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Recording Advice Needed

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sjohnson
Recording Advice Needed
on Nov 26, 2007 at 4:13:49 pm

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.

Any advice would be greatly appreciated!


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Matte
Re: Recording Advice Needed
on Nov 26, 2007 at 11:43:28 pm

A good "studio condenser" mic will have the wide frequency response and high sensitivity to pick up any audio that's there, but...

No matter what kind of mic you choose, the resulting audio from just recording the "room tone" will not represent any kind of actual conditions in the home (or other location.)

You will likely have to increase its level by many times to allow the low sound it to be heard clearly on the audio track.
There would be no "reference" to tell just how loud the actual audio was at the location. (The playback system would also be a factor in being able to hear the sound clearly (if at all.)

---------------------------

For "legal" purposes, you need to contact a company that can bring out a Sound Pressure Meter (or Sound Level Meter) that can determine and isolate the frequency and amplitude of a given audio source.

Those kind of SPM units are calibrated to a reference level before being used in the field, so that the info they provide is useful in a laboratory, case study or courtroom.

The SPM's also usually have an audio (mic or line-level) OUTPUT, so that a technician, audio engineer or camera operator can record the audio that the unit's mic is picking up.

You could then state (in numbers) the actual measured dB level of the sound, and play your "enhanced" version of the sound for demonstration purposes.


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Ty Ford
Re: Recording Advice Needed
on Nov 27, 2007 at 2:34:42 am

Rent a Neumann TLM103 with a suspension mount with a pop filter and a digital HD recorder witth 24-bit 44.1 kHHz or better capacity with a good mic preamp. Get a set of Sony MDR 7506 headphones.

Go into each house and record a sample then turn off the HVAC and the refrigerator and any other motor-based devices and record again with the mic in the same place. Turn the mic to different positions.

Setup outside each house and record again.

1/4 mile is a long way for LF to travel. It'd have to be pretty durn loud at the site itself.

Regards,

Ty Ford

Ty Ford's "Audio Bootcamp Field Guide" was written for video people who want better audio. More at: http://home.comcast.net/~tyreeford/AudioBootcamp.html
or http://www.tyford.com


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sjohnson
Re: Recording Advice Needed
on Nov 27, 2007 at 3:34:25 am

So, I like the idea of using a SPL meter. I think this makes a lot of sense. But I'm pretty sure the one I have will not be good enough. I have a regular digital one you can buy at radio shack. Says it has a range of 50-126dB, but I don't think it has the accuracy I need. (+- 2dB @ 114dB) That's pretty high for a sound that is so quiet. I need to do some "google'ing" on that one. It could be a good way to get a reference sound level, but it would be a level of the room, not just the tone. (at least with the one I have) That's a very interesting train of thought. I wonder if anyone reading this might have a reference for this. Sounds like I may be searching ebay..

I have a few good condenser mic's, although they would have to be amped up pretty hot to hear the drone. I'm concerned about all of the hsssssssssssssssssss I'm going to get that might just step on the low-freq sounds. I guess I was hoping for an easier answer with a specialized mic, but I have to say I expected that this was the way to go for picking up the audio. My yamaha preamp and mixer might not be up to the task of driving the gain up to --30+'ish. (yeesh)






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Ty Ford
Re: Recording Advice Needed
on Nov 27, 2007 at 3:51:08 am

[sjohnson] "I have a few good condenser mic's, although they would have to be amped up pretty hot to hear the drone. I'm concerned about all of the hsssssssssssssssssss I'm going to get that might just step on the low-freq sound"

Like I said, use a Neuman TLM 103.

Ty Ford

Ty Ford's "Audio Bootcamp Field Guide" was written for video people who want better audio. More at: http://home.comcast.net/~tyreeford/AudioBootcamp.html
or http://www.tyford.com


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Nate Graham
Re: Recording Advice Needed
on Nov 28, 2007 at 4:26:43 pm

[sjohnson]"I'm concerned about all of the hsssssssssssssssssss I'm going to get that might just step on the low-freq sound"

For those rare times when I've needed some serious gain, with extremely low noise, I've used a Millenia pre amp and been very impressed. That is one pre amp that will not add any other noise to the recording. But as Ty said, you're going to need a quiet mic, too.


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zach smith
Re: Recording Advice Needed
on Nov 28, 2007 at 5:26:50 pm

If you are recording this for legal purposes I would imagine you would need to use a mic with a flat frequency response or a reference mic, some manufacturers are Earthworks, or DBX, the 103 drops in frequency response from 70 hz to 20 hz by almost 10 dB and the charts dont even show below 20 hz for the 103. Since this is a lf hum and is traveling 1/4 mile you're probably looking at very low frequencies so you would need a mic that is accurate in this range.


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