Piezo as a microphone

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I have a project where I want to connect a piezo to a microcontroller to measure sound level to detect when water is flowing through a pipe. The idea is to do this without tapping into the piping while keeping the microphone immune to external noise.

Is there any suggestions as to what piezo buzzers I can use for this? Should the piezo disc be internal or external drive? What should I be looking for in a piezo to choose the best suitable one in terms of sensitivity?

I don't think there is any reason to use a piezo, a simple electret capsule will probably do fine depending on how you mount it.

Thanks for the reply. Trust me - where I want to use this project, there is very loud noise. It is at a fertilizer factory, the machinery are so loud that you need ear plugs to enter the site as the volume level of the noise is very harmful the ears. A Piezo is certainly the most suitable for the project. Like I said, it is immune to noise, but is excellent at picking up noise emitted from a solid object. An electret MIC is VERY sensitive to loud noise and thus unsuitable.

I did see this project, but I do not want to know how to do it, I am seeking advice on how to select a Piezo, something which is not just as simple as using "instructables.com".

Let me rephrase my first post.

Piezo discs are mostly used for buzzers, which emits sound. So a Piezo (when used as a buzzer) is selected based on Internal or External drive circuitry as well as the tone (determined by the frequency it emits). I would like to use the Piezo as a MIC and thus seek advice from people out there with more expertise than myself, that have done this before, to advise me on what kind of Piezo would be best suited for the application I would like to use it for. I do know how to build the project, I just need to know what specifications to look for in the Piezo to make it the most suitable contact MIC as selecting the right Piezo for the job will make the project more accurate and easier to program.

What kind of Piezo will be the best,
Internal or External drive?
I assume a larger diameter Piezo disc is better.
Will a Piezo with a higher or lower dB sound emission rating be better at picking up audio (remember it is now being used as a MIC, not a buzzer)?
electret capsules with >140 dB 3% distortion points are available

lots of different flow sensors too - but developing from scratch is often way more difficult than some might think

it is often worth the price of a proven industrial sensor from a respected and experienced manufacturer

even a few $k is tithe of the value of the time to really develop, characterize your own home rolled solution
Piezos are also used as tweeters, the very common Motorola piezo now made by CTS are real cheap, and I have taken them apart to steal the piezo disc in the past. Piezos are also used as microphone, that is what is inside a cheap "crystal microphone".

The drum pads on electronic drum kits use piezo discs on a circle of wood.

Piezos work both directions. Put a voltage to it and it changes shape, a varying voltage then will make sound. But put sound to it or vibration, and it generates a voltage. SO you probably do not need to worry whether the piezos you find are for buzzers or speakers or microphones or drum pads. I have seen the same piezo discs in all of those.

Here is a piezo vibration sensor. You might be able to use it or take its idea and adapt your own.

Digikey has lots of vibration sensors:
Vibration Sensors | Sensors, Transducers | DigiKey

And just to offer up an alternative, does the pipe change temperature when whatever is flowing through it? There are many inexpensive solid state temperature sensors that could be used easily if so. And ambient sound or vibration from passing trucks would have no effect on them.
it is immune to noise,
think again ;)
but is excellent at picking up noise emitted from a solid object.
true, it will also pick all noises picked up by that solid object

To determine whether the project is feasible or not:

1) Epoxy piezo disk to pipe.

2) wire a shielded wire to a high impedance preamp, 1M is good, 10M may be better if sound you need to detect is very low frequency.

3) scope the output, see whether you can visually determine whether there is water flowing through pipe or not; you will also see background noise, even if you do not use an acoustic pickup such as an electret, the kind of high SPL ambient noise you mention will vibrate/shake the pipe and that will also be picked by the piezo.

4) if there is a reasonable signal to noise ratio, say water flow vibration is at least 3 or 4 times higher than "sneaked in" ambient noise, then you can process that signal and do something useful; if signal to noise works against you, there's not much you can do .

5) in any case, post results , they may help others with similar needs, even if to avoid wasting time.

PD: I have a problem with this project: if water has laminar flow/low turbulence through that pipe, there will be very little vibration/noise to pick, if any at all.

A Venturi would be way more sensitive to even small and smooth flows ... just saying.
I am not going to use this project at home. It will be used for something else at a manufacturing plant.

I know for sure that there will be interference. The piezo is immune to sound passing through air. In other words, sounds not loud enough to vibrate the piping. Also, it will be connected to a processor that is able to do fourier transform and other forms of DSP, so I can easily distinguish between external noise / interference and water flowing. I am not at all concerned of noises that may cause a false alarm. Using AI and DSP I am pretty 101 % sure this will work. As long as I choose the right mic, hence the reason why I am looking at a Piezo.

Also, the rate of flow will remain constant, so at the point where the water starts to flow (even 10 seconds after the valve opened), this should not be a problem. I don't think the project itself is a problem - this will be a prototype of a prototype. This is something I can change later on as well.

But like I said, I am at this stage not concerned about noise / vibration on the piping. I will be able to program the processor to distinguish between all this and identify when the water is definitely flowing. Installing a flow meter is not an option, I do not want to measure flow and I certainly do not want to tap into piping that is not mine. This will basically be a yes / no device. Even if the water changes temperature, it will be only a slight change.

Google was not of much help regarding the use of a Piezo disc as a mic, but I guess I will get a few different ones and then test them to see the results.

Basically Enzo has answered the greatest part of what I wanted to know. As far as I understand, the Piezo discs used in buzzers are the same as the ones used for pickup mics. So it doesn't matter if I buy a buzzer and remove the Piezo element or if I buy just the bare Piezo element (which will be cheaper).

Unfortunately it is no where near as exciting to "just always buy a solution". It is also boring to do that - no one will ever learn anything or gain any experience that way. This is where great inventions come from. Imagine if all inventors in history thought that way. What if Newton or Einstein just said "let's do it like the others and not ask questions". I like the fun and game of doing this, hence the reason for becoming an engineer. I do not want to make money, impress girls and be remembered. I just really like experimenting.
Really? Google (piezo contact microphone) you have to be kidding.
Honestly, I think it makes sense that if I said Google was of no help, then I obviously meant it in the light of my first post. I mean the questions I asked there.

I do know what a Piezo is, I do know that it can be used for a mic and a buzzer. But in the first post I think it becomes evident that is not what I wanted to know - that is what Google is for and that would make the purpose of forums like these useless.

Let me sum up what I asked in the first post. I wanted to know whether I should use the element of a Internal or External drive Piezo. Googling "piezo contact microphone" no where answers my question as to even how to choose the correct Piezo disc. All Piezo elements are made of Piezo material, but even though they are all Piezo, they don't all have the optimal frequency response at the same point.

So basically Google doesn't show me the kind of data that I need. The equivalent of looking in a datasheet of a transistor. Google it for a Piezo and you would see why I asked advice here.
I wanted to know whether I should use the element of a Internal or External drive Piezo.
As far as I know, there is no difference in the actual piezo disk, which is the part you want. "Internal" means there is an electronic oscillator circuit inside the housing, as well as the piezo disc. "External" means there is only the piezo in the housing, and you have to drive it with an externally generated signal.

From your point of view, an internal oscillator is unnecessary, and if it costs more, that's unnecessary, too. All you need is the piezo itself.

I think you may find the project more challenging than you anticipate, but it seems you don't want to discuss that. So let me just sincerely wish you good luck.

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