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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!


rockstardave Mon, 11/26/2007 - 14:09

wow great question. i'm anxious to see what everyone thinks.

i'd assume some sort of large diaphram condensor microphone, but i wouldnt know which ones to go to...

speeddrmmr1200 Mon, 11/26/2007 - 22:35

i would think if you recorded something you cant hear....your not going to be able to hear it. there are tools you can make to show the existance of the frequency being there but what you hear is what you get. if its possible id say an ultra sensitive large diaphragm mic with basically no self noise

stjohnson Mon, 11/26/2007 - 22:49

I can actually hear the drone. My wife acts like she's straining to hear it, and then says it wouldn't bother her. I guess we have differing levels of tolerance, but it is definitely a noticeable frequency present. 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. That's the best I can describe it. I guess I'm more sensitive to it being more of an audiophile than my wife.

I've been pointed to a Neumann TLM 103 as a good mic to pickup low freq with a very low noise floor. I might give that a shot, but the problem is that it won't be representative of the actual db or sound level of the noise. I need to have in addition to a recording sweetened to highlight the hum, some sort of measurement of the sound. So I'm researching SPL meters now. The one I have from Radio Shack is not going to cut it. But perhaps they make one that I can tune to a specific freq (or something, maybe octive) and get a measure of the actual sound and frequency. I think this will hold up more in court for my client than a general recording would. Or at least it would add some merit to the recorded track.

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Boswell Tue, 11/27/2007 - 05:32

You need a measurement or calibration microphone coupled to a recorder with an adequate low frequency response.

The Behringer ECM8000 is a perfectly adequate measurement microphone, but needs phantom power, which limits your portable recorder options. Don't get the Zoom H4 recorder even though it can supply phantom power, as its low frequency response is not up to the task in hand.

Conventional microphones such as the the Rode NT4 have a low 3dB point of 20Hz and can be battery powered, so one of those running into a Sony MZ-RH1 minidisc recorder would do your job.

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JoeH Tue, 11/27/2007 - 05:54

Actually, you'll need a SMALL Diaphragm microphone, something like the earthworks or the DPA 4006's. THese microphones were originally calibration mics, and go all the way down to 20 HZ and a bit lower, and you'll need tha accuracy when doing something like this. (DPA used to be B&K - which is now exclusively a calibration & testing microphone company.)

Believe me, I understand EXACTLY what you're trying to capture here. I'm currently sufferening from something very similar to this. About .2 miles from here is a new construction site where they are building a 2 or 3 story office campus. It will be fairly eco-friendly and will be tucked behind a huge berm and lots of trees, but in the meantime, the construction process is intense/insane.

Any given day, there are at least two to four large earth movers, backhoe's and levellers pushing dirt around and digging trenches. The beeping is one thing, but the combined low end is absolutely astounding. The morning hours are the worst; I can barely sit in my studio (which at the moment is a semi-finished garage, while I'm buidling my new space). Delivery people have stood at my front door and remarked how bad the bass is at my house (which seems to work as a resonator) and how they can't hear it on the street - even CLOSER to the construction site! If you stand outside, in front of my house, it's not bad, but when you come INSIDE and feel the resonance, it's ungodly.

Fortunately, it won't last forever, and they're done working most days by 4, so I'm trying to take it all in stride.

But in your case, you've got to document this acoustically AND visually. (With charts and graphs, etc.)

If there's an attorney or class action suit involved, you should get some "Up front" money to cover your expenses, perhaps from the law firm handling this.

You're going to need the following to do this properly:

Professional Mic's and pre's with a Firewire or USB (Digital!) interface to your computer's recording software. (Do NOT use the analog inputs, no matter how tempting) to capture this low end properly. I'd suggest renting it all for a couple of days, esp if you're not going to need this beyond the capture/documentation stage.

Software to properly show the frequency response in a vivid, visible way. Any good spectral editor will really wow them. I have Algorithmix (which is about $3k stand alone, you can probably get the same thing from Adobe's Audition - a spectral editor/viewer that does similar stuff for a lot less). You will want to do a screen capture and print it out too.

The low end stuff will be immediately visible to anyone, including audio amatuers, with just a little bit of help: The spectral analyzer shows them frequency, time and amplitude in a very easy to understand way. (Left to right is time, Top to bottom is freqs., and COLOR is the amplitude/intensity.)

And last but most importantly, if you're going to show anyone (a judge, an attorney, or the company owner) what it all FEELS like, you're going to have to get a subwoofer (with calibration documentation to prove you're not hyping it!) to reproduce this sound in a neutral environment, away from the er, crime-scene. Perhaps a local studio or someplace that's very quiet and still. - when you switch on the Low end noise, it should be dramatic and very impressive.

Then while the noise is on, perhaps you can attempt to show them what it's like to live with this sound day in and day out. (Maybe a tv show, phone call, or even a simple meal with this going on.) If you're creative, you can possibly show something even VIBRATING to the low end to give it visual impact.

I would think the first person you should talk to is the attorney for the person with the machinery. A couple thousand in rubber motor mounts or factory retro-fits would be alot cheaper than a lawsuit, then LOSING the lawsuit, and then paying for the repairs anyway.

Good luck, and drop me a note if you want any more ideas on what it's like to live with constant low end noise. :roll:

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MadMax Tue, 11/27/2007 - 05:55

I'm not an expert in forensic audio, but these are the issues as I see em';

Your recordings will have to be made for at least for 2-24 hour periods...

You will have to record the rooms/structures in their "natural state"... e.g. all standard appliances, etc running. Then you will have to record with all of said appliances off.

Then you will have to produce waveforms that show the difference.

As you pointed out, you are looking for a microphone that has a much lower frequency cut-off than your typical mic. This isn't going to be available in your typical banjo center. If the frequency is actually as low as I think it is, you are likely looking at a fairly sophisticated transducer that would be better attached to the structure than a microphone in free air.

The first thing to do is forget doing this yourself. Hire a licensed acoustical firm to come do the testing. Since they do not have a vested interest, they will give you both the source of the noise and a plausable solution. Besides, they already have all the gear to monitor and test.

Split the cost of their services among the neighbors who have a complaint. You should get reimbursed by the court when you take a class action suit against the warehouse.

Even if you are able to record this accurately, but then enhance the problem frequency in the wrong way, or present it in the wrong way, you will lose your case as soon as it's presented.

The frequency of the noise is the issue. Think about how low this frequency is. It's traveling over a quarter of a mile! This falls in the range and classification of industrial noise. Realistically, you don't stand a chance of fighting this without proper professional assistance.