building a very sensitive listening device

[Amaan]

Hey guys hi,

I'm trying to build this circuit (the one given by ronv in post #18 named 'Leak2').
The idea is same i.e. to build a water leak detector for underground pipelines. But i'm planning to add more functionalities to it, like a lcd display to give an indication of the sound frequency being measured so that the operator need not use the headphones all the time.
But that is the later part.
What i am struggling with now is the sensor.
I'm just not able to think how to go about it as i'm dealing with sensors for the first time.
Also i would like to give more details about the problem area which has prompted me to build this project.

A couple of months ago i saw how leak detection is carried out in my city. I was shocked to see a primitive thing like a sounding rod (a listening stick) being used for this purpose. Not only is it a very painstaking way of detecting leaks but also a not so effective one. So i thought of taking this as my academic project. My city looses a lot of water to leaks (about 25% of what it supplies).
I wanted to induce electronics into the same old concept of 'sounding'... as it's called. I was not finding anything satisfactory until i came across ronv's circuit. It was really looking promising.
I'm working on it now but the sensor is something that is bugging me.

Please help.

Amaan

 

[audioguru]

The sensor in post #18 is an inexpensive electret mic that is used in toys and modern telephones.
But the first post talks about using a piezo transducer (speaker) as a sensor. A piezo transducer is sometimes used to pickup vibrations from the body of an acoustic guitar or a drum. A piezo might need some simple changes to the circuit.

I don't know how the mic or transducer can be held against the leaking pipe.

 

[Amaan]

That is exactly what is confusing me.. what to use whether an electret mic or a piezo disc ?
i have brought an electret mic but i rather find it useless.. there are no details about it's range or sensitivity.
So that is an another question 'should i go ahead with just any electret mic or should there be some specifications?'

I am planing to keep the sensor in some housing so that it can be placed on the ground surface (it is not going to come in contact with the leaking pipe as what i am dealing with are underground pipes).

 

[audioguru]

Most electret mics have almost the same sensitivity. They pickup sounds through the air. They are difficult to strap to a vibrating object to pickup vibrations and even then they will pickup background sounds through the air.

A piezo is made as a beeper with a built-in oscillator or as a transducer (speaker). A piezo transducer has a horrible frequency response that is poor for low frequencies and the midrange and high frequencies have many resonant peaks with nulls between the peaks. But they work well to pickup vibrations from car engines and the body of an acoustic guitar.

You should research the type of mic and its method of mounting for a commercial pipe leak detector.

Here are the frequency responses of a couple of typical piezo transducers:

 

[ronv]

Hey guys hi,

I'm trying to build this circuit (the one given by ronv in post #18 named 'Leak2').
The idea is same i.e. to build a water leak detector for underground pipelines. But i'm planning to add more functionalities to it, like a lcd display to give an indication of the sound frequency being measured so that the operator need not use the headphones all the time.
But that is the later part.
What i am struggling with now is the sensor.
I'm just not able to think how to go about it as i'm dealing with sensors for the first time.
Also i would like to give more details about the problem area which has prompted me to build this project.

A couple of months ago i saw how leak detection is carried out in my city. I was shocked to see a primitive thing like a sounding rod (a listening stick) being used for this purpose. Not only is it a very painstaking way of detecting leaks but also a not so effective one. So i thought of taking this as my academic project. My city looses a lot of water to leaks (about 25% of what it supplies).
I wanted to induce electronics into the same old concept of 'sounding'... as it's called. I was not finding anything satisfactory until i came across ronv's circuit. It was really looking promising.
I'm working on it now but the sensor is something that is bugging me.

Please help.

Amaan

The sensor attachment may be the tricky part, but I think I would just try epoxy around the edge of the microphone attached to the top of a piece of rebar.

 

[Amaan]

How about a high-sensitivity piezoelectric accelerometer (about1V/g) as the sensor ?
Should some changes be done in the 'Leak2' circuit if i use an accelerometer?
Also how do i include an Anti-clipping circuit to protect hearing ?

 

[audioguru]

I have never played with and have never seen a datasheet for an accelerator sensor.
My latest RC model airplane is stabilized by three of them and they do the job very well.

You can clip the earphones signal to limit the loudness or use an AGC (automatic-gain-control) circuit.

 

[Amaan]

Hey hi ronv and audioguru,

I have come up with a plan... i am thinking of buying a conventional sounding rod.. then i'll place a sensor (not decided yet what to use.. an electret mic or a piezoelectric accelerometer) near it's brass cup (where the ear is placed to listen to the leak sounds) and then i'll feed it's output to the 'Leak2' circuit.
What do you guys say ?

 

[Amaan]

Here are a few accelerometers I've found on the net:

1.**broken link removed**

2.http://www.digikey.com/catalog/en/partgroup/ldt-series/13276

3.http://www.meas-spec.com/product/t_product.aspx?id=2474

 

[Amaan]

and this:

http://www.meas-spec.com/product/t_product.aspx?id=5411

 

[ronv]

Good idea!

Use the microphone where you put your ear and you will be happy. I think it will be better than the piezo you picked out. You might want to put a cover over it to try to keep regular noise out of it.
Maybe Audio has a clamp circuit in mind. If not I will take a look.

 

[audioguru]

An electret mic picks up sounds all around it. It will produce too much background noise.

 

[Amaan]

Then what would be your suggestion audioguru?

 

[ronv]

Maybe you can use a more directional one like this and put a little foam box around it.

https://www.electro-tech-online.com/custompdfs/2013/03/em10_wm556a103_dne.pdf

 

[audioguru]

Then what would be your suggestion audioguru?

You need a vibration sensor, not an air microphone.
Look in Google for Piezo Contact Microphone. Many links show how to attach a piezo transducer.

Double-sided tape is used to fasten it on this You Tube video which shows it on the body of an acoustic guitar, a drawer, a comb and on the guy's forehead. It shows the schematic of a Jfet high input impedance preamp:
https://blog.makezine.com/2011/12/20/collins-lab-diy-contact-mic/

 

[ronv]

Might work better. Let us know how it turns out.

 

[sanket30]

Guys, Me and amaan are working on this project together.
Today, I tried the leak2 circuit on breadboard. But circuit seems to be not working exactly as i expected. At output terminal i.e. across the headphones no sound is generating (though i am getting minute voltage reading across it ). The voltage across the mic and after 1st amplifier is almost same i.e. no amplification is taking place. I am using 9v DC battery as a supply voltage.
where am i probably going wrong?? please suggest something.

and what changes are required to make in circuit if Pizo plate is to be used as a sensor??

 

[audioguru]

How can you "measure" the voltage across the microphone when it is only 0.005V? The circuit should work fine.

You must read about the "piezo contact microphone" article I posted to see how the piezo is connected to an amplifier.

 

[Amaan]

Hey audio please check your PM

 

[Amaan]

Guys please help me..

Is there any change required in the 'Leak2' circuit if i am using a piezo disc as the sensor instead of the electret mic ?