Reccomend a Wired Lav NOT a Sony That Doesn't Pick up Ultrasonic Motion Detectors?

[Richard Crowley]

I didn't see any reference to exactly which mixer is being used, but it is quite possible to have a steep low-pass filter installed in one or more channels. They could be made switchable. Filters like this are pretty easy to make INSIDE a device because you don't have to deal with unpredictable source and load impedances.

[Greg Miller]

Richard,

The OP said in his earlier thread that he was using a couple of Shure mixers, presumably that's still the case although that hasn't been confirmed.

I was thinking along those same lines: a LPF added inside the mixer.

Ideally the mixer would have mic input transformers, and you'd add the filter immediately after the transformer secondary. Once that high level HF noise gets into the first active audio stage, it could cause some bad IM distortion that would make the desired audio sound awful. Once that happens, if you filter out the HF at a later stage, the audio is still crap. If the mixer's mic input jacks go directly to "active balanced" inputs then all bets are off; the noise could saturate the active balanced stage and cause IM there.

So yes, it's a possibility, and might be what I'd do myself. But whether the OP can send out his mixer for mods, for some open-ended timeframe, remains to be seen. I doubt that many people would be comfortable with that.

Actually I think the simpler solution would be a different mic (preferably dynamic, which would produce little or no output at 25 kHz) or else a properly designed passive filter between mic and mixer.

When you stop to think about it, if the HF noise is sufficiently high level, it might even be causing IM distortion inside the mic itself, which would be entirely impossible to filter out.

EDIT / ADDENDUM:

After thinking more about what I said in the last paragraph above, I realize that if the SPL is sufficiently high, the ultrasonic noise could indeed be overloading the mic electronics. And we do not know the SPL. So tackling this electronically may not be as simple as originally envisioned.

One thing that would help: post several seconds of sample recorded with your mixer's limiter off... maybe this will give us a clue about what is actually happening.

The other approach is to abandon the Sony electrets, and try a relatively small dynamic clip-on mic. It will be more conspicuous than the Sonys, but you're not doing a broadway musical so you don't need to hide the mic in someone's hairpiece. Even using this mic will be a bit of an unknown at first, since its response is not specified at 25 kHz, but I would be willing to bet that it's a lot less sensitive than the Sonys at that frequency. If you test the new mic in a couple of the troublesome rooms, and the problem is gone, then you can rest easy.

[Derek Heeps]

In many cases such as this , the best solutions are often the simplest ones .

I would try some sort of acoustic filter rather than an electrical one : a sheet of thickish sound absorbent material placed between the subject and the emitter - say a moderately thick rug , piece of thick card , or suchlike suspended overhead between a couple of lighting stands , high enough to be out of shot ; maybe you can sit off to one side away from the emitter and place the rug between subject and emitter ? It does not take much material to effectively attenuate high range audio frequencies .

This way the mic can still pick up full range from the speaker , but the unwanted signal will be blocked .

If hanging a rug or similar is impractical , try wrapping the mic in felt , cotton wool , or experiment with other materials until you find something that attenuates the unwanted frequency without affecting voice pickup - obviously , in vision the appearance of such a device is a consideration .

Hope this helps .

[Derek Heeps]

I just read a little bit more of this thread , and see that it is a board room type scenario where too much paraphenalia might be inappropriate .

I wonder , then , if another type of microphone than a Lavalier might be more suitable ?

I appreciate that for video the mic being 'invisible' is a consideration , but I wonder if some other type might work better ?

For example , a boundary mic placed on the table , with some kind of shield placed above it to shield from the overhead ultra AF signal might work ? I have never tried this , but it is just an idea . Boundary mics are pretty omni directional and might pick up reflected ultrasonics no matter where it is placed , besides sensitive to room accoustics .

More likely , some kind of tabletop hypercardiod mic which will pick the speaker up clearly whilst keeping the unwanted signal well off axis might work ?

I am thinking of the likes of the AKG C747 ( there is an Audio Technica equivalent , similar to the AT 935 lectern mics I have a couple of , but I can't remember the type no ) , and no doubt others that might work equally well placed on a discrete table stand some two or three feet in front of the speaker .

Just a couple more avenues to explore .

If you are stuck with a Lavalier for some reason , you could look at the likes of the old AKG D109 , which is a true 'Lavalier' with the mic suspended round the neck in a 'cup' which could be packed with something to block ultrasonics . Rather than 'tie-clip' condensers such as the Sonys already mentioned with their much more extended ranges ( I also have the likes of Sony ECM 55B and Sennheisser MKE 2-2R which are very nice mics but probably unsuitable here ) .

As with all the above suggestions , you would have to take them into the empty room and experiment on your own to see what works best .

[Greg Miller]

Originally Posted by Derek Heeps

For example , a boundary mic placed on the table , with some kind of shield placed above it to shield from the overhead ultra AF signal might work

Most boundary mics I can think of are electrets, so they will likely have extended HF response, just like the problematic lav. And since the boundary mic will be much farther from the speaker's mouth, the S/N ratio will be much worse.

Of course an appropriate acoustic shield over the mic might help. For starters, I'd suggest a wool fedora. If that doesn't provide enough attenuation, you might try a trombone plunger mute. Either of these will provide an interesting atmosphere to the shoot.

Originally Posted by Derek Heeps

(I) see that it is a board room type scenario where too much paraphenalia might be inappropriate

That's precisely why I suggest trying a dynamic lav. The physical mass of the moving coil assembly will create a mechanical low-pass filter, which hopefully will reduce the problems with the ultrasonics. The D109 you suggeest might work, although it's pretty big. The Shure SM11 is a current mic, much smaller than the D109, and fairly wearable with a lapel clip. The published response curve doesn't extend to 25kHz, but I suspect sensitivity is pretty far down by that point.

[Ken Reeser]

Okay gang, thanks for all your input. I am about to start a new thread on the subject of this issue and specifically low-pass filters, since this is no longer about recommending a different microphone. It will be about how I can get a low-pass filter made based on a design I got. I'll add the link to that thread here once I get it posted.

Meanwhile, about the Shure SM11: It's range is listed as 50 to 15,000 Hz, almost the same as the ECM-44B with 40 Hz to 15 kHz. Keep in mind that the ultrasonic signal is not just there, is is at an ear-splitting screaming level. I suspect that the problem would occur with the Shure mic as well.

[Greg Miller]

Originally Posted by Ken Reeser

I suspect that the problem would occur with the Shure mic as well.

"Suspect"???

Do you mean to say that in the eight months since the last post, you haven't bothered to rent an SM11 and try it? Apparently this isn't very important to you.

Look at the response curves for those two mics.

At 15kHz, the Shure is already down -10dB. It has a relatively large mass moving coil assembly, and you're already well above the self-resonance (which appears to be around 9kHz). The mic's mass and compliance forms a mechanical low pass filter; the response will fall off rapidly above this point.

At 15kHz, the Sony is at reference level 0dB. It appears to be falling, but probably not nearly as abruptly as the Shure. The Sony is an electret, with a moving mass that is many times smaller than the Shure. The Sony will roll off gradually compared to the Shure.

And, as I mentioned in post #32 (above), if the ultrasonic level is loud enough, it might even be causing some overload and IM distortion in the FET that is inside the Sony mic itself. If that's happening, no filtering in the world is going to solve the problem.

Honestly, before you invest any time or money on filters, I strongly suggest you get just one SM11 and try it, so you will have some concrete data to start with. While you're at it, try any old dynamic mic you have lying around ... an SM58 or whatever ... to see whether that will pick up any of the offending noise. Then, once you actually know something definite, you can make an informed and intelligent decision about how best to proceed. I really believe in the scientific method!!!