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Circuit For. Listening To Very Faint Sounds

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zunebuggy

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I have built a crude and noisy but functional bat detector and it is interesting.

Now I want to go to my garden and with a microphone, amp and headphones, I'd like to listen to and record the sounds of an insect walking on a leaf.

The closest I can come is a vintage handheld Sony cassette recorder that I have. If you flip open the case and push in the record protect tab (without a cassette) and press record you will hear pretty good when you turn up the volume. You will really notice when you just tap the mic a little and you have headphones on. It'll make you jump. I also can literally hear a pin drop. But it's not quite as powerful as I want. I took apart the cassette recorder and part of the board had a 741 op amp chip. I am impressed at how low the noise is, that is unless it is windy out.

I want to know, is there an amp circuit schematic around that is even better as far as super high gain and ultra low noise so I can record the mostly unheard world of insects? (i.e a butterfly flapping it's wings, footsteps of an insect, an earthworm moving in the dirt, etc.)

Thank you.
 

dknguyen

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Are we just talking something that just amplifies regular sounds within human hearing? Or frequency shifts it down into human hearing?
 

rjenkinsgb

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The LT1115 is pretty much the lowest noise opamp available at present:
https://www.analog.com/media/en/technical-documentation/data-sheets/lt1115fa.pdf

However, most really high quality microphones or small signal preamps use discrete transistor preamps rather than opamps, at least in the first stages.

That's either a FET such as a 2SK107, 2SK118 or, oddly enough, a small high current transistor such as a ZTX851; they appear to have exceptional low noise characteristics.
ZTX851_vs_2SD786_Noise.JPGZTX851_vs_2SD786_Noise.JPG
 

unclejed613

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it would be easier to answer the OP's question if we knew whether or not the "bat detector" is shifting the frequency down from above 20khz to below 20khz. as far as amplification goes, most op amps will be sufficient, the problem of background noise will be the real challenge. the trick might be to use a noise cancellation design such as using two of the same model microphones wired out of phase (or the microphones are connected to a differential input). any background noise will preferably be the same amplitude, and will cancel, but a low level sound source closer to one of the microphones will not cancel. it takes careful balancing of the microphone signals, as well as careful placement of the microphone near the sound source you are interested in.

insect footsteps? i don't know... such sounds are made by something that usually weighs a few milligrams, and has legs with very small dimensions. any sounds produced will likely be several tens or hundreds of khz just because of the physical dimensions being so small. a standard microphone element might not even respond to such sounds because the wavelengths are so short and the mass of the microphone element is very large in comparison to the mass of air being moved. back in the 60s and 70s, some TV remotes used ultrasonic transducers for their remotes (usually at about 40khz), so ultrasonic transducers might be not difficult to find if you know where there's some very old TVs.

you might have some luck if you can get the insect to walk on the microphone element.
 

atferrari

Well-Known Member
When experimenting with a directional microphone I built, the preamp shown here (post #2) once properly adjusted worked OK.
 

Mickster

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If OP has the facility to program a PIC, Bruno Gavand has a 'Picobat' project, using just a 12F683, an ultrasonic piezo receiver and a piezo speaker:
http://www.micro-examples.com/articles/index.php/PicoBat
It uses the ultrasonic receiver to provide clock cycles to the PIC and scales the frequency down by 4.
C source code is provided.
This might be able to be tweaked/adapted to suit OP's needs somehow.
 

audioguru

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I live in a city and a few blocks from a major road. I can hear the traffic sounds. Stupid cars and motorcycles modified to make as much noise as is possible are deafening.
Maybe the background noise in farmland will be low enough to use high microphone preamp gain.
 

dr pepper

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A 741 operating at high gain and quiet, that is unusual.
The Ne5532 is an 8 pin high performance low noise dual op amp (not pin compatible with the 741 as its has 2 amps in the chip), its old but good, be carefull there are counterfeits out there.
Mounting the mic on some very resilient silicone mounts is a must, wind protection is also important.

My mrs wants me to build a bat detector, we have bats, I was thinking of using a Ne602 to pitch shift a couple of octaves, and instead of using a mic to pick up the handed little beggars actually use a ultrasonic detecor, like what used to be used for remote controls on Tv's.
 

audioguru

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A 741 operating at high gain and quiet, that is unusual.
I agree that the old 741 opamp has lots of noise, but it also needs a high supply voltage.

The Ne5532 dual opamp is an 8 pin high performance low noise dual op amp
Nope. The NE5532A is the only one selected for low noise. The NE5534A is a single old low noise opamp and its minimum gain must be 3 or more because its slew rate is better.
 

dr pepper

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Very interesting that chem, I've recently been playing with a switching mixer for a Vlf receiver project, I like the long tail pair on the i/p.
With a couple of little mods and a fet front end that circuit would probably go to 200khz.
 

zunebuggy

New Member
Are we just talking something that just amplifies regular sounds within human hearing? Or frequency shifts it down into human hearing?
I do not want to add this to my bat detector. I want to greatly amplify sounds that are within the audible frequency but go otherwise unnoticed due to their very low volume level. I want to do so without a great deal of noise.
 

zunebuggy

New Member
I agree that the old 741 opamp has lots of noise, but it also needs a high supply voltage.


Nope. The NE5532A is the only one selected for low noise. The NE5534A is a single old low noise opamp and its minimum gain must be 3 or more because its slew rate is better.
Thank you. I think I'll give this chip a try.
 

zunebuggy

New Member
If I go with the NE5534A can anyone recommend a good mic with a built in preamp? Thanks again. I'll post here later with the results.
 

rjenkinsgb

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The first stage of the preamp is the most critical part for a low-noise circuit.

Most cheap mics (and some not so cheap ones) use an electret capsule which has a built-in FET preamp. Those can vary tremendously in noise levels and quality.

High-end mics tend to be either dynamic (and low sensitivity, for such as live / stage use) or use a large, externally biased condenser type capsule plus a discrete component preamp.
A really good one of those starts at about 200-
eg. http://recordinghacks.com/microphones/MXL/V87

(For info, most "condenser" mics pushed for amateur use are scams, with electret capsules in them).

The cheapest approach is possibly to buy a bulk batch of electret capsules and just see which one out the batch gives the best result..
Some of the supposed "high quality" ones on ebay boast 60db signal to noise, which is not good compared to a decent condenser mic - but at less than 10- for batch, you get what you pay for..
eg.
https://www.ebay.com/itm/20pcs-Prim...icrophone-60db-s-n-Capsule-Japan/121924034703

(One of my part time hobbies is comparing and improving mics..
Some of my collection here: https://flic.kr/p/UuiU61 )
 

audioguru

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The Jfet inside an electret mic has very little voltage gain, it is an impedance converter. You still need a low noise preamp with lots of gain.
I would not buy an ebay mic which might be a manufacturer's reject or a fake. All electret mics sold at Digikey are excellent.
 

dr pepper

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I lashed up the bat circuit on breadboard, put a fet buffer after a ferrite ant on the i/p and it worked, I dont have any unbuffered logic so I deleted the amps, and replaced them with a single tranny after the switches.

As for listening to insects the mic is everything, also noise is largely a problem if its in the range of what your listening to, you can equalize some of the noise out, if your sounds are not in that range.
 

schmitt trigger

Well-Known Member
Just an idea;
To improve both linearity and acoustic gain, you could try a parabolic reflector.
At 20Khz, the sound wavelength is already down to about 1.715 centimeters, and it will become shorter with increasing frequency. The reflector size will be small.

Electronically, you may also want to employ a high-pass filter after the high gain amp.
 
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