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Acoustic Surveillance Device/Equipment

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AMSA84

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Hello guys,

Hope everything is running great.

Let me see if I can make myself clear: I would like to know if someone knows if there is any equipment or device in the market that, when pointing directly to a particular place, allows us toh ear the sound in that same place from a reasonable distance. When I say a reasonable distance, I mean something like 20/30/50 meters? How those are called?
 

bigdogguru

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They are referred to as Audio/Acoustic Surveillance or Eavesdropping Equipment, which is why I change your thread title.
 
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    AMSA84

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AMSA84

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Thank you. I took a look to some of those equipment's and they are very expensive for what they have inside (from what I have searched around the web). I found some tutorials that they teach how to build a device like that using LM386. However those circuits are not stereo.

That said, does anyone knows any references to build this kind of circuit but working in stereo mode?

Regards
 

Dan Mills

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Stereo does not really buy you anything as your target is effectively a point source at long distance.

The real key to such things is the acoustic design of the microphone or mic array (depending on how you are skinning it), parabolics are often better then interference tube designs, but are bigger (which helps them gather more sound and be more directional), and there are various phased array tricks that may or may not be useful.

30m in open field is easy, 10m in downtown NY during rush hour is going to be severely problematic.....

Regards, Dan.
 

AMSA84

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Okay, thank you.

So you suggest to keep up with only one microphone (I will be using the Omni-Directional Electret Condenser Microphone type) and one channel headphone?

When you talked about the acoustic design of the microphone, I did a little search and a found this:


Is this what you are referring to? Or only about the way I will be capturing the sound? (parabolic or tube).

Regards.

EDIT: I forgot to say one thing, that is, the microphone that I will be using it only has a frequency spectrum from 20Hz up to 16kHz. That should be enough, right?

One more question, regarding the noise. I read some comments regarding the noise in those tutorials, where people said that they were getting a bit of noise. I wounder, if, using a pre-amp, then a low-pass filter, with a cut-off frequency round 18kHz, then the actual power amplifier, would attenuate the noise?
 
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betwixt

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The image is just a standard electret microphone. It converts the sound arriving at it's front (the side facing away in the picture) in to an electrical signal which you can then amplify until it is loud enough for your needs. The acoustic design is the mechanical or in some cases electrical part that narrows the beam it can hear through so only the source of the desired sound arrives at the microphone. It can be a parabolic dish which has the advantage of working like a big ear, it collects sound waves over a larger area or, it can be done by selectively picking up the sound pressure waves at selected distances so undesired ones are cancelled out. Dishes can be big and expensive, directional 'stick' microphones might be cheaper but may not perform as well.

Stereo is pointless (forgive the pun).
Your awareness of stereo depends upon the sound arriving at different times in each ear. Your brain calculates the source direction by knowing the speed of sound and the distance between your ears. In this case you are listening to a sound from a point source so the difference will be constant. Over even 20m the angle between the source and each of your ears is so small you wouldn't be able to discern it anyway.

Brian.
 

Dan Mills

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Also, you probably don't want to be using an omni as a starting point, velocity rather then pressure transducers offer rather more scope....

Speech at 30M is not going to happen with a little element like that unless you get it very much closer or the target is an opera singer, or you use significant acoustic gain (equals large assembly any way you cut it).

Parabolic mics are this sort of thing:
http://www.canford.co.uk/Products/2017665/53-3901_KLOVER-KM-26-PARABOLIC-REFLECTOR-MiK-26
Interference tube mics are this sort of thing:
http://www.canford.co.uk/Products/9...CROPHONE-RF-Condenser-super-cardioid-long-gun

You will want both high and low pass filters probably before you apply too much gain so you don't have near by traffic noise clip the signal chain, 300Hz - 3K is normal for communications quality speech, and you will want some compression and limiting.

Regards, Dan.
 

SunnySkyguy

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  • find an old sat. dish
  • replace LNA with mag coil mic such as those used from old reel to reel tape recorder eg Panasonic, TDK etc
  • Alignment is critical. then you can hear 10 to 50x better.
  • put soft baffle material on back of dish and add handle
 

Audioguru

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My new hearing aids have a very sensitive mode where I can hear people talking very far away without a dish and without any electronic noise.
I was shocked the first time I used it when a woman behind me called my name. I turned around saying hello but she was very far away talking on her cell phone, not talking to me.
 

SunnySkyguy

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I think m webcam mic can pick up the TV in the basement, But like a satellite antenna ,the detection depends on the signal to noise ratio, in this case via 16 dB gain parabolic dish
 

AMSA84

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My new hearing aids have a very sensitive mode where I can hear people talking very far away without a dish and without any electronic noise.
I was shocked the first time I used it when a woman behind me called my name. I turned around saying hello but she was very far away talking on her cell phone, not talking to me.
Hi audioguru,

Are you serious? LOL That's a good one.

By the way, are you aware of those circuits, simple ones, with the LM386/LM386N? Have you tried to build one of them? Did it work? If not, do you think that a circuit like that can work properly? (with satisfying results)
 

Audioguru

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Are you serious? LOL That's a good one.
Yes, I am serious. I use the very sensitive mode of my new hearing aids to pickup voices of the people mumbling far away in a conference where they push away the microphone for the PA system. I am probably the only person who can hear them.

By the way, are you aware of those circuits, simple ones, with the LM386/LM386N?
They are toys, designed by a 10 years old kid and posted as an Instructable:
1) An LM386 is a cheap little power amplifier with plenty of noise, not a low noise preamp.
2) There are many low noise opamps available to be used as a microphone preamp.
 

AMSA84

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I already suspected.

There is any good low noise preamp that you could suggest? By the way, to design this circuit there is any special circuit (like special techniques, etc) that must be implemented or just using a low preamp + power amp is enough?
 

Dan Mills

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Careful choice of gain distribution, matching current and voltage noise levels to the source impedance, appropriate filtering, appropriate dynamic rage reduction, the usual high gain layout fun, just the usual small signal audio stuff really.

THAT corp make some nice preamp sand, as well as a rather good dynamic range control chip, but there are plenty of others.

The key is the microphone itself, as I say a cheap omni is very unlikely to do it.

Regards, Dan.
 

I14R10

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Yes, I am serious. I use the very sensitive mode of my new hearing aids to pickup voices of the people mumbling far away in a conference where they push away the microphone for the PA system. I am probably the only person who can hear them.
Well, we will soon begin to implant sound amplifiers into our ears to hear from 1km distance. We are turning into Borg :)
 

AMSA84

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I have searched around the web and I found some low noise amplifiers, that are available in some vendors here.

NE5532P: http://www.farnell.com/datasheets/1836402.pdf
OPA2134 and NJM2068D.

What do you think about this one?

- - - Updated - - -

Careful choice of gain distribution, matching current and voltage noise levels to the source impedance, appropriate filtering, appropriate dynamic rage reduction, the usual high gain layout fun, just the usual small signal audio stuff really.

THAT corp make some nice preamp sand, as well as a rather good dynamic range control chip, but there are plenty of others.

The key is the microphone itself, as I say a cheap omni is very unlikely to do it.

Regards, Dan.
Okay, but those that you have suggested are extremely expensive. Do you have any suggestion in the range of $10?

- - - Updated - - -

By the way, what are the main characteristics of a electret condenser that I should look at? For example, S/N ratio, sensitivity, frequency response?

In the vendor here next to me, they have for example this microphone:

Transmission band frequency: 50...16000 Hz
Impedance: 2.2 kOhm
Noise/signal ratio: 60 dB
Sensitivity : -52 dB
Sensitivity tolerance: ±3 dB
Operating voltage range: 1.5...10 V
Operating current: 0.8 mA

Is this any good?
 
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Audioguru

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I have tried cheap electret mics and expensive ones and they all performed the same. I found a small one in a Name Brand cell phone and it also performs the same as a cheap normal size one.
The OPA2134 has two opamps in it. The OPA134 has only one opamp that you will use. They are low noise and use a fairly low current. The minimum supply is 5V so a 9V battery will be good.
you can use an LM386 power amplifier with a gain of 20 to drive headphones. You cannot use a loudspeaker because the microphone will hear it and cause feedback howling.
 

chuckey

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Say you experts, how does the output of an electret mic compare with a good ol' crystal mic? Certainly easier to use then electret and what with FET opamps, high Z is no problem. Sig to noise is a problem, but if the system is any good, the noise of the wind and the rustle of leaves will overcome it. In fact I reckon that some ALC would be advantageous.
Frank
 

Audioguru

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I last saw a crystal mic about 55 years ago. It sounded awful because it is a piezo that has a frequency response that is full of resonant peaks with nulls in between.
An electret mic is common today, is easy to use and sounds great.
 

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