Originally Posted by baniels
I understand that this is just noise - but where is the noise coming from? Old thread - I know.
Here's an RTA with no signal - just ambient floor noise.
And this is the same window, with pink noise generated but with the receiver MUTED.
And this is a 0-500 sweep with mic and soundcard cals shown. Again, the speakers are MUTED, so there is no sound here other than the noise floor, which is a little above 40 dB.
I don't see how this could come from the soundcard calibration, as the soundcard loop measurement from which the cal was made is down 0.6 dB at 10hz and down 3 dB at 4hz. And the resulting loopback measurement after applying the calibration is flat as can be.
Could this be some sort of cross talk between the input and output within the UCA-202?
Here's another one.
These are saved from REW's RTA. Blue is just the noise floor of the soundcard, my computer, whatever. Pink is with full-band pink noise being output from the UCA-202. The inputs of the UCA are empty - no mic, nothing.
Doesn't this look like a big problem? Even at high measurement levels (which I'd prefer to avoid), at what point do you think these results are trustworthy?
Here is a combined chart of the above with some close-mic measurements of 0-500 sweeps (on top). Not exactly the roll-off I'd expect for a sealed box.
If you use a mic with a variable preamp, you can calibrate your mic at a higher level to run your mic signal hotter if you want to better signal to noise numbers at low measurement levels.
The issue is that the apparent (because I only see it on some systems) noise floor of the UCA202 starts to rise below about 80 Hz, and gets to a point where the headroom available for measurement drops to around 25 dB by 15 Hz, which buries a low-level signal below the noise floor.
Yes, this is definitely a limitation when it comes to measuring things at low levels below 40 Hz or so, but as I said in my previous post - I only see it with some combinations of hardware and software. I am looking into this as I can.
I confirmed that it happens with my XP laptop while running both the Java and the Behringer ASIO driver. With the same laptop and different sound cards, I see "proper" noise floor behavior. I can also confirm that I am seeing similar behavior in both REW and ARTA under XP using the Behringer ASIO driver, so this is not an REW issue. So far, I am not seeing the same behavior with Windows 7 (32-bit) and either the Java or the ASIO4All driver, but I have not tried the Behringer ASIO driver under Win 7 yet.
Anyhow - this seems like "new" behavior to me. While I don't have screengrabs to prove it, I don't recall a noise floor like this in my early use of the UCA202. One of the first things I do with any sound card I'm planning to use for acoustic or electrical measurement is a set of basic measurements: Loopback response, noise floor, and a quick check of THD at 0 dB and 1 KHz.
While it isn't as good as my Emu 0202, the Behringer measured well enough for me to work with it and recommend it for several years, and it is widely available for less than $30. Since I don't see it in all cases, I do not think it is hardware-related, so I hope this is as simple as a setup or driver issue.
Anyhow - I am looking into this, hope to have more information soon.