Recording Advice Needed for tracking a low frequency sound

Posted: Wed Nov 28, 2007 8:57 am

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.

Posted: Wed Nov 28, 2007 11:03 am

VonRocK wrote:

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.

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.

Posted: Wed Nov 28, 2007 12:02 pm

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.

Posted: Thu Nov 29, 2007 6:54 am

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.

Posted: Thu Nov 29, 2007 8:00 am

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.

Posted: Thu Nov 29, 2007 8:30 am

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:

Posted: Thu Nov 29, 2007 9:05 am

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.

Posted: Wed Jan 02, 2008 4:19 pm

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

Posted: Thu Jan 10, 2008 11:19 pm

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.

Posted: Thu Jan 10, 2008 11:50 pm

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.

http://www.geophone.com/Category.asp?CatID=1&SubCatID=3

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.

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