Mystery low rumble

[Elly]

Can you help me identify this strange low rumble I seem to be getting in my recordings?

I noticed it when I was editing. On silent bits, the waveform sometimes seemed to be very lively. Peaking up to even -3db. I couldn't hear anything, there was no actual noise there, just these strange peaks. Then the silences would go back to just their normal peaking at -30/-25db ambient background. It turns out, you can actually hear it if you turn up the volume. It's a kind of a low rumble. With headphones, it sort of sounds like the workings of my inner ear. Weird. So must be a low frequency thing.

I have uploaded a sample from when I was about to start recording today and was getting some roomtone. It was particularly noticeable. You'll hear a couple of bits of me talking to myself (!) in this recording. And, at the end, I've just cut in a very short piece of my actual recording I did today, so you can hear what my set up sounds like when I'm recording something. It's for an audio book (not music).

MY EQUIPMENT
Microphone: Sontronics STC-2 Cardioid Condenser
Amp: ART Tube MP Tube Mic Pre-Amp
Soundcard: Creative Soundblaster X-fi Titanium HD
Software: Adobe Audition 3.0
Connections: XLR, apart from two phonos into the soundcard

SET UP
Recording in a separate room from the computer (due to fan noise). Shielded cable runs out of one window into another (it says Canford HST on it, recommended by a BBC engineer)
Microphone sits in a cradle (the one that came with it) on a tablestand
Monitoring via remote desktop over wi-fi via MacbookAir, plugged into mains
One reading lamp (LED, but was same with my old lightbulb lamp), plugged into mains
Kindle to read from on battery power
Everything else in the room unplugged from the mains apart from two table lamps over the other side of the room which are turned off
Sound dampening supplied by a homemade "tent" made of felt material and suspended over two clothes racks

Would be grateful if you could give me some pointers.

Jane

 

[gecko zzed]

Do you have a draft in your room?

That's the kind of noise I get when I leave a window open and I get the faintest of breezes blowing into the mike.

 

[JimmyS1969]

That definitely sounds like air movement to me as well as some transfer of vibrations from the desk at the beginning. Easy fix though is a high pass filter (HPF or HP). You will want to use one to some degree on just about anything you use a mic for.

Description HERE.

Usually for vocals you will want to pull up the frequency of the high pass point until it starts to affect the sound of your voice (the low end gets thin), then pull it back to desired frequency. Depending on the voice, this may be anywhere between 100-400Hz. Way above the rumble frequency which will then be so far below the recorded sound that it will not be an issue.

I would also get that mic off the desk stand and use a boom stand on floor so that any kicking or arm resting on the desk will not transfer to the mic.

 

[Elly]

Thank you for all your comments.

I have discovered there is a high pass filter switch on the microphone which was turned off. This filters at 75Hz. I shall try flipping the switch this evening to see if it gets rid of the problem.

I have just bought the desk microphone stand as those boom arm microphone stands (one of which I have) is useless for this type of microphone. The microphone is top heavy and falls over, often bashing itself on the table. The spider shock absorber (cradle) should avoid any rumble coming up the stand.

The noise is definitely not wind noise, which is actually audible on a track (where this is kind of sub-audible, if you know what I mean). Besides, in my recording tent inside a room with closed windows, closed curtains and the door closed, there is no wind!

Hopefully, the switch will sort it. I will update you on success or not