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

This is actually a follow-up to a post of mine from 2008:
http://www.dvinfo.net/forum/all-thin...buildings.html

The cause of the problem was, as Ty Ford suggested, the ultra-sonic motion detectors used in some rooms. Moreover, the problem was most bothersome when a small room is equipped with a detector made for a large room, which emits a much more powerful signal. The problem is worst when the detector is right over the participants.

(To sum it up for those who won't read the previous post: my microphones were picking up this signal and this made the VU meter on my Shure mixer swing wildly and hover in the upper ranges, causing the limiter to kick in intermittently, playing havoc with my recording quality.)

Once I confirmed this, I discovered that my hand-held AKG C1000 did not pick up the signal at all, even when pointed directly at the device. Obviously a microphone has to be sensitive to the frequency in order for it to be picked up and passed onto the mixer. This gives me hope that perhaps other brands of lavaliers might not be affected by this particular high frequency, yet still be as good as or at least comparable to the Sony 44b's and 55b's I have been using.

So I am seeking a recommendation, but only if you have some specific experience with this problem and know that a particular lavaliere won't be affected by the signal AND that it's a good microphone.

Anyone? Thanks.

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

Do you have any idea, or is there any way to find out, what range of frequency(ies) are used by the detectors?

I would think a good universal solution would be to build a high-cut filter which can be inserted in line between the mixer and the mic. This would be similar to the low-cut filters for reducing room rumble and wind noise, except that it would cut the highs rather than the lows. Ideally, the filter would hopefully pass everything up to around 15 kHz, and then drastically cut everything above that.

Of course to design a filter, one needs to know where the offending frequencies are located. If they are very close to the audio band (for example, if they're around 25 kHz) then designing a sharp enough filter would not be trivial. OTOH, if they're at least an octave above the audio band (say around 45 kHz or higher) then the filter design becomes simpler.

Indeed, there might be some mics out there that are not capable of picking up these detectors, but I imagine this is a rather unusual question, so most mic specs might not predict this with any certainty or accuracy. It's certainly nothing I've run into, so I can't give any advice about specific mics. But I suspect lavs, which are typically small diaphragm condensers, might very likely have some response up above 20 kHz; clunky old large-diaphragm dynamics are probably much safer in general.

Sorry I can't advise re: specific mics, but I would be interested to know what frequencies we're talking about.

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

As a matter of fact the frequency is exactly 25kHz.

So, add to that the fact that building any kind of filter would be difficult for me, perhaps that's not such an easy solution. I have looked for just such a solution, and the internet is full of schematics, designs and forums dedicated to building such things. I can't understand more than three words in ten. If there were such a design available that listed the parts in normal English, or at least by names I could look up and order, plus a visual diagram of how all the parts are connected together (not schematics), then maybe. But I see nothing of the kind.

However, I have discovered this little item:

http://www.bhphotovideo.com/c/product/275823-REG/PSC_FPSC0010E_ALPF_Low_Pass_Filter.html

Which is the one and only low-pass filter commercially available as an in-line XLR filter, which is exactly what I need. However it does not filter from, say, 20 kHz and above, but at 7 kHz. It says the cutoff is -3dB. Is that drastic enough? It sounds like that ought to do the trick, especially since it was designed to filter high frequency noise. And my research tells me that most human voices fall within the 1kHz to 5kHz range, so there should not be any effect on the voices of my subjects. In theory.

It might help you to know that the full audio range is not important, just the full human conversational voice range.

I am on the verge of buying one of these and testing it, but if it fails I'd like to know if there's a simple answer in getting a different mic that doesn't pick the sound up in the first place. Hence the purpose of this thread. But if you or anyone has comments on the filter idea, please do.

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

For speech intelligibility, you only need frequencies up to about 3500 Hz. However, the human voice certainly does contain frequencies above that, and above 7000 Hz as well.

If you apply a sharp cutoff at 7000 Hz, and compare the audio to unfiltered speech, you certainly will hear a difference.

On the other hand, if you start a [i]gradual rolloff[/u] at 7000 Hz, the difference might be nearly imperceptable.

So we need to know the actual corner frequency, and slope (dB / octave) of this filter. I wonder where you found the 7000 Hz figure that you quote. I saw absolutely no specs on the B&H website, and a quick internet search didn't find any specs.

By all means, if you find a link to any specs or to the manufacturer, post them, and let's give it some more thought. And you could also call B&H and ask them if they'd let you return it if it doesn't solve your specific problem.

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

Ken,

Thanks for the nod. :)

Are the 44b and 55b two or three wire mics? I'm wondering, if they are two wire mics, maybe the addition of the 3rd wire would allow common mode rejection to cancel out the 25kHz, or shield it from the audio. Perhaps it's also a function of the transmitter input topology. What transmitters are you using? If the transmitter has an unbalanced input, it might be allowing the 25 kHz in. I guess if the diaphragm is picking up the 25 kHz as audio, that may not be a solution.

The only 3 wire lavs I know are the Sanken COS-11 and Sennheiser MKE-2. Might give them a try. Maybe rent a couple and see.

Or find someone who can build you some inline Low Pass filters that would cut off at 15 kHz-16kHz before the transmitter. I used to have problem with LF on an Audio Technica U100 wireless. They were so wide open, that with Countryman B6 mics, the LF from HVAC systems and other large industrial electromechanical devices would generate enough LF to make the compander wiggle. A mic with a more restricted LF response scraped off the LF before it got to the transmitter. You may be having the same problem on the other end of the frequency spectrum.

Regards,

Ty Ford

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

Based on Ken's original description, I'm guessing that the mics are picking up the acoustic signal (just slightly above the range of average human hearing) and then converting that to an electrical signal... that's what mics do.

If that is the case, then balanced or unbalanced won't make any difference. And in fact the lavs that he has been using (Sony ECM-44B and ECM-55B) are already balanced. This just reinforces my belief that the problem has an acoustic origin, and not a problem with unwanted electrical pickup.

And I don't believe he's using any transmitters; he specified "wired lavs" in his question, so it's not a question of transmitter electronics.

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

Thanks for the all the ideas!

The website I got the 7kHz from is this:
Adapter Barrels

And I will be writing an email to them right after this. But all they say in the text is:

"Low Pass Filter: Provides high frequency cutoff, -3dB @ 7 KHZ, to reduce high frequency noise."

Nothing about roll-off, corners, etc. Not explicitly. Still, I think I will give it a try, as it does seem to be designed to do what I want, albeit at audible frequencies. Oh, by the way, its a special order, so no returns.

That is correct: the 44b and 55b are wired mics, no transmitters involved.

Ah! That would be excellent! But I have no idea where to go to find someone who can build something like that. And it sounds expensive. Still, if you know of anyone or anywhere, I'd look into it.

Maybe this is what I'm looking for:

Youspice: 20 KHz Second Order Low Pass Filter

Again, terms like "second order" mean nothing to me, but it's one of many schematics I found in my search.

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

So the turnover frequency of the FPSC0010E is specified as 7 KHz. If we assume a simple 3dB per octave from a passive single-pole filter, that yields around -10dB at 25 KHz. Quite possibly not enough attenuation @ 25KHz to eliminate your stated symptoms. That Youspice circuit has a much higher turnover frequency and an equivalent slope, so you can see that it is only ~5dB down at 25KHz. The PSC adapter barrels appear to be made for line-level use and may not operate as expected at mic levels (or impedances).

Assuming you can't turn off the detector, I would try using a different room (without a detector) or a different microphone (many are not nearly that sensitive at such a high out-of-band frequency). Or you could use a light stand to hold something over the emitter (like a big foam hemisphere or a thick hat, etc.) IMHO, using a low-pass filter is a fiddly and dubious way to solve this problem.

Note that this situation is possibly an environmental health hazard as this high-level noise is impacting your ears even if you don't perceive the sound. That can't be good for your ears.

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

Richard,

I like the hat idea!

Get a bumpy foam mattress sheet. G-tape it into a cylinder and g-tape it to the emitter.

Regards,

Ty Ford

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

Hmmm... well, I'm not sure what -10dB actually means to the signal I'm getting from the mics, but it sounds like a heck of a lot. And even a little would help alleviate the real problem of the limiter kicking in because it is detecting a screaming match in the room.

I have emailed the maker with my concerns. But in the meantime, I think it is safe to assume that this device performs some useful function. So if it doesn't reduce high-pitched noise significantly without causing too much change in the quality of a person's speaking voice, then what does it do?

If, however, the filter is designed for line-level and not mic level, perhaps that would be a problem. But what are you reading that suggests this is the case?

As for working in a different room or rigging up a cone of silence in the middle of the ceiling - the most immediate and obvious effect would be to turn the lights off - neither of these are real options to me. So far the limiter seems the best bet. Still accepting suggestions for different lavaliere mics, though.

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

Okay, got a reply to my inquiry from Mr. Ron Meyer at the PSC company (at least I'm assuming he's a Mr.; odds are he is, but I might be wrong), who says:

"I think our barrel would work great for your application. It rolls of at 6dB per octave with a -3db point at 7 Khz. At 14 Khz the signal will be down approx 9dB and at 25Khz it will be down approx 14dB. There is very little audio energy from the human voice at 7Khz......almost none. Keep in mind the telephone system runs out at about 3.5Khz."

So, It's off to the internet with me and my credit card, assuming we've passed the sabbath. I must say that was very nice of him or her to reply on a weekend.

I'll let you know how it works out. That might take a while. It may be weeks before I get back into such a room again. But I'll be ready.

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

Filter performance is affected by the source impedance (the mic) and by the load impedance (the mixer/recorder input). So unless all these impedances are specified, it's really hard to predict how a given passive filter will work.

(By the way, a single pole filter has a slope of 6dB per octave, 20dB per decade.)

Still, $36 is trivial, so if you have the time and inclination, it wouldn't hurt to try a test.

To be really scientific, I'd tape the mic to a desk stand (so the position remains fixed), turn on the recorder and observe levels (from the ultrasonic system), then put the filter in line and observe levels again.

Of course another approach would be to have the building electrician disconnect the ultrasonic system and bypass it so the lights stay on.

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

Whether or not the filter works will be immediately apparent, believe me.

It also seems apparent that many people don't understand what an ultrasonic motion detector is, how they are installed and why. These things are designed to be on all the time to turn the lights out and save electricity. If they could be turned off, tenants would leave them off and they wouldn't do any good. And no, they aren't a hearing hazard.

I can't call the building maintenance department and demand that they disable the ultrasonic motion detector. I can't attack it with a screwdriver or redesign it so it can be turned off. Nor can I move the meeting to another room, which, in all likelihood, would also have a motion detector anyway. And try to remember that motion detectors detect motion in order to decide whether or not to keep the lights on. Blocking the motion detector is not an option.

Can't turn off, disable or block the motion detector; it's here to stay. Can't move the room. The solution has to be something I can do to my equipment, is least expensive and most permanent. That means either A) New mics, B) New mixer, or C) Low-Pass filters.

While I am still looking for A), a lavaliere microphone of good quality that will not be affected by the signal, C) became my best option after I finally found such a filter. At $38, it is by far the cheapest solution, and I could get by with two sets of three, and could use them on whatever mic or mixer I used, now or in the future. Call it fiddly if you like, it is actually a precision solution that hits the trifecta: inexpensive, little added time and goes wherever my kit goes.

Knowing the limits of the problem is the key to finding effective solutions.

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

Did you ever identify exactly which model lav microphone you are using? I tried to scan back through all the dozens of messages and didn't see it. I continue to be rather surprised that your microphone is so sensitive at 25KHz. I don't think I have ever used any lav mic that was that sensitive to ultra-sonic frequencies. How do people use ANY microphones in those buildings? Telephones, cell phones, etc.

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

I'm not sure what has prompted this heated-sounding response.

I certainly do know what they are. I know they are used to control lighting, HVAC, and security systems. I know that there is not a simple switch where a tenant can turn them off. I didn't suggest that, nor did I suggest your "attacking" or redesigning them. I did suggest having a building electrician temporarily disable them.

I used to work in a large conference center with a lot of kinky electronics. If someone was coming in to shoot video, and asked for our cooperation, we gave it to them. If the air handler was too noisy, we'd temporarily turn it off. If they got flicker from fluorescents, we'd turn them off. If they needed special power or a feed of some sort, we'd take care of their request.

Certainly if you're just renting a hotel room for the normal day rate, it might be unreasonable to expect help from the building engineers. But if there's a room where you frequently shoot video, and the ultrasonics are causing problems, I would hope (at least from my past experience) that a reasonable request might get results from the house staff. There is no "tenant operable" switch, of course, but it would be relatively trivial for a building engineer/electrician to temporarily disable/bypass the system.

The key is the mic's inherent frequency response. Dynamic mics have much higher mass, given the moving coil. That will limit the high frequency response. It's a real challenge to get a dynamic with good, flat HF response. A dynamic mic will be bigger and heavier than a newer electret mic.

The electret has extremely low mass (only a thin metallized diaphragm) so the response is going to extend out a lot further, and roll off a lot more gradually, at the high frequency end. Now if the manufacturer anticipated problems with ultrasonics, they might have designed the mic's integral electronics to have a steep rolloff at the HF end of the spectrum. But if they did not intentionally include sugh a filter in the mic's electronics, then an electret will likely be more problematic in your situation.

The filters you found are not quite ideal, from a theoretical perspective (see comments in my previous posts) but let's hope that they solve the problem for you! I will be interested to hear your findings, especially if you perform a semi-controlled test as I described earlier.

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

Richard,

He stated in his first post that he's using Sony ECM-44b's and ECM-55b's.

Telephones and cellphones are sharply band limited at around 3500 Hz at the top end, and typically around 300 Hz at the bottom end.

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

Not heatin'. Just sayin'. Your experience notwithstanding, turning off the motion detector is just not gonna happen. The building maintenance department is not at my disposal.

I was also bemused, not heated, at suggestions that I rig up a cone of silence over the motion detector. Can you imagine the partner walking in and seeing something taped to the acoustical tiles or propped up with a light stand in the middle of the conference table? And the lights not coming on for some reason? I think she would look at me dubiously and fiddly. I also wouldn't have to worry about any problems at that law firm anymore.

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

Come to think of it, a loud ultra-sonic noise in the room would be an interesting anti-bugging counter-measure. Sort of an electronic "cone of silence".

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

Ken,

Not to worry. Sorry to hear that "customer service" is not on the priority list of the building engineering staff. The "ultrasonic blocking device" was an interesting theory, but it would, indeed, take some finesse of design and construction to avoid it looking tacky. And the resulting blackout would have been less than ideal.

Hey, keep us posted about the XLR filters! I'm very curious about the actual attenuation.Yes, they will slightly roll off the very top end of the voice, but in fact you could restore that with some EQ in post.


Richard,

High-level ultrasonics would be a good anti-bugging measure ONLY if you know the bugs use electret mics with extended high frequency response. Obviously you could still bug the room with a cellphone or an SM57.

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

It is very difficult to find a small microphone these days that is NOT electret (or magnetostrictive, i.e. "piezo" or "ceramic"). And both of these types are good at high-frequency (ultrasonic, even) pickup. Indeed the devices themselves probably use that kind of microphone.

OTOH, I have used microphones in places with motion detectors without any of these symptoms. But I suspect there are more PIR (passive infra-red) sensors than active ultrasonic ones.

I'm trying to picture a bugging scenario where you could plant an SM57 without it being noticed as out of place. :-)

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

I'll bet there aren't many piezo mics around any more. The response tends to favor the HF end, and at least in the old days they were rather fragile.

I'll bet the motion sensor does use a piezo transducer, at least as the "loudspeaker" element. Motorola started making piezo tweeters about 30 years ago, and I think that technology proliferated at least for a while.

An SM57 bug... hmm, yes, that does challenge the imagination, doesn't it!

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

Well remember that a cylon planted an interstellar sub-space tracking device using double-sided sticky tape right on the base of the tactical display station of the CIC on the Battlestar Galactica, big as a jumbo First Alert smoke detector, and nobody even noticed!

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

The specification of the barrel adapter lists 3db at 7khz. That is the breakpoint in the response. The response rolls off at 6db per octave. Have not put a calculator to it but it should be down approx. 12 db @ 25khz, which should be sufficient.

A word on lavalier microphones: I have used the Audio Technica ATR3350 wired lavaliers for years. The frequency response is 50 to 18000 HZ. It is powered by a 1 1/5 volt hearing aid battery, that lasts for hundreds of hours of use. They used to be sold at Radio Shack as well as AT dealers. The best part is that they sell for $35-. I have never picked up any interference with them.

Douglass Spotted Eagle also uses them. I have done a stealth recording of a symphony orchestra with a pair of them; those who have heard it think it was professionally recorded. My point, don't waste your money on lavalieres costing several hundred dollars unlewss you have nothing better to do with your money!

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

Yes, that was stated on July 15. That's what it says. What I want to know is: how does it perform in reality, in this situation?

As I stated in one of my earlier posts, the behavior of any passive filter is very much dependent on the source impedance (the mic's actual impedance as a Thevenin generator) and the load impedance presented by the input circuit of the mixer/recorder.

For example, older dynamics and ribbons frequently had an actual source impedance around 150 - 200 ohms; present condensers with active electronics frequently have a much lower source impedance. Older boards typically had inputs that matched the mics, in other words were around 150 - 200 ohms. Newer boards frequently have "bridging" inputs that can be 1,000 ohms or more. All these variables will affect how the filter actually performs.

I don't recall seeing any specs for the source impedance, or the load impedance, for which that particular filter is designed and for which those specs are quoted. So when Ken gets one, I will be quite interested to see how it actually performs.

And now that I think of it, it's been ten days since Ken threatened to order one for evaluation. Ken... are you still out there, and what have you discovered?

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

I ordered it, but its a special order so it will take some time to get to me. Then I have to wait until I am in that room again or another equipped with the same kind of motion detector. Could be a while. I don't see it on the calendar for months.

But I'll let you know.

(By the way, I posted what a rep from the maker told me which is that it would be -14db at 25Mhz.)

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

Ken,

Thanks for the update. I wasn't aware it would be so long before you had a chance to repeat the situation.

Yes, I saw your posting of the predicted attenuation at 25 kHz. In theory it would be -9dB @ 14 kHz (one octave above the knee frequency), and -15 dB @ 28 kHz (two octaves above the knee), so -13 or -14 seems about right at 25 kHz... in theory. So it will be interesting to compare the actual performance with that prediction.

Keep us posted, and meanwhile happy shooting!

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

I wouldn't dismiss the "cone of silence" foam attenuator placed on the device itself. Reducing any distracting sound at the source is always preferable, and high frequencies can be blocked without a heck of a lot of material. Even just a few layers of white gaffer tape might make a significant difference.

I'm not convinced that covering the sensor with foam or similar would turn off the lights; it might be overpowered enough that enough signal gets through your foam for it to work, or it could sense the reflections from the foam as occupants.

As far as filtering, your best bet would be a notch filter centered on the particular frequency the sensor emits (25k), but if a 7k rolloff filter solves your problem then that's great.

Re: Filtering Ultrasonic Motion Detectors
 

UPDATE:

I ended up purchasing two of the barrel filters and finally had an opportunity to test them out. The results: Not enough filtering.

I set up my kit and placed two microphones on the table - without the filters - and observed the VU meter, which was swinging wildly to the end of the red. I then connected the filter between the microphone barrels and the XLR cable, being careful to not move the microphones themselves. I saw an almost imperceptible decrease in VU activity. I then turned one mic down to zero and then placed both filters on the one microphone path. That showed a noticeable drop in the signal on the VU meter, but it was not enough to help with my problem.

By the way, I did not detect one bit of effect on the quality of people's voices from these filters. So that issue we can put to bed.

I wrote to the manufacturer, and he suggested that I need a 3 pole active filter that could give me 18db roll off per octave, which they do not make. Nor does anyone else, it seems.

So now I am once again in search of someone who can make these kinds of things. This device is always just out of my grasp! I have found plenty of designs and white papers on this topic, but can understand none of it. It is obviously not an exotic technology. But the demand for such an audio filter is so low that no one makes them commercially, and no one ever thought of reducing a design to plain English for a hobbyist to make use of. I know it can be done, I just need someone to do it.

If anybody has any suggestions on where I can find such a person or company, I'd be very grateful.

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

Ken,

Thanks for the update. For the custom filters, let's see.....maybe Scott Dorsey. He hangs out at rec.audio.pro.

Regards,

Ty Ford

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

Hi Ken,

First of all, thanks for the update.

Sorry to hear the bad news, but I'm not terribly surprised. That's why I kept asking about the actual performance as opposed to the published specs. Passive filters were fine back in the day of known, fixed impedances (line level circuits usually 600 ohms, mic level circuits usually 150 or 200 ohm). God knows I built enough of them. But they are a real guessing game with circuit impedances all over the place as they are today.

I would still like some numerical data on your filters, using your mic and your mixer. Here's how I would go about making this measurement.

1.) Place the mic in a fixed location, connect to mixer.
2.) Power on mixer, slowly increase mic gain until ultrasonic signal bring meter up to 0 VU.
3.) Power down mixer, without changing gain settings. Insert one filter in mic line.
4.) Power up mixer, and read new level on meter.
5.) Power down mixer again, without changing gain settings. Insert second filter in line with first.
6.) Power up mixer, and read final level on meter.

That will, for a start, tell us how these filters actually perform.

As another approach, I suggest you contact the filter manufacturer and ask for a schematic of the filter. That will give us some more info as a starting point, if we perhaps want to modify these filters that you've already bought.

As far as active third order filters, that's something that I could design and build. But my gosh, when you factor in the time to design the circuit, lay out the PC board, the price of etching the board, plus the chassis and connectors and all the components... and then the assembly time, you're getting up into the stratosphere.

We could also pursue other mics, although I think the selection will be quite limited. Please remind me again exactly what you are recording (depositions, audio books, etc.?) and whether the size and appearance of the mic is critical.

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

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.

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

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.

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

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 .

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

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 .

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

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.

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.

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

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.

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

"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!!!