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

[Greg Miller]

Originally Posted by Richard Crowley

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. <snip> How do people use ANY microphones in those buildings? Telephones, cell phones, etc.

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.

[Ken Reeser]

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.

[Richard Crowley]

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".

[Greg Miller]

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.

[Richard Crowley]

Originally Posted by Greg Miller

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.

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. :-)

[Greg Miller]

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!

[Ken Reeser]

Originally Posted by Richard Crowley

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

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!

[Bill Wilson]

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!

[Greg Miller]

Originally Posted by Bill Wilson

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.

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?

[Ken Reeser]

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.)

[Greg Miller]

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!

[Tom Morrow]

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.

[Ken Reeser]

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.

[Ty Ford]

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

[Greg Miller]

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.