What is the most effective measurement process from a practical standpoint, for normal living conditions vs. a studio?
All just my quick and rambling random thoughts, not to be considered an expert opinion.
- Do you get all of your treatments out of the room and measure a baseline?
Hopefully you measure before adding a treatment and that is your baseline. In my case I did that, then a computer crash lost the data.
I have no plans to pull treatments down, however. At this point I want to optimize the room I have, could remove treatment if needed, of course.
- Do you measure one speaker at a time? From the listening position? From other positions? From how many positions?
I measure the L/R pair at the main listening position, then the L/R pair at other listening positions. I may adjust things to get a better average across the seats but in my case I have enough treatment that things do not vary much.
I tend not to worry much about surrounds more than a basic spot check. Their position is usually fairly fixed by the room layout and I am much less sensitive to their sonic attributes for movies. I will tweak the center to get it aligned with the L/R mains.
- Do you add your treatments one by one, and re-measure?
Well, this is a function of how much time you have and your knowledge of what needs doing. I would typically add a couple of corner bass traps and perhaps treat the first reflection points then measure again. From there it depends on what the measurements show and what I want in the sound. I would not generally add single panels (absorption or diffusion), usually go in pairs in corners and at the sides and ceiling. Unless you mean specific points by "one by one", as in treat the side reflection points, then ceiling, etc. I tend not to do that for lack of time (too busy, too impatient, take your pick
).
- What in the measurement data determines if you need to absorb or diffuse? Specific peaks or nulls?
Cost is often the criteria as diffusors tend to cost much more than absorption panels... However, in general higher-frequency issues can be dealt with by either type so it depends upon my budget and desired sound: diffusion for a more diffuse "all-encompassing" sound field; or, absorbers for a tighter image with sound field essentially determined by the recording. Diffusors must be a significant fraction of a wavelength so tend to get too large to help in the bass frequencies in most rooms, so absorbers are used. Of course, you need a lot to deal with a deep null...
Reverberation time may also play into my decision; I prefer not too long as it muddies the sound so in a very live room where I can't (or can't afford to) place diffusors everywhere I will lean toward absorption.
- How do you know from the data if you should tweak the position of the speaker or move the treatment?
Well, you can calculate wavelengths and see if moving things around will help. For bass room modes about all you can do is move the listening position (or place a sub near-field to help compensate a null). Drawing ray diagrams should determine where treatments should be placed so there should not be a lot of guessing... Again, speaker placement is usually dictated by the room and so minor speaker placement tweaks can be useful but chances are there's a fairly narrow range of adjustment.
- What data do you look at to determine if it's the floor, ceiling, or sidewalls that are causing problems?
Look at the frequencies and compare wavelengths to distances (speaker and reflection points to listening position). For room modes, room dimensions determine where peaks and nulls will be (physically and at what frequencies).
- How do you know from the data how many diffusors to add?
Based on initial measurements you may decide which of the first reflection points to trea. From there it is usually a matter of taste to establish the reverberant field desired. I don't have a specific rule of thumb for the number. Iwill typically treat first reflection points (side walls and ceiling) and the wall behind the listening position unless it is a goodly distance away. I will also treat the wall behind dipole speakers, unless the room is large enough to get them well out from the wall behind to minimize filter/cancellation effects.
- What don't you know from your measurement data?
Most measurement systems work with frequency response, leaving time domain a guess. REW and others do perform decay/reverb measurements so you can help determine how much diffusion/absorption to add at what frequencies but for most audiophiles determining exactly how much reverberation desired is probably not obvious. The final imaging and sound field (spatial field) is often hard to determine from FR and waterfall plots, though I am sure others on WBF are much better at reading and applying waterfall measurements than I (30+ years ago I did not have tools that generated waterfall plots so, while I have used them for RF and more recently acoustics analysis, they are still new to me).
I use a system that performs impulse and step (time) response as well as frequency response and waterfall plots. That said, except to time-align the speakers at the crossover point, I don't do much with it. Largely because my current room is fairly small and has enough issues that I just added absorbers everywhere.
Gotta' run, I am sure my much more learned colleagues will add a lot to this discussion! - Don
