Can a Piezo Buzzer be Used to Spy on People through Walls and Doors?

While working on the Piezo Buzzer to write the Open Project Journey, I wondered if people could use the Piezo Buzzer with good enough op-amp to hear through walls and doors.

Piezo Buzzers are used to produce “buzzing” noise through an applied voltage. Piezo Buzzers are also capable of producing a voltage across its terminals proportional to the vibrations it experiences. Voices are made by creating vibrations in the mediums around us. Sound vibrations travel best through solids and a lot less through vacuums. This would mean that if the vibrations from our spoken words hits the surfaces around us, then the Piezo Buzzer should be able to pick up those vibrations and we could decode them back into sounds.

Why not test this theory out? Let’s begin. This is going to be a written and video journey. Which means you will get to see through the whole process of how I designed the prototype circuit to test this theory. You will also get the chance to contribute and be part of the IoT Simplified grid!

Piezo Elements is a serious transducer that is very helpful! Check out this research related to Piezo Elements:

Concept and Evaluation of a New Piezoelectric Transducer for an Implantable Middle Ear Hearing Device

Theory

People’s voices are just vibrations in the atmosphere that travel through mediums. A Piezo Buzzer can detect people’s voices through thin walls and doors.

Experiment Begins

What kind of signal comes out of the Piezo Buzzer before any amplification? This is what we need to find out first.

We used a Piezo Element from Amazon, you can get your as well so you can follow with us and conduct your own testing!

As tested, the voltage across the load resistor (1M ohm) was very low. At loudest sound vibrations created, we were able to record the following oscilloscope output:

Oscilloscope output analysis of the Piezo Buzzer

As you can see from the figure above, the signal has a lot of noise covering what we are truly looking for. What we are looking for is a small voltage change of about 1 mV. This is going to be a very hard task to try and clean the signal from all the noises in the atmosphere. Just to be clear though, I am not talking about the noises we hear. I am talking about the noise the electricity makes. As you all know when something oscillates it creates noise in the atmosphere in the form of electromagnetic (EM) waves. These EM waves hit the surface of the Piezo element and convert to a voltage drop that shows on our O-scope reading.

To get the same great output waveforms as shown above, we used RIGOL from Amazon. This oscilloscope is both very cost effective and anyone can afford it, and VERY easy to learn and get used to. The amazing part about it too, is that you can actually hook it up to your PC using a USB cable and with the program included you can get LIVE feed on to your PC. I know I use that feature a lot and it is very helpful. Accuracy is pretty good, just enough for my work and testing.

Different examples of EM noise sources that are hitting the surface of the Piezo Element and is causing the harmonics at the output.

Two problems are introduced here and we need to solve them before we can prove the theory.

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Problem 1: Very low output voltage signal from the Piezo Buzzer

We can solve this problem using an operational amplifier (Op-Amp). An op-amp can be used to amplify the voltage that is produced by the Piezo Element. The op-amp used in this experiment is LM358 which is a General Purpose Op-Amp. The general approach to amplify using Op-Amp by using two resistors to control the GAIN. Which is the number to multiply the input voltage by. Using the following equation, we can use to calculate the values of the Resistors.

SIDE NOTE

The op-amp is in the non-inverting configuration.

Eq. 1 – Non-inverting Output Voltage Equation for a Op-Amp

In Eq. 1, we can see that the R2/R1 is what we can control to change the Gain. The output signal of the Piezo Element is 10 mV peak-to-peak, we can use a 100 V/V gain to get an output of about 1 mV peak.

SIDE NOTE

The output will cut the negative signal since the Op-Amp will saturate at anything less than 0 V. Since this is a single power source.

To get the 100 V/V gain, the following values for the R1 and R2 are chosen:

R1 = 1k Ohm (5%)
R2 = 100k Ohm (5%)

Do not forget to place an order for a Resistor Assortment Kit and the LM358 General Purpose Op-Amplifier, shown below.

Below is the circuit design of what we will be testing. V2 is representing the input voltage coming from the Piezo Buzzer. Here we are using a single power source of +12V.

Circuit Schematic for Testing the Output Voltage after Amplification of 100 V/V Gain

Here I am actually using a DC Single Power Supply from Lycow. Great and stable power supply and I have been using for a year now without any issues! It also has a smart cooling system!

Breadboard View of the Circuit Built for Testing

The breadboard you see up there is one of my favorite and they are not one of those really big ones that take up 50% of your workspace. I love how small it is and that is also has a double sided tape. I got my Breadboard from Amazon and it took them a day to get here!

Results from Solution to Problem 1

The output from the Op-Amp have shown a signal that is directly proportional to the noises made. The only problem is that it might be masked by the bigger noise signal. In which we will discover how we can try to eliminate as much as we possibly can.

SIDE NOTE

We will not be able to eliminate the noise completely, unless we use an Instrumentation Amplifier.

Comparing the output signal after amplification when no sound is made and with sound made

Problem 2: High amount of noise at the output signal from the Piezo Buzzer could be masking more interesting information

Before we try to solve this problem. Let us take a look at what frequency is being induced on the output signal.

Frequency of Ch 2 signal is shown to be 60 Hz.

The 60 Hz is hitting the Piezo Element and causing that signal to ride on that bigger wave. The reason behind this 60 Hz is most likely the noise coming in from the power supply. Our homes/offices are powered by a 120 Vrms and 60 Hz frequency. So that makes sense why it would show up on the Piezo Element. So how do we remove it and focus only on the other frequencies?

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Contribution and Learning Opportunity using Piezo Buzzers

Further research needed to find out how to minimize the noise from the environment.

This is your opportunity to show the world what you are capable of and your research and development skills. Enhance your skills with us by submitting your research findings and we will post an update under your name (your website). If you have a website and you will do your own research independently. We ask that you would tag us or add the link to our website so we can continue to do research and open it to the world to learn!

Findings Based on our Simple Experiment with the Piezo Buzzers

It is clear that it is possible to detect voices from objects around by placing the Piezo Buzzer on it to feel the vibrations. Noise makes vibrations, and therefore it should be detectable by the Piezo Buzzer. Thus far, the information and based on our very simple experiment we can see that it is possible to detect voices behind walls. There would need to be further improvement and research to support this statement. This is why we are asking all the students, hobbyists, and engineers to come together to prove our statement while learning along the way.

Further Improvements to the Experiment Needed from our Members

  • Improve the amplification circuit
    • It maybe better to pick an instrumentation amplifier
  • Choose a better more sensitive sensor (instead of the Piezo Buzzer shown in our experiment)
  • Remove unwanted noise from the circuit (because of how sensitive it is)
    • Maybe a good idea to use a Band filter circuit
  • Use a MCU to apply advanced analytics on data and process the data very fast
  • Use an output speaker to allow for the voices picked up the Piezo Element to be heard

Please learn more about how you can contribute to this experiment. Remember, we will take ALL experiments and tests no matter what they are and even if they fail. We can always help you!

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