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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

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.

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

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.

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.

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. :)

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