[Carey Carlan]

"HiC" wrote in
oups.com:

On Feb 6, 7:06 pm, Carey Carlan wrote:

Noise increases 25 dB when microphones are attached but muffled in
pillows and sleeping bags. 25 dB is a LOT of noise to add. Have you
looked at the frequency spectrum of the noise? Is it heavily
weighted to treble or bass or is it wideband?

Running spectrum analysis on one recording of "air" in the room shows
"prominent frequency -79 db at 47hz" I got another that indicated -80
db at 22hz. While not exactly clear how this is arrived at as a
"prominent frequency", I gather it's pointing at the low frequency
end?

Apparently you're correct. I find that just by engaging the 70hz
rolloff switch, it drops the noise floor by about 5db. Fooling with eq
drops it even more. This seems promising. I'll be curious to see how
close I can get toward my goal just by tweaking the eq & optimizing
mic placement.

This is typical and harmless. Unless you're recording something really
deep like bass violin or organ, you can roll off everything 47 Hz and
below. Likely culprits are your furnace, refrigerator, traffic, low flying
helicopters, etc. which are gently shaking your house.

My "rule of thumb" bass rolloff is an 8th order Butterworth filter below 64
Hz.

In fact, you'll want to start your high pass filter higher. I edited a
concert this week of ladies' chorus singing a capella (no instruments) with
a rolloff at 140 Hz. As long as you leave the fundamental of the lowest
note, you're usually OK.