RTA Questions

[strider445]

Hi guys,

Something seems a bit wrong with the RTA for me. The variance or dB range is about 15dB in a quiet room, and when I unplug the microphone completely, the graph still shows ~30dB and fluctuates rapidly. Additionally, I suspected something was wrong when testing my sub amp in the low range (20-150). I saw no change in the RTA (using the Behringer ECM 8000 mic with a M-Audio MobilePre). I'm hoping someone can help me or tell me what I'm doing wrong here. I'm using an old computer running windows XP and wondering if that has anything to do with my current dilemma. Thanks for the help!

 

[Wayne A. Pflughaupt]

Make sure Windows is recognizing the MobllePre as a recording device in the Soundcard settings (right-click on the Speaker icon, then left-click on “Recording Devices”). Here are John’s recommended settings from a number of years back.

Regards,
Wayne

 

[strider445]

Make sure Windows is recognizing the MobllePre as a recording device in the Soundcard settings (right-click on the Speaker icon, then left-click on “Recording Devices”). Here are John’s recommended settings from a number of years back.

Regards,
Wayne

Thanks for the quick reply. Do you have any idea why the RTA meter would be displaying fluctuating data when the microphone is unplugged? I can't seem to figure out why any data would be presented in the microphone's absence much less the dynamic fluctuations that are observed.

 

[strider445]

Am I correct in assuming that if the microphone is off and/or unplugged that the RTA should be a flat line at 0 dB?

 

[Wayne A. Pflughaupt]

I dunno, I’ve never unplugged the mic. And personally I am not curious enough to worry about anything I might see with the mic unplugged, since I only care what it gets me with the mic plugged in.

That said, the graph should be in the 25-100 dB range for any in-room measurement. If you’re scaled at -30 dB you’re probably, like, 100 miles below the noise floor of any device, so that’s probably what you’re seeing.

Regards,
Wayne

 

[John Mulcahy]

An unterminated mic amp input will be noisy and prone to picking up electrical interference from the environment. 0 dB is just an input level, like any other, the level on an input with no signal will depend on the noise floor of the input stage.

 

[strider445]

Thanks for the prompt and thorough replies to my prior inquiries. If my goal is to determine if the RTA is accurately displaying the sound in a room, what would be the best way to check that?

 

[Wayne A. Pflughaupt]

Keep the mic plugged in. If you’re checking room noise (i.e. with no audio signal from the speakers), scale your graph the lower limit of your graph no lower than about 20-25 dB.

For speaker measurements, scale the graph for 45 dB lower limit and 105 dB upper limit.

Regards,
Wayne

 

[Matthew J Poes]

Having spent a lot of time measuring room noise floor for my own sound proof theater but also a number of office spaces, studios, and other theaters, I've come to the conclusion that these inexpensive micriphones are inadequate. The setup you use is far better than the USB mics, but I still find they have way too much self noise to be accurate. After a great deal of frustration and feeling that what I was measuring did not equal reality both in spectrum and absolute levels, I rented an Nti room measurement system and took measurements of a number of rooms in my house, theater included. It seems, from what I can tell, that the inexpensive mics have a raised LF noise floor, overall raised noise floor, and frequently an uneven and raised high frequency noise floor. The USB mics look like a smiley face. The Mic you have looks like a hockey stick with a bump in the high frequencies around 3-5khz (at least on the sample I had). Overall the noise floor was about 10db's above the noise floor of the quietest room I measured and in other rooms that I measured noise hovered around the system noise at times. The noise floor of a typical USB mic seems to be in the neighborhood of 35-45db's on average with less noise in the midrange. The Behringer and Dayton non-USB mic seems to be between 25-35db's but with a bit less noise in the mid and high frequencies. The better mics in the 1K range seemed to drop this another 5-10db's, none of which is appropriate for room noise measurements unfortunately.

When I get a chance to go through all the data and create images or graphs I can post, I'll post what I found in my theater. Briefly I'll say that using the USB mic caused ARTA (ARTA autofits a noise curve) to score my room's NC level at NC40. Switching to a better mic improved things to NC35 and NC30. Switching to the NTi device gave NC20. I can only assume that the NTi, designed for this exact purpose, is the accurate device.