Strange spike on RTA.

[ozzy9832001]

Hi all,

Today, I noticed a few strange spikes when just having the mic doing an RTA without actually playing anything. I noticed the noise floor was acting strangely, so I tried to do some investigating and I've run out of solutions.

According to the RTA I have this very weird spike at 58hz and 107hz. I figured the 60ish hz one is probably power related, but I can't figure out the 107hz one. I thought it might be the microphone malfunctioning, but a simple RTA app on my phone shows roughly the same results. Then I thought ground loop, but I don't hear a hiss or anything coming from my speakers, certainly not at the levels it proports to be at.

Basically, what I'm trying to figure out is if this is throwing off the measurements in any (no pun intended) measurable way.

Any thoughts on this one?

 

[Blumlein 88]

Maybe some low frequency noise getting into the room and those peaks are resonant modes of your room. Do wavelengths for 58 and 107 hz work out to something near your room dimensions? Also try again later. Maybe noise of a distant loud object (someone's outdoor AC unit next door) or a fridge running you didn't notice.

 

[sam_adams]

It will have minimal effect on your measurements. Understanding how 'decibel arithmetic' works is the key to assuaging your fears.

Decibel Calculator - dB Calculator - Addition and Subtraction of dB Values

The Decibel Calculator shows the addition and subtraction of dB values in the usual acoustic range of 0 to 200 dB. An explanation of decibel addition and subtraction is included.

www.noisemeters.com

 

[boxerfan88]

Try using your phone measuring app and measure, if the spikes are also showing up in the phone measuring app, then the mic is good.

I am guessing here -- could the second spike be air conditioning or fan noise or mechanical noise from some equipment in the vicinity ?

 

[ozzy9832001]

Try using your phone measuring app and measure, if the spikes are also showing up in the phone measuring app, then the mic is good.

I am guessing here -- could the second spike be air conditioning or fan noise or mechanical noise from some equipment in the vicinity ?

I probably should have been more specific when I posted. I was half asleep.

At first, I thought the 100hz noise was the PC fan(s), maybe the motor from one of them. I cut the breaker to the room to see if that helped, and it didn't, so there has to be something else in area or something in the line. Maybe something in the USB.

More than it impacting the sound or measurements, I was more curious what it was and if anyone had any idea because I thought it was odd.

Not sure huge deal, but thought it was a cool investigation, but I'll be damned if I can figure it out.

 

[DonH56]

They are close enough to power-line components of 60 Hz and 120 Hz (e.g. created by a full-wave rectifier in a power supply) that I suspect the measurement time base is off a bit, or perhaps those are the FFT bin centers, and you are really seeing power supply bleed/leakage from someplace in the system. Could be an external source, like a fan or fridge, or internal like the power supply for the mic or mic pre (or anything else along the way to the RTA).

 

[ozzy9832001]

Found it!

What a journey.

After killing the entire breaker to the whole side of the house, it finally went away.

There is a roof exhaust fan in the crawl space which had the wiring duct taped together (unreal). Never noticed it before when I was up there because the wiring is buried. I can't believe it didn't catch fire. After removing the duct tape and using properly wire connections, it went away. The wiring was tied into the same run as all the wiring for the outlets in my office.

So, while it may not have caused any sound issues, it definitely led me to an obvious issue which very well could have started a fire.