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[SOLVED] RF Emission from Piezoelectric Buzzer

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EricaS

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Hello

Is it possible for a pizeobuzzer ringer to emit audio as RF?

Here are some pics of the buzzer. (Black object).

IMG_20150331_154704.jpg

IMG_20150331_155654.jpg

Someone once had this to say:
"I have no direct experience but a change in the position of the diaphragm will certainly change the impedance of the coil which will affect both its Q and resonant frequency. A nearby transmitter may well have a detectable audio modulation of its frequency or power. That is after all how a metal detector works!"
 
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Vbase

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The buzzer creates sound by vibrating and moving the air, these waves hit nearby buzzer that converts the vibrations to electricity.
You can do it with low frequency RF piezo crystals.
 

EricaS

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That's not my question.
My question was whether the audio detected by the buzzer can be emitted as RF?
 

Vbase

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Thats true, I didn't answer your question because I suspected that you don't know what you are talking about, now I know that you don't know what you are talking about!
 

EricaS

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Let me elaborate further.
My telephone has a piezoelectric buzzer as the ringer.

I think someone is using the buzzer to transmit room audio as RF to eavesdrop on me.
 

Vbase

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It is not likely the the buzzer is used to pick up the sound in your room, it is easier to do it with the microphone in the handset. It is possible that someone planted a bug in your phone or anywhere in your room, that bug uses RF to transmit to where he is.
There are other ways he can eavesdrop on you, by putting his ear on the wall or by taping into your phone line. If he gains access to your phone line then he can listen to all your calls without changing your phone.
If he is clever then you wont be able to detect his listening device by using an oscilloscope.
 

EricaS

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Is this correct:

"I have no direct experience but a change in the position of the diaphragm will certainly change the impedance of the coil which will affect both its Q and resonant frequency. A nearby transmitter may well have a detectable audio modulation of its frequency or power. That is after all how a metal detector works!"
 

Vbase

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Metal detectors work by creating magnetic field, when a metal enters that field it effects the way the field is spreads. Metal detectors detects metals by sensing the modification of the magnetic field. Metal detectors don't use RF.
Piezo doesn't have inductance. Piezo changes shape when you pass current through it, if the current is RF then it vibrates at RF.
Piezo also works the other way around, when you bend it or put mechanical force on it it generates electricity, like in lighters. If sound hits it it generates electricity that follows the sound waves.
A piezo cannot transmit RF or detect metals.
The piezo in your phone is designed for audible frequency and it has no relation to RF.

- - - Updated - - -

Is this correct:

"I have no direct experience but a change in the position of the diaphragm will certainly change the impedance of the coil which will affect both its Q and resonant frequency. A nearby transmitter may well have a detectable audio modulation of its frequency or power. That is after all how a metal detector works!"

When I look at this again I think it must be an answer to a question if metal detector can pick up RF from mobile phone or another transmitter. In this case the answer is yes, metal detector can pick up RF if it's near. It is possible that someone can hear your mobile phone on his metal detector but not well enough to understand what is said.
If you change your phone to different network it is likely to stop.
 
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betwixt

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Forget it EricaS, for three reasons:

1. Piezo elements make very poor microphones because when mounted they only work over a very narrow audio range, that of the tones used for ringing.
2. They are connected inside the telephone in such a way that any voltage they produce never reaches the phone line wires.
3. The only frequency they produce, except possibly for distortion, is that of the sound waves hitting them.

The "someone" you quote has no idea how metal detectors work, their statement is nonsense. Apart from not being technically possible, why would a transmitter detect anything? It's a source of signal not a receiver of one.

There was once a case of someone caught eavesdropping by using a system called secondary modulation but it is technically quite difficult to do and I suspect you would notice the two large dishes pointing directly at your house :shock:

Brian.
 

Audioguru

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Somebody can shine an invisible infrared beam onto your window or through it onto the glass on a photo on your wall that is vibrated by your voice and the glass reflects the modulated infrared to a receiver.
 

EricaS

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Forget it EricaS, for three reasons:

1. Piezo elements make very poor microphones because when mounted they only work over a very narrow audio range, that of the tones used for ringing.

Attached below is a recording i made using the ringer as a microphone


I think its a pretty good microphone.
 

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  • Piezo Output.zip
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EricaS

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Somebody can shine an invisible infrared beam onto your window or through it onto the glass on a photo on your wall that is vibrated by your voice and the glass reflects the modulated infrared to a receiver.

I'm 99.9% confident that this phone is being used to eavesdrop. Transmitting room conversations.
 

EricaS

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Some background:

I've already been threaten by the people behind this. They suspect I know something but I can't yet prove audio is transmitted from the phone.

I've check the telephone using a CPM700 (one of the most respected TSCM devices) for RF (AM and FM), and carrier current signals. Nothing.

I've even checked the exit line with an oscilloscope nothing.

I've inspected about 70 of these phones (at the office and a department store) all the clean phones have the negative lead (black wire) of the ringer connected to the positive terminal on the PCB and the positive lead (red wire) connected to the negative terminal of the PCB. The phones that I suspect are bugged have the polarity correct, I.e. red wire to positive terminal and black wire to negative terminal.

I believe its no coincidence that the bugged phones have different polarity connection than the clean phones. But some have said this means nothing, is that true?

Have any heard of the Great Seal Bug?
Maybe this piezo is stimulated to transmit some kind of RF?
 

BradtheRad

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Your audio clip is intelligible speech.

The Great Seal Bug was embedded in a wall decoration which the Russians gifted to an American embassy. The bug did not transmit anything. It was a tuned-cavity resonator which reflected rf beamed at it from a nearby building (where a transmitter was operated by Russian agents).

If you measure the size of your piezo beeper, you'll get an idea what wavelength could resonate inside the piezo beeper. It will be a very short wavelength, which means a very high frequency. Can you bring in an rf meter tuned to that wavelength (or 1/2 or 1/4)?

I suppose the cavity in the beeper must be lined partially with metal, in order to reflect rf back to the transmitter. Can you examine the beeper, to look inside for a metal lining?
 

chuckey

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" impedance of the coil " your quote, piezeo transducers have no coil. They are really poor microphones. They need a transmitter to generate RF because they can not do it them selves.
You might be bugged, if you think its the telephone, give the eavesdroppers a test message then you can see if there is any response. Repeat the experiment with the telephone unplugged from the line and any internal batteries disabled. Or just buy a new telephone, in the UK they are £10.
Frank
 

EricaS

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" They are really poor microphones.
Frank

Have you not listened to the recording I made using the buzzer? See my earlier post.

- - - Updated - - -

Your audio clip is intelligible speech.

Its near perfect!

If you measure the size of your piezo beeper, you'll get an idea what wavelength could resonate inside the piezo beeper. It will be a very short wavelength, which means a very high frequency.

Are you refering to the diameter or height of the buzzer? Any idea how high? Perhaps > 1 GHz but < 2 GHz? Will this be FM or AM modulated?

Can you bring in an rf meter tuned to that wavelength (or 1/2 or 1/4)?

I've tried using an RF frequency counter, trouble is I think the ringer emitts very low power RF and the detectors are not so sensitive to distinguish between background RF and the ringer. The only way is to check each frequency with either a spectrum analyser or radio scanner. But is it AM or FM or something else?

I don't know what u mean by 1/4 and 1/2.

I suppose the cavity in the beeper must be lined partially with metal, in order to reflect rf back to the transmitter.

What or where is the transmitter? I cannot see any transmitter in the phone. Are u referring to the receiver of the RF?

Can you examine the beeper, to look inside for a metal lining?

This is what it looks like inside

https://obrazki.elektroda.pl/9989391600_1431070817.png
 
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betwixt

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ALL that is in those piezo buzzers is a brass disc (the sounding board), a thin layer of ceramic and on that a thin layer of metal flashing. When a voltage is applied across the ceramic sandwich , it bends and as a consequence pushes the air around it, producing sound waves. They make terrible micrphones, the plastic cavity around them and diameter of the outlet hole is designed to make them resonant at ringer frequency. If you remove the plastic casing they WILL work as a crude microphone but the ringer volume would be almost non-existent. They can be wired either way around so the wire polarity is unimportant, they have to be driven by an alternating voltage to produce repeated vibrations (the tones) they produce no sound if DC voltage of either polarity is applied across them.

In a conventional telephone there is a circuit to make the ringer unresponsive to small signals on the line, basically a blocker that stops anything less than about 4V AC reaching the ringer circuit and it works both ways around, preventing any audio pick-up getting back on to the line. So rule out the line itself being monitored. In any case there is an oscillator circuit between the ringer voltage and the piezo to convert the low frequency ringing voltage into to 1KHz or so needed to drive the piezo resonance.

Having worked extensively in the telecoms industry, I can confirm that SOME telephones can be monitored remotely without any alert at the phone itself but it can only be done on 'office' phones connected to digital switchboards and only if special commands are sent down the wires. The principle will never work on conventional phones. If you have a normal domestic style phone, buy another one and carry it with you when you leave the building.

The "secondary modulation" technique I referred to is the same as the IR beam and "Great Seal" methods mentioned. It was also used to excite resonant ash trays around a conference table I believe. Unless you have some serious issues with secret agents I would rule that system out. It requires complicated equipment and a considerable amount of setting up, it also needs a narrow beam transmitter and a narrow beam receiver spaced some distance apart but in visible range. Spies used it by aiming signals through windows of tower blocks that were convieniently positioned to see the windows of the room they were observing but the geometry is critical and it wouldn't have been possible if the street layout was different.

If you really are being bugged, and a radio transmission is being used, a telephone is probably the worst place to hide a transmitter. They can be placed anywhere in the room and a favorite spot would be behind a mains outlet where there is a hole for the sound to pass through and a steady power source. You will not find one with a frequency counter, you need a wide-band RF field strength meter or better still a spectrum analyzer. Be prepared to spend money if you want decent ones!

Brian.
 

EricaS

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ALL They make terrible micrphones, the plastic cavity around them and diameter of the outlet hole is designed to make them resonant at ringer frequency. If you remove the plastic casing they WILL work as a crude microphone but the ringer volume would be almost non-existent.

I don't understand why you say "they make terrible microphones". Did you listen to the recording I made using the ringer?
 

betwixt

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Clearly, we have different opinions of what constitutes good quality. Was that recording made with the piezo still element still inside it's plastic shell and installed in the phone?

Regardless, it is not capable of radiating RF and it is not capable of sending sound down the phone line. The picture you show in post #1 has 3 ICs, the larger black one (IC3) is a standard DTMF tone generator to produce the dialling key tones, the one under the board is the audio controller and the small black one (IC2) is a ringing tone generator which you can see is wired to the piezo. This is a normal POTS analog phone like millions of others on the market.

Do you suspect you are being spied upon ONLY when using the phone or at other times as well? It is possible (if you have access to it) to tap into a phone line quite easily but it will only let you hear a conversation while the phone is off-hook.

Brian.
 

FvM

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Microcphone operation of the buzzer is surely possible, despite of different opinions about the quality. But I'm missing any idea how the phone circuit should generate RF.

Did you actually try to detect RF emissions with a frequency counter? That's hardly possible.
 

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