise, experience, and patience have been a huge help.

[Richard Crowley]

There are also ferrite filters that are cut in half so you can snap them around the cable. If you have much of a cable collection for your computer(s) you may already have a cable or two with this kind of thing around it. Either a clip-on version (which can be removed and tested with your microphone) or the molded-on kind (which are not "hackable").

I just did a search on eBay for "snap around ferrite" and got 230 hits.

Remember that you can significantly increase the effectiveness by looping the cable back around through the ferrite bead.

[Greg Miller]

Originally Posted by Luke Arndt

If I'm understanding correctly, the reason the SM11 is "special" is because it's a balanced dynamic mic as opposed to an electret?

That's correct. A dynamic element is just a coil of wire, so it is balanced by default (although you can ground one terminal if you really want unbalanced). An electret element, by its very nature, is unbalanced. Most small electret mics also include a FET impedance converter, which is also unbalanced. They then feed the unbalanced signal down an unbalanced cable to the "power pack" which usually converts to balanced at that point. But there is a lot of unbalanced circuitry in an electret mic.

Originally Posted by Luke Arndt

I was sifting through this thread, and I couldn't see any mention of any other mics like the SM11, but I was wondering if such a thing exists.

I had never given it any thought until I read about your problems. At that point I said to myself, "what Luke needs is a balanced lav, which would have to be dynamic. A few minutes on Google found the SM11, but I've looked since then and have not found any other dynamics that are comparable size.

Originally Posted by Luke Arndt

The SM11 looks like it will suffice; I'm just wondering what other options there are before I buy anything else.

It sounds as if you've already researched that and have found the answer: essentially none.

Originally Posted by Luke Arndt

Is there any specific type of ferrite bead I should be looking for? Richard mentioned a ferrite torroid, but was saying I would need a large one that would allow the microphone's head to pass through it.

The previous suggestions were good ones. If the RF is traveling down the shield of the [thin] cable and getting into the "power pack" then ferrite might help you. I suspect, though, that that's not the modality of the problem. I suspect that the [thin] cable is acting like an antenna, so there is RF voltage at the capsule end, relative to the power pack ground. Since the capsule is unbalanced, that RF voltage then becomes superimposed on the audio output from the capsule, travels down the center conductor of the [thin] cable, and enters the "power pack" on the hot lead. If that's the case, ferrite will not do much good.

Apparently you're not happy with the sound of the SM11, or you wouldn't be looking at $500 E-V microphones. Have you tried my previous suggestions about equalizing the SM11? I think you should try that before doing (or buying!) anything else. Let us know exactly what you do, and how it sounds.

[Luke Arndt]

Okey dokey. More news.

I did get my hands on a few snap-on ferrite beads from different cables. I tried putting three of them on a mic, wrapping the thin mic cable around each one a few times, but unfortunately that didn't make any difference.

Two videographers (Pete and another fellow I know) have told me they use Audio-Technica's AT899 mics (http://www.bhphotovideo.com/c/product/266176-REG/Audio_Technica_AT899_AT899_Condenser_Lavalier.html) for depos with no problems, so those look like real possibilities.

What is interesting, though, is that Audio-Technica has a feature they call UniGuard (Audio-Technica - Microphones, headphones, wireless microphone systems, noise-cancelling headphones & more : Audio-Technica’s UniGuard® technology) that seems to do just what I am asking. However, the ES943C (Audio-Technica - Microphones, headphones, wireless microphone systems, noise-cancelling headphones & more : ES943C Cardioid Condenser Lavalier Microphone) is the only lavalier mic of theirs that I could find which has UniGuard... leading me to believe that the AT899s do not have it. As an aside, I can't even find any site that sells that ES943C.

Now, naturally if the AT899s are impervious to the cell phone buzz, I don't care what they call the technology. I'm just curious if somebody who knows more about this stuff than I do can read the specs of the AT899s and understand how they could be possibly built/shielded differently to keep out the interference.

By the way, the Shure SM11s aren't BAD. I'm just looking at my different options.

[Greg Miller]

Originally Posted by Luke Arndt

By the way, the Shure SM11s aren't BAD. I'm just looking at my different options.

Did you try equalizing them as suggested, to restore some of the missing bottom end?