Psycho-acoustics
The human ear, and brain, is capable of great sensitivity and ability to filter nuances from what we hear. This is a wonderful ability that enhances listening to music but it can be very deceptive when attempting to use hearing to measure acoustics. Our ears don't lie, but we often hear only what we want to hear or even hear what we thing is there but is not.

Measurements
Precision, measurement instruments are necessary tool which, when used correctly and knowledgeably, can give us a more UN-biased perspective.

RPlusD / ETF-5
This is a very useful application for measuring room acoustics from Acoustisoft; written by Doug Plumb. However, room acoustics measurement is not a completely intuitive process and the application can feel complex because of it's many features. So, here is a simplified view along with a set of directions which may help to more quickly get some useful information about a room.

The Concept
Fundamentally, ETF sends a complex signal of pink noise and a swept-frequency sine wave to one of your monitor speakers and collects the room response signal via a flat-response measurement microphone. Note a few details about this:

• Note: Your monitor speaker is assumed to have an ideal, flat-frequency response. It doesn't ... but it is likely much better than the room's response -- so ETF ignores any speaker error. It is also very difficult to measure a speaker because you need a near ideal acoustic space and a precision, calibrated measurement microphone. This is another reason why good near-field monitors are well worth the investment.
  
• All the various plots are derived from the same measurement data. So, after everything is set-up, take a measurement and save the data. That data can be recalled later for further analysis.
• You don't need an expensive (>$1k) mic to get measurements of sufficient accuracy and resolution to find problems with room acoustics. The $50 price class Behringer ECM8000 does very well.
• Generally, take 3-5 measurements which all look similar; avoid or reject the really 'odd' plots as noises can disrupt the measurement. As long as you can easily get a 3-5 series of plots that look really similar, the data is probably a valid representation of room acoustics -- at that spot.
• Average 3-5 plots. Their differences are meaningless and just part of the noise.
• Measurement position matters more before the room has been acoustically treated. So, it is best to just place the measurement mic at the "listening position" and that will give you easier comparisons.
• An initial "loop-back" configuration with the sound card in your computer IS NECESSARY. This is how a reference is established for all additional measurements.
• All measurements are the ratio between the Left Channel Reference (hard-wired loop-back) and the Right Channel Measurement (using the microphone).