Is there a way to test for "low frequency noise in the room"?
Yes, use the REW Spectrum Analyzer.
Once your levels are setup for a normal response measure, then select the Spectrum tab and the following settings.
Mode = Spectrum, FFT Length = 65536, Averages = 2 or 4, Window = Rectangular, Y Axis = dBFS
Select the Signal Generator and select WhitePN (periodic White Noise).
Press the RED record button on the Spectrum page.
It lets you examine the room for noise energy that you may not be aware of.
Below is a pic of my office with the Spectrum analyzer running with no signal being generated.
You can see my furnace is introducing some low frequency noise. If I shut it off, the noise is eliminated.
You can see I have the typical poor PC computer 60Hz hum and its related harmonics at 120Hz, 180Hz, 220Hz etc.
You can also see (amazingly), my NTSC RPTV CRT's horizontal oscillator frequency of 15.750Khz present in the room.
The TV is several rooms away and happened to be turned on, but the oscillator noise is present in my office (not that I can hear it, but there it is). Shut the TV off and it goes away.
The Spectrum analyzer can tell you a lot about your room and system before you even do a measure.
Sometimes you'll see a waterfall with a strange signal that rings out for many hundred of milliseconds. If you looked at the spectrum, you might see the source, rather than incorrectly deciding it comes from a modal resonance.... My furnace is a case in point on that issue.
The RTA is also quite useful in REW for dynamic measurements, rather than a simple static measure.
Below is a response measure and then a quick RTA. Once this is running, I can then move the microphone around to see what different listening positions look like (all in real time). It is revealing to see the difference a few feet make when you move the mic, and to compare against the simple response measure on the screen at the same time for comparison.
The RTA also allows you to change filters or move equipment and watch the RTA screen for the real time changes. It's also really useful to adjust phase on a subwoofer for the best crossover response. You simply watch the RTA screen as you dial the phase control. A lot better than taking a bunch of measures to accomplish this task.
Here's a pic of RTA and a response measurement on the same screen (measurements are selectable to be placed on the RTA screen for comparison).
I was sure not to move the mic when I took the measure and then started the RTA.
Pay attention to set the RTA up properly. See the setup values on the plot below. (note that the RTA requires switching the Signal Generator to PinkPN (periodic Pink Noise)
I understand having some room gain, but 70-72dB at 18Hz!?!
Well it's about 15dB above the noise floor. Remember, if there's a high noise floor (typical 45dBSPL, but can be much higher), then the meter calibration file will raise the signal level as the frequency drops. A response chart signal can be mis-interpreted as actual signal, when it's really just noise being raised by the meter calibration file. Consider an old analog Radio Shack meter is adding +30dB offset at 10Hz from the cal file. A high noise floor will show as signal. This is the case with the MBM plots on page 1 of this thread. See how the signal drops nicely until about 30-35Hz and then begins to rise as the frequency goes lower than that. The signal below the drop is noise......... ignore it.
brucek