Piezo contact mic and amp recommendations? (wall)

[gtsfan]

Hi all. Any suggestions on a good cheap piezo contact mic and amplifier? Preferably for the amp I want something that I can plug into an electrical outlet 24/7 (not something that requires batteries or recharging). This would ultimately be fed to a computer which would record.

I know most contact mics on Amazon are for musical instruments. I have read that this might not be the best for wall applications because they might not be as sensitive.

Cheap is a real plus. Also I prefer to buy the parts (mic, amp) separately for deniability as opposed to as a "spy device".

Thank you for any hints. I've read many topics in search but the info seems dated and not complete. I'd especially be interested in people with real world experience giving equipment suggestions.

[gtsfan]

I'm very interested in this and have probably read every post on the subject in the forum using search.

Absent any exact hardware recommendations, does anyone have any technical information relating to this? I see some premade equipment on sale on ebay or amazon but it seems most are battery operated or use a charging port. That's not ideal for my situation. Also from reading around the quality on these seems sketchy at best even when they are overpriced.

I am strongly leaning towards buying components individually or even making something entirely from scratch (save for the amp) but just do not have enough knowledge yet on what to look for. For instance from what I read most of the piezo contact mics at Amazon would not be ideal because they are built more for instrument use. Also with getting an amp my concern is that these would be targeted towards musical use and thus not ideal for this application.

Any and all technical information or tidbits related to this would be most helpful. In return I promise to share the final products I go with and review them for the group.

[gtsfan]

So I did some more research off the site and I found that I probably do need to go with a preamp (as another poster on a different topic in this forum suggested) for the purposes of getting the piezo contact input to a usable line quality. Also there is a severe impedance disparity which tends to lead to part of the low signal getting chopped unless this is addressed.

So far this looks like what I probably need for the preamp:
https://www.amazon.com/rolls-DB325-Inst ... 00IMO0IH6/
(rolls Instrument FET Preamp-Di (DB325) ) $99

This has the benefit of having a wall power supply but can optionally use a 9v battery.

I'm thinking with a good piezo contact mic, the required connectors/cords, I ought to be able to just connect the pickup to this preamp and then the preamp to the line in on my computer running audacity. This should get me something workable to be able to find those leaking pipes or to detect "faint footsteps". Please correct me if wrong.

[reggind]

That looks like a nice contact mic preamp from the picture. It is hard to say how well it is going to work as far as what you are trying to do with it. It should have the high input impedance that the contact mic needs, and it should have a low output impedance suitable for going into a notebook or portable digital recorder.

The parts that I am not sure of are the noise floor and the gain. Contact mic's for instruments are not starved of acoustic excitement. That is they are usually in direct contact with the instruments body and often times in places to maximize the amount of vibrations they will pick up. Think directly under the bridge in most acoustic electric guitars and basses. You are never going to get that same kind of vibration where you will be, so you are going to need a lot more gain, and with that gain you will get both a lot more electric noise from the preamp and a lot more noise from the room, ars passing by, people walking down the halls, HVAC system and fridges in the rooms etc.

I do not want to sound negative here, but I want you to have realistic expectations of what you are likely going to get.

[gtsfan]

reggind, to the contrary, I do not see you as being negative. I appreciate the input as I'm coming from a position of very little knowledge of this. Part of this for me is a learning experience just to learn what is possible so at this point I will just see the $130 so far that I spent as the tuition. So far I have learned a great deal just from reading posts here and researching.

Based on what you are suggesting and what I have read elsewhere I am thinking I will probably at least need a good amp too. But I will hold off on that for now and see what happens with just the preamp. I'm thinking I should be able to possibly clean up the signal in audacity a bit too. I plan on using the auto noise activation feature (have it automatically record and stop based on the DB readings) though so that may make this even more of a struggle. I'll definitely report back as to what I find.

[gtsfan]

Also here are the specs for the DB325 I bought:

SPECIFICATIONS
Input Impedance : 4 Meg Ohms
Frequency Response: 20 - 20kHz +/- 3 dB
Output Impedance: 1K (XLR 600 Ohms)
Max Output Level: +8 dBm (XLR -12 dBm)
Max Input Level: -13dB
Gain: 20 dB, (XLR 0 dB)
Signal To Noise: 106 dB
THD: < .570%

from https://www.rolls.com/pdf/M_DB325.pdf

Seems pretty decent but I'm pretty new to this stuff. The SNR of 106dB seems a little higher than what I usually saw in other preamps. The gain of 20 dB also seems okay compared to others which were merely 12 dB. But I'm guessing 50 dB gain would have been a lot better for this?

I'll have to wait a bit until I receive this but in the mean time if anyone has any suggestions for amps or equalizers etc. (in case I end up needing it), then I'm all ears. Price is a big factor but I wouldn't mind spending another $100 maybe $200 if it makes a big difference.

Also I ordered two pickups. One coated and another uncoated with a 3 in 1 design. From my readings the raw uncoated piezo discs are more sensitive and require less amplication than the coated ones. So I figured for our purpose it might be better. Also from what I understand the three in one design makes it a little more sensitive. Please do correct me if I am wrong on this!

[shulrab]

Perhaps some helpful links for the DIY'ers:

https://www.zachpoff.com/resources/alex ... amplifier/

http://www.richardmudhar.com/piezo-cont ... e-version/

https://zeppelindesignlabs.com/product/ ... ntact-mic/

[gtsfan]

So initial report here after a few hours of having the equipment:

The results are better than I expected but not yet great. I have a lot of learning to do with audacity and sound recording though. Right now I can hear through the next room through the wall slightly better than I can holding my ear to the wall. That's not great but it is SOMETHING. I found having the preamp turned all the way up makes a big difference. The model I bought has a gain of 20 dB so I suspect getting something with more gain (say 30 dB) would help significantly.

The problem I suspect is with the line in/microphone to my computer. Technically the rolls preamp puts out a line level output. In audacity it seems to be indicating that the input is seen as a microphone (as is it indicating so in my computer OS) but I'm not sure. What I notice is that with the microphone input at 99% level the amplitude of the sounds generated in Audacity is very low, between +-0.15. If I turn the microphone input up to 100% it then generates three times the noise but also three times the amplitude. The signal coming in to audacity (@ 99% microphone and 100% preamp gain) is about -22 dB with spikes depending on loudness.

My original intent was to set this up so that Audacity would utilize "Sound activated recording" so it would only record during detection of significant sound but so far I am finding that a challenge because of the faintness of the signal and the noise. It might be that I will be able fix this but for now it looks like manual recording will be needed.

I suspect the main thing I need to do now is find ways to reduce the noise.

Overall though like I said though I am happy enough right now. This is definitely better than holding ones ear to the wall for a multitude of reasons. It will be an interesting project to try to improve things and tweak them. I'd also welcome any input or hints too of course.

----

On the Rolls preamp: It's adequate and better for our purposes it seems than almost everything else out there on Amazon or Ebay (at least for under $200). It seems to be pretty well made and does come with an AC adapter so you do not have to go through 9V batteries. Unlike some of the other cheap specialized devices you see for this sort of thing it appears this should last even with being on 24/7. A bonus with it as I understand is that all the outputs work separately. So you can hook up multiple outputs at once (to say your line in on your computer or recorder and probably to your headphones).

[reggind]

You are seeing the problem that I described, the more gain you have the more noise. This is why having a mic with very little self noise and a very low noise preamp is important. The good news is it does not sound like you are overloading the mic preamp on your recorder. As an aside on recorders line level inputs are a crap shoot as far as how they are wired. Some of them skip the mic preamp which is the better option, but it is (sadly) not uncommon for them to take the line input and attenuate it and feed it into the mic preamp. This is much worse from a noise perspective. As you can see, the best thing to do is get more signal into the preamp. You can look for a more sensitive mic, or ask the "talent" to play louder. In the case of what you are trying to record playing with mic's is probably the only option as far as getting a hotter signal. A lot of people have turned the piezoelectric disks in piezoelectric beepers into mics. If you can solder and can make a cable you can experiment. The good news the beepers are inexpensive. Google is your friend on that one. If you lucked out and got a schematic of the preamp you bought you may be able to get some more gain out of it with some modifications, but as I said before, that will also add to the noise, and by noise I mean any extraneous room noise that gets picked up, any self noise from the mic, and any noise the preamp generates itself. As I said in the first post, sadly we are trying to pick the needles out of the haystack so to speak. The equipment you have is really built to be affixed to a musical interment, and get vibrations that are thousands of times more vigorous than you are getting on the far side of a wall with people speaking in normal tones in the other room. One thing that will help a bit but will also negatively effect the fidelity is to use the high pass and low pass filters on Audacity with aggressive rolloffs at roughly 300 Hz on the low end and 3000Hz on the high end. That narrow 300-3000 frequency band is where most of the intelligence is in human speech. Filtering it down to that results in the sound quality of an old telephone however. Decidedly not hifi. But you do nix a lot of the noise. And if you want to use the noise reduction in audacity, getting rid of as much of the noise with the filters first helps a lot. That is use the high and low pass filters first and the noise reduction second.

BTW, on the outputs, they may be electronically isolated but they probably are not. If plugging in a pair of headphones knocks the audio level down going into the recorder they are simply wired in parallel and are not truly isolated from each other. If you have the schematic of the preamp that will be really easy to spot.

[gtsfan]

Reggind,

As always thank you very much for your knowledge. By using your tips I tried setting preamp gain to MAX and the microphone to 100%. This amplified the sound and the noise by 300%. Then I ran the high and low pass filters between 300-3000hz with a 24 dB dropoff. Then I applied a noise filter. The result is an amazing difference! There is pretty much zero noise post processed and the amplitude is decent at around +- 0.40.

If I could just figure out how to always apply the high low pass filters with noise reduction (based on profile) while live it would be pretty much absolutely perfect. Sadly I don't think this is possible with audacity based on initial readings but even if so I feel I will be getting some great recordings soon of some rats. I will definitely be sharing them but will probably have to re-register under a different moniker for security purposes.

[reggind]

I am glad I was able to help, and I am really glad you got some better results. I have an idea that I want to try at some point in time, but I just have never gotten around to trying. I have some very snazzy high end ultra low noise preamp chips. I want to try using two mics but out of phase with each other. I am curious to see if I can put one on a wall or part of wall that should have similar ambient sounds on the other side of it, and subtract that from the mic that has the sounds we are looking for. In theory, they should cancel. It is just a matter of adjusting the levels carefully. In practice there are a lot of reasons that won't work as well as you might think. The two mics not getting the same similar signal and little time (phase) delays between the sound pickup on the two mics. I think it is worth playing with at any rate I also have some pro quality voltage controlled amplifiers, these are the heart of automatic gain systems. I doubt your system has any AGC on it. If the talent starts getting really loud you may have to turn the volume down to keep it from distorting. AGC takes care of this for you. I would also build hardware hi pass and ;ow pass filters in. As you might guess, I have been pondering this for a long time. I have procured the expensive pieces. One of these days I may even get off my ass and dig into it. The one thing this would not have that I should also think about would be a high impedance FET front end that could be switched into the mic preamps for use with piezo contact mics. And I would also have truly isolated headphone and recording outputs. The down side is just the raw parts make this an expensive project.

[gtsfan]

Yes, AGC (automatic gain control) would be VERY useful. I've made some adjustments to my set up and have better perfected it. It's a huge difference. I'm now to the point where it is basically like having an open mic in the other room! Just with using a 3 in one raw piezo and the Rolls Preamp above I now have to turn down the volume on the microphone or I can get clipping with moderate talking. Also knocking sounds (such as a headboard) are causing clipping as is the TV volume. Not sure if I can do much about that unfortunately.

As a tip for anyone else doing this I found piezo placement can make a big difference. I am seeing best results from placing the disks vertically (I have 3 in 1) at about the middle of the wall about 8-12 inches apart from one another. Also from reading, evidently the smaller the piezo disc the more sensitive it is. However bigger tends to give better quality sound. I am to the point where I would probably benefit from using better piezo discs.

As far as adding an amp or needing more gain, in my case I definitely do not need any more gain. With the adjustments if I turn the preamp all the way up and the microphone to 100% it will actually start clipping with very moderate talking. I now highly recommend this preamp.