Skip to main content
Community forums, blogs, song critique, press & more...

low frequency recording

Hello all,

I have an issue I'm not sure how to tackle, so I'm searching for some advice. I need to record for legal documentation a very low-frequency low-amplitude drone that is present in some households. Apparently it is caused by some fans in a warehouse that is located about .25miles away from a neighborhood. But, I'm not sure of the best mic (omni, amorphic, cardio) to buy to record this. Is there a good mic someone knows that is particularly sensitive in very low frequencies?

My task will be to go into people's homes into different rooms, make the house as quiet as possible (turn off all appliances and A/Cs) and start recording. Then I will need to pull up the drone sound and record for a few minutes.

The audio is so low that many people don't hear it. I think some people are just less sensitive to it than others, but that's the type of noise I'm trying to pick up. So I need to be able to document the noise-floor of a mic to prove that the noise recorded is not actually mic-hum, but rather the low-freq from the warehouse.

Also, it would be absolutely wonderful if I had a mic (shotgun type) that I could bring to the warehouse, target one of these external fans, and pull a sample that I could filter out all other frequencies and prove the sound print was identical to what was sampled in people's homes.

The high frequency sounds are lost in space from the warehouse to the homes, but the bass travels through the walls and insulation into homes. So that low sound I have to record.

I would like to go through my mixer and record to disk on my macbook's analog input. But if recording to HDV or DVCAM tape is better, I could bring a camera along soley for it's audio inputs. Not really sure what provides the best signal to noise ratio.

I would like to do this w/o breaking the bank, and have a mic that would be good for other purposes as well, but I'll take what I can get.

Any advice would be greatly appreciated!

Comments

stjohnson Tue, 11/27/2007 - 08:53

Boswell, this is a good point. I am looking at studio mic's. I should tune my search a bit, so to speak. I'm ok with needing phantom power, as I have field gear and equip that should be fine if I need to go away from an outlet.

Joe, that is insane info. Incredibly useful. You do understand the noise! It's exactly like that for about 4 households that have stepped up. I'm not sure if there are more, but for now, it's not a huge legal issue since it's such a quiet drone. People are more passive about it than they should be in my opinion. I have the simple Adobe CS3 sound booth software and it does have the sonar feature you mention (which I couldn't live without). I was definitely planning on using that to filter the high frequency sounds. GREAT freken idea with the showing people what it's like. I definitely don't have a good enough sub, but I could stop by the local guitar shop and rent one of those for a demo if needed. It would be clunky and difficult, but man that would be awesome!

Max, thanks for the insight. I don't think I'll be able to sample people's houses for 24 hours! But from what I'm told this shouldn't be necessary. Just proof of the sound and level is what is needed. I'm doing this myself because I have technical knowledge of audio engineering, but just not the gear to accomplish this task. Also, it hasn't escalated to a class action yet. This is the basework to establish a case, but will be used primarily at this point to solicit the company owning the warehouse to modify their ventilation systems. If no go, then this becomes real and suits will be filed.

But ALAS I've come across a company that will set me back ~$1k, but has the exact tool I'm looking for. Just thought I'd post it here for anyone else's info.

http://www.linearx.com/files/pdf/RTAjr_Brochure.pdf

These guys have an ancient ISA-Bus architecture product called the RTAjr. It looks like they can not only give me SPL per frequency but also the dB per freq (dBspl). This is exactly what I'm trying to accomplish. With the right filters on this audio I can also record to an offboard recorder through their line out. It sounds too good to be true, but I'll find out more very shortly. My last concern is the mic. The transducer might in fact be a better route to go, but I'm going to go trial and error on that one and find out what gives the best overall and repeatable response.

Profile picture for user Boswell

Boswell Tue, 11/27/2007 - 09:24

If you want to go down to really low frequencies, then a velocity device like a Geophone is the thing to go for. It's used for such things as seismic investigations and quantifying the vibration effects of trains in tunnels on buildings above and around the tunnel. Best used with a data acquisition board (which goes down to d.c.) rather than an audio interface.

Click for help files

Don't bother with the RTAjr gear, unless you are tuning up for a car stereo loudness contest.

fredmushell Sat, 04/26/2014 - 01:37

Years ago (just after the dinosaurs disappeared) I was working at a post production house in NYC. After an expansion to the facilities we started having low frequency noise problems. Every so often a rumble would go through some of the editing rooms. Nothing you could really hear but you could feel it in counter tops and maybe a picture on the wall or a spoon in a coffee cup would buzz a little.

dpd mentioned a piezoelectric detector. What we came up with as a measuring system was a children's cheapo portable record player with a crystal pickup cartridge. We put a record on the turntable and put the needle on the record but the record player was not turned on or spinning. We connector a Tektronix oscilloscope to the output of the cartridge as a DC preamp. The scope had a Channel A Output which we connected to a Sound Stream digital audio recorder. The system seemed to have an output probably down to 0 Hertz. What we found was the rumble had a frequency between 0.7 and 3 Hertz with a peak at about 1.5 Hertz.

We had a 50 ton cooling system that worked great so when we expanded we bought a second 50 ton system, just like the first one. What we figured out was that the blower fans, in the two systems, were running at about the same speed depending on how many of the air dampers were open. Both systems were connected to the same air handler system (so if one system went down we would still have some cooling). Because the two blower were running at about the same speed their sound was beating with each other and that the air handler system was resonant at about the beat frequency.

We solved the problem by changing the fan motor pulley sizes so that the beat frequency between the 2 fans was about 15 Hertz. Because the 15 Hertz was not resonating in the air handles system it was undetectable in the edit rooms.

DonnyAir Sat, 04/26/2014 - 03:15

Cool post....

Just letting you know though, that you are responding to a thread where the last post before yours was over 6 years ago ;)

Not that there's anything wrong with firing up an old thread. I just wanted you to know as a new member, that many of these threads will appear new to you, as you have never seen them before, but they aren't necessarily current. ;)

Welcome to Recording. :)

stjohnson Tue, 11/27/2007 - 20:50

I don't really need low frequency recording. The frequency will be 50-60hz. The amplitude will be very very low. Some of the hardest signals to pick up are the low freq low volume ones. Looks like the RTA people will allow me to eval a system before buying. Funny what they say about car stereo engineering, but I still think it might be a good solution if it does the other 1/2 of what they claim.

RemyRAD Tue, 11/27/2007 - 23:04

I would have also recommended the B. & K./DPA/Earthworks microphones, since their early existence was that of calibration and noise pollution measurements.

The problem with what everybody is recommending here is that, you really aren't going to be able to play back this annoying sound as it appears in people's homes. This is akin to trying to record the extremely low-frequency communications of whales & elephants. First, like we recommended, you need to record the source sound in the offended locations. Then, that recording needs to be speeded up along with a spectral display and appropriate scientific explanation with the double speeded audio played back so that it can be properly audibly realized.

Trying to give people the experience of what this sounds and/or feels like is like trying to reproduce the sound of a Shuttle launch, in a courtroom. Right, impractical at best. And that's why we have all of this wonderful equipment to be able to manipulate and display sound in a scientific way. Why not just have some idiot with an overblown car stereo play some rap and/or hip-hop while driving by the courthouse outside? Maybe that's what it is?

I believe filtering anything out would make your demonstration invalid. All of the noise can be scientifically identified in spectral displays/analysis. And so, you really need to contract a forensic audio expert from the NTSB that can validate this noise problem.

Thinking outside of the noisy box
Ms. Remy Ann David

Profile picture for user VonRocK

VonRocK Tue, 11/27/2007 - 23:34

stjohnson wrote: I don't really need low frequency recording.

You are not being subjective with this statement, and in my opinion, not justly serving your clients.

You have stated that some people can not hear this noise, as it is too low in frequency. Then you state that you really don't need low frequency recording?

It is logical to assume that a large part of this noise is lower than most people's ability to hear, as you stated that your trained ears can hear it, yet your wife's cannot, and some people do, and some people don't.

Either way, a fully subjective study of the issue is desired.

Perhaps you should pass this job on to somebody else. Seeing as you "don't need low frequency recording"

Is there any such thing as a forensic audiologist? At the very least, I'm sure these people would not be too impressed with their audio expert seeking microphone advise on a public forum.

I know at least one lawyer that would tear you "expert opinion" apart based on this alone.

Profile picture for user VonRocK

VonRocK Tue, 11/27/2007 - 23:42

Crap, Remy's was typing and posting before I saw her....

the gist of the message is, "it is beyond your scope of expertise" and buying a microphone is not going to change that.

RemyRAD Wed, 11/28/2007 - 00:33

Yes VonRocK, is basically saying the same thing that I said. Great minds think alike especially when they're professionals like we are. You have a fool for a lawyer if you are attempting this yourself. You need it backed up by a professional from a professional.

I understand, you don't need low-frequency recording. But you need to record the low-frequencies, so that the spectral energy, along with all of the other crap, can be properly displayed by an audio forensic professional. And yes, forensic audio professionals exist! The NTSB has enlisted forensic audio professionals to determine particular cockpit voice recorder sounds in the background prior to a catastrophic incident. It was even suggested to me to go into that field by Robert Hager, of NBC News, because I was an NBC audio engineer that was already working with computer audio back in the mid-1990s, before NBC was playing with computer audio devices. I wasn't interested much. I'd rather record rock-and-roll than listen to people losing their lives. Maybe that's why I'm not much into death metal? Been there, heard that.

Enjoying lively rock-and-roll
Ms. Remy Ann David

Profile picture for user JoeH

JoeH Wed, 11/28/2007 - 06:58

We should all realize, of course, that most of this will come down to the attorney for the suffering parties sitting down with the attorney for the factory with the fans, having lunch, and figuring out the best/easiest/cheapest way for the factory people to stop the noise.

If they have any brains, that is.

The mark of a truly failed negotiation is litigation in a coutroom. At that point, it's just a slug-fest, and the one with the best representation wins.

As others have pointed out already, maybe you should get an attorney invoved FIRST, before you buy or rent anything, spell it out for him/her as to the problem & solution, and see what they say. An hour of an attorney's time up front may save you and your friends a LOT of hassle in the long run.

One of my clients does forensic audio for criminal law in NJ and NYC, and most of the projects he brings in for documentation and analysis never make it trial; once the lawyers look at the evidence (or lack thereof), many of these things are settled quietly, far away from a judge or courtroom.

Hopefully your noisy neighbor will see the light (or hear the rumble) and take care of it before things get ugly.

stjohnson Wed, 11/28/2007 - 09:03

VonRocK wrote: [quote=stjohnson]I don't really need low frequency recording.

You are not being subjective with this statement, and in my opinion, not justly serving your clients.

You have stated that some people can not hear this noise, as it is too low in frequency. Then you state that you really don't need low frequency recording?

It is logical to assume that a large part of this noise is lower than most people's ability to hear, as you stated that your trained ears can hear it, yet your wife's cannot, and some people do, and some people don't.

Either way, a fully subjective study of the issue is desired.

Perhaps you should pass this job on to somebody else. Seeing as you "don't need low frequency recording"

Is there any such thing as a forensic audiologist? At the very least, I'm sure these people would not be too impressed with their audio expert seeking microphone advise on a public forum.

I know at least one lawyer that would tear you "expert opinion" apart based on this alone.

haha.. i love the Internet and all the opinions out there. Its a beautiful place.

I've never said that the low frequencies are invalid, but I don't need them. For all of you shadetree lawyers out there, do some research and you will find that the range of human hearing is between 20hz-20Khz (don't shoot me down if I'm not 100% accurate here, haven't looked in a while but I know I'm close). Your lawyer that would tear my "expert opinion" down because I didn't include a full spectrum would be immediately put in his place when I was crossed and revealed that any missing information in my presentation isn't even audible to us! Most of these geophones being recommended are insanely low frequency sensitive, and not valuable for this case. The lowest I need to go is 20hz. Not 4, not 7 or 11 like some of these mics and people are suggesting. I would dare anyone to bring a lawyer to challenge that. So I reiterate, I don't need low frequency recording, I need down to about 20hz. This is not such a tall task. The hard part as I've said previously is the noise floor of the mic. Just because it's sensitive to 20hz, doesn't mean it can hear 20hz @ -80dB. The spec for human hearing is a range of frequencies, not amplitudes. The same holds true for Mic's based on their noise floor.

The idea of duplicating for play back is far fetched, like you say Remy, but it is not far from reality. The idea of getting a subwoofer and reproducing it identically is of course not going to happen. Seems like most people can't hear the tone anyway, so why reproduce something people can't hear? That would be a funny piece of evidence. In all actuality, upon displaying the soundprint spectrally, and identifying it's specific wavelength and dBspl at different sites. it would be easy to play it back at an amplitude that was easily audible. And it would not be invalid as it would simply be those frequencies that would be boosted, rather than filtering out the surrounding noise. This is perfectly valid for demonstration purposes. It doesn't filter or alter in any way the evidentiary sound, rather just emphasizes the documented sound for a judge, should it get that far.

You won't hear me say that the NTSB or some other "forensic audio firm" is not valid, they are valid, if only for their accreditations. But for now, there's not much they can do that we can't. If it gets to the point where we need to go into a court of law, yeah, it makes sense that if you put two people on a stage, one has a degree, so to speak, and the other doesn't, the people probably will tend to believe the one with a degree. That's a flawed perception, but unfortunately true. We would get our results certified in that case.

Profile picture for user VonRocK

VonRocK Wed, 11/28/2007 - 10:02

I love the internet too!

You asked, and you received opinions from this forum. These opinions all range from complete amateurs (like me) to very knowledgeable recording professionals. All of them, a valid contribution with varied point of views.

The subject has peaked interest here, and in so doing created individual thoughts and a discussion concerning very low frequencies. I will be doing some of my own experiments for my own curiosity later on today with very low frequencies, and exploring an avenue with my equipment that I may have never thought to go. Thanks!

Now, back to your issue;

I really don't think that you should be neglecting to look for offending frequencies below the spectrum of human hearing. I can't see the flu virus, it's beyond the ability of of my human vision, but it sure has had an affect on me this last week!

None the less, I hope that you are able to find an adequate solution, and would appreciate a continued dialog and updates as this issue develops.

Profile picture for user Boswell

Boswell Thu, 11/29/2007 - 04:54

stjohnson wrote: I've never said that the low frequencies are invalid, but I don't need them. For all of you shadetree lawyers out there, do some research and you will find that the range of human hearing is between 20hz-20Khz (don't shoot me down if I'm not 100% accurate here, haven't looked in a while but I know I'm close). Your lawyer that would tear my "expert opinion" down because I didn't include a full spectrum would be immediately put in his place when I was crossed and revealed that any missing information in my presentation isn't even audible to us! Most of these geophones being recommended are insanely low frequency sensitive, and not valuable for this case. The lowest I need to go is 20hz. Not 4, not 7 or 11 like some of these mics and people are suggesting. I would dare anyone to bring a lawyer to challenge that. So I reiterate, I don't need low frequency recording, I need down to about 20hz. This is not such a tall task. The hard part as I've said previously is the noise floor of the mic. Just because it's sensitive to 20hz, doesn't mean it can hear 20hz @ -80dB. The spec for human hearing is a range of frequencies, not amplitudes. The same holds true for Mic's based on their noise floor.

The focus of this thread seems to have moved from audio recording to capture of legal/forensic evidence. Even if you disagree with that, I think there are several flaws in your argument.

1) The point of this thread is that you are being troubled by a low-frequency acoustic emission which you believe comes from a warehouse in the neighbourhood. You "need to record for legal documentation" this acoustic emission. The annoyance factor of low-frequency emissions is not just in the range normally assigned to human hearing. A large fraction of the acoustic energy is at frequencies lower than 20Hz. The annoyance is caused not just by what you can hear but what you can feel. If a diesel-engined truck pulls up outside my office and the driver leaves the engine running, I often have to get up and go elsewhere until it departs. This is because of acoustic energy (that I have measured to be in the 7-10Hz range) that becomes painful. The levels of emissions above 20Hz (the nominal lower limit of hearing that you quote) are much smaller. You need these lower frequencies to support your case.

2) Even if you were concerned only with frequencies of 20Hz and above, you would not record and measure these convincingly with equipment that itself was struggling to reach down to 20Hz. You would need 20Hz to be in the passband of the recording equipment, which generally would imply 2 octaves above its nominal lower -3dB point (so 5Hz in this case). A competent lawyer would shred a case where there was doubt about the equipment's ability to capture the offending sounds.

3) Notwithstanding the hearing/feeling split form (1), the Fletcher-Munson curves for human hearing show that the sensitivity of the ear falls off rapidly at low frequencies compared with what it can detect in the region around 1KHz. Recording equipment does not follow the F-M curves, and any recording gear appropriate for this task would have plenty of headroom above the noise floor at low frequencies compared with the ear.

4) You must not filter or remove frequency ranges from the recording. Firstly, it would be regarded as tampering with evidence if you wanted to bring a legal case, and secondly, you are pre-judging what it is in the emission that is causing the annoyance. You said "...It's just like someone has a high voltage 50-60hz transformer attached to the frame of your home in the basement, and the hum from the transformer is being transmitted into the walls which you can pick up on from other rooms." It seems you have made this analogy and now you want the recordings to fit your model. Similarly, you want opinions from this forum to back up what you have already decided.

As JoeH, VonRocK and others have said, we are here to help, and we take time and effort to give you what advice we can based on our knowledge and experience. It's your choice whether you take any of it on board, but if you are looking to get some action against your warehouse, the opinions here in this forum are friendly, but only the first test of your case. You will meet much sterner examination if you take it further down the legal route.

Profile picture for user MadMax

MadMax Thu, 11/29/2007 - 06:00

Boswell's dead on the money.

You cannot quote me or even refer to my reply (or any of em' for that matter) to this post with any legal credibility, but one thing I think you will find is that the real frequency of your noise is going to be in the sub 20Hz range.

You are probably looking at nodal issues where the buildings are acting as node points like the endpoints of a vibrating string. There, the energy is transfered into the structure, and causes the structure to vibrate at a harmonic of some fundamental within the individual structures.

This is a seriously complex industrial noise issue.

If you want to go ahead and jeopardize your case, proceed. If it were me and my money... I'd hire a professional.

Profile picture for user Thomas W. Bethel

Thomas W. Bethel Thu, 11/29/2007 - 06:30

stjohnson

Please take my advice. I have been called as a professional witness in a a couple of forensic cases and I would suggest that you....

Hire a professional.

They will have the equipment and expertise to do the measurements correctly. And they will know how to present these to the court so they can be understood by a layperson without advanced degrees in physics or acoustics.

First thing...what people are telling you is basically what any acoustical expert would tell the lawyer for the building. The acoustical/vibration expert would shoot down any "home made" evidence within the first hour of the trial (if it ever got that far) Don't forget that there are "hired guns" (acoustical vibration experts) that are professionals that can prove or disprove just about anything having to do with sound or acoustics.

You are to be commended for wanting to DIY this yourself but on the other hand.....you don't stand a snowball's chance in he!! of going up against a pro. The first thing that they will want to know is "when was the last time your equipment was calibrated and is it in compliance with the NBST standards" A rock and roll microphone will not be calibrated and unless you have access to some very sophisticated equipment you will not be able to calibrate your microphone yourself and it will not be traceable to the NBST. A pro will be able to do that and have a calibration certificate from an approved lab.

The second thing is the 24 hour testing. You have to prove WHEN this was happening and if it was happening when the plant was in operation or was shut down so you can't do that in a couple of minutes in someone's house.

Thirdly if you start to play around with frequencies you are going to get into BIG trouble very quickly. The judge will want to know why you manipulated the "evidence" before presenting it. The consul for the factory will use his expert witness to basically say that you were manipulating the evident to prove your case and if this is a jury trial the jury will be skeptical of any additional evidence you present.

Forth. The human ear CAN hear frequencies between 20 and 20 kHz frequencies below 20 Hz are technically classified as vibrations and not sound. The person who said that you will have to measure down to 5 Hz is correct. The MIDBAND of your testing will have to be 20 Hz so you will have to go from 5 Hz to 200 or above to make the measurement mean anything. When you get down into the very low frequencies you may have to use equipment that is very expensive and may have to be used by a trained operator to validate the results and to provide the necessary graphs and technical details of the vibration and sound. He will have to do this over a 24 hour period and take readings when the plant is in operation and when it is shut down ( you have to PROVE beyond a reasonable doubt that it is the plant in question is the one making the noise and this will involve having a background noise sample without the plant operating so the acoustical expert will have something to compare the reading to). He may also have to do this in a several locations to validate the results and this could get expensive very quickly.

My best friend is an acoustical engineer and gets called in on all types of noise problems and sometimes he has to turn things over to someone with more sophistcated equipment than he has and he has some of the best equipment around.

Two cases come to mind that were similar to what you are expereinceing.

A recording studio owner purchased a building on a seldom used road. They were within 40 feet of the center of the road way. When they purchased the building the seller conveniently "forgot" to tell them that there was going to be a major road widening and becasue of the road widening the traffic increased from nearly nothing to considerable with in a few weeks and all of sudden large trucks were using the street where previoiusly they did not use it. Obviously the owner of the studio was upset and told the person that he bought the building from he had two options. Give him his money back and he would find quarters elsewhere OR fix up the building so that the road noise would not interfere with the recording. My friend was called in to do the necessary measurments and had to go there both in the day time and at night to get background readings. It is still being debated so there is no clear settlement.

The other involved the building of some acoustical barriers around a night club. The club started up about 7 pm and ran into the wee hours of the night. This club was close to a residential neighborhood and the people in the neighborhood were understanably upset with the noise and also when the owner of the night club decided to convert a back parking lot to a "outside patio" complete with outdoor speakers and sub woofers so that people could dance outside. My friend was called in to do some acoustical measurements. When the residents went to the city council and showed them the results the city council passed a noise ordinance that was contested by the night club owner saying that they passed the ordinece AFTER giving him a building permit to build the outside patio. It went to court and the night club owner lost. The night club owners solution was to build a 6 foot high barrier dam around the patio and then put a 6 foot wodden fence on top of the barrier. Since the weather got cold there has been no outside activity so the residents cannot access the difference. Next summer will be the next time they can check out what changes this will bring to the noise problem.

I wish you well and hope you can get this done and prove that this is a problem and get the plant owner to do some maintenance on his equipment that will allow it to function but quietly.

Please do yourself a favor and HIRE this done. The frustration that you will get from trying to DIY will not be good and you will NOT be able to go up against a hired gun that has all the time and equipment he needs to prove you wrong.

Before you do ANYTHING you should talk to a good lawyer to find out how much of a case you have. It will be money well spent.

:wink:

qveda Wed, 01/02/2008 - 14:20

Recording low frequencies ~ 10-100 hz

Found this thread through a google search for info on recording low frequency sources. Not trying for true pro quality. But I'd like to know if reasonable results can be expected using "conventional' gear to record low freq "found sounds" for my avante garde music.

e.g. if the mic, recorder (in my case Edirol R-09), and the rest of the audio chain is rated as 20-20Khz , or better, will that produce reasonable results ? (sorry, I know I'm using subjective terms).

Or, are there special considerations for making amateur field recordings for home studio music that would include low freq sounds?

thanks!
-Qua

taxman Thu, 11/29/2007 - 07:05

Based on the nature of this forum, and the specific question you posed, you are getting advice about how to record the offending sound. At this point, you shouldn't bother. Doing it youself will not be helpful to establishing a useful record, and hiring an expert may not be necessary to accomplish the result you want, which is remediation of the offending noise pollultionl.

The FIRST thing to do is speak to a competent lawyer in this subject matter. There is NO need to bring him any recording or other technical evidence. Your lawyer should certainly believe your representation as to the offensive nature of the noise. You must find out what remedies are available. I suggest an attorney familiar with land use and zoning issues, and environmental law.

You may find that the company has complied with every rule in the book regarding appropriate use of its facility. If that is the case, all the recordings won't help one wit. Of course, you could post them on this forum for all the recording gurus to hear, but that is hardly the point.

This will cost you some money, but the first money spent should be to find out what your rights and potential remedies are. Then you can decide if it is worth hiring the expert.

Yes, I admit it, I am an attorney.

Profile picture for user dpd

dpd Thu, 01/10/2008 - 21:19

To prove that the signal you recorded at the remote location matches the signal at the suspected source will, obviously, require recordings at both locations using an identical experimental setup. Then, you are part-way there. I would NOT recommend doing any spectral shifting. What I believe you must do is to cross-correlate these two signals - a process that compares similarities and then normalizes them in a spectral display format. Additionally, spectragrams (time, frequency, instensity) displays are excellent for this. Record as standard .wav files at 44.1 K and then get them processed by a degreed engineer using a software package called Matlab.

My best advice is to solicit the help of an engineering professor at the closest local university or, a Registered Professional Engineer. You try to go it alone, you'll have your lunch - and more - handed to you by any competent attorney.

Profile picture for user dpd

dpd Thu, 01/10/2008 - 21:51

Boswell wrote: If you want to go down to really low frequencies, then a velocity device like a Geophone is the thing to go for. It's used for such things as seismic investigations and quantifying the vibration effects of trains in tunnels on buildings above and around the tunnel. Best used with a data acquisition board (which goes down to d.c.) rather than an audio interface.

echo audiofire 10

Don't bother with the RTAjr gear, unless you are tuning up for a car stereo loudness contest.

Wow - I haven't seen a geophone used in over a decade. Piezoceramic sensors are lower cost to manufacture and can easily out-perform these things.

x