Room acoustics

Shadorne,
Thanks for pointing this out: indeed there are other systems as well. REW is what I use and, I think, most people. Of course, there are also other automated measurement and equalization systems (I use one myself). My plan was to leave those for later.

OK, for a kick off: measurement. To measure and interpret in-room response, you need a measurement microphone and a system to generate test signals, to record the response, and visualize those data in a graph. These days, that can all be done with a free software package: Room Equalization Wizard (REW), for Windows, OS or Linux. All you need in addition to a computer and this software is a calibrated measurement microphone. Personally I use the microphone that came with my Antimode 8033 room eq (plus its MicAmp), but for most people the calibrated UMIK-1 usb microphone is their best bet, at $75.

By the way, is there a way to post graphs here?

The discussion about weighting is how much different frequencies should be weighted to measure total sound level. That is a different issue and not relevant here.

The result is likely to horrify you, and serves to underscore the importance of speakers and of optimizing their in-room response. It is not unusual to measure in-room peaks and dips of +/- 10 dB. Compare that to the peaks and dips of good electronics - usally +/- 0.2 dB over much or all of the audible frequency range. That 0.2 dB limit is important because it represents about the smallest level difference that humans can recognize. So the peaks and dips in listening rooms are many times larger than that. A significant part of those unwanted deviations from a flat frequency response originates in the room itself, and not in the speakers. Good speakers are not nearly as flat as good electronics, but usually manage to stay within +/- 3 dB for much of the full range of human hearing. Usually only the bass region is much lower. Compared to good electronics, +/- 3 dB is still not very good, but far better than their real life performance in an actual listening room. So the challenge is to get them as close as possible to their best performance in an anechoic room, or free field outside in the open air.

And here for tips from the AV Nirvana forum: https://www.avnirvana.com/forums/official-rew-room-eq-wizard-support-forum.10/

See here for some practical advice about using the UMIK-1 measurement microphone with REW: https://www.minidsp.com/applications/acoustic-measurements

I bit of bass lift is not quite the same as a bass peak. These peaks generate one note bass.
As a practical solution in real life where you want to maintain an elegant decor, I think the combination of two smallish subwoofers plus an automatic equalization system like the Antimode 8033 is quite realistic. It does not involve ugly bass traps or huge subwoofers, is not a great hassle to set up at all, and is relatively affordable. Depending on room size and desired levels of bass extension and loudness, two small subs and an Antimode 8033 would cost from about $1300 (more if you want the subs to go lower and louder to fill a bigger room). Push the subs into two adjacent corners of the room, do the Antimode measurement ritual (half an hour’s work, including reading the manual), set the level and crossover (that is the hardest bit), perhaps with a measurement microphone, and you are done. With my B&W PV1d you get precise instructions for slope, phase and crossover settings for the specifications of your main speakers. They turned out to be spot on.
In my case I am still using only one sub, and even then the equalization is a big improvement. The Antimode allows you to optimize for a wider area, and that has worked pretty well. Even so, a second PV1d sub is the first item on my shopping list.
As for the additional analogue to digital to analogue conversion, with the Antimode 8033 this only happens in the bass region reproduced by the sub(s). The main speakers receive the same signal as before.

I have to sign off now (it is evening where I live) because I am expected at a birthday party. Please continue.

Indeed Randy (I have an open plan as well), but real life rooms are almost invariably more complex than simple models. Therefore, rather than measure dimensions, I prefer to measure (and adress) the response.

OK, maybe we need to look into that then, because graphs are very illustrative of potential problems. Most telling are waterfall graphs, as these show that peaks also linger on. If people talk about slow bass performance of a speaker/sub they are wrong to blame the speaker. It is the room that makes the sound linger on. And that is very audible.

This may be the moment to introduce the so-called Schroeder frequency, named after the German physicist Manfred Schroeder. The Schroeder frequency denotes the crossover frequency between the chaotic behaviour of sound waves above it and the discreetly spaced peaks and dips/nulls below it. Above the Schroeder frequency sound waves bounce around the room, producing many small spikes that are closely packed along the frequency range. Below the Schroeder frequency the peaks occur at the resonant frequency of the room's dimensions, and create big peaks at particular frequencies and again at their upper harmonics. These so-called room modes are large and pretty far apart, so they are quite obvious to the listener, with a boomy bass that often lingers on at particular frequencies. See here for more explanation: https://www.soundandvision.com/content/schroeder-frequency-show-and-tell-part-1
The Schroeder frequency can be calculated approximately, and depends on a room's dimensions. The larger the room, the lower the Schroeder frequency, from about 200 Hz in smallish rooms down to about 100 Hz for a large room. See here for a calculator: http://www.mh-audio.nl/sg.asp
Knowing the Schroeder frequency of your room is important, because treatment above it has to be quite different from treatment below it. See here for more discussion: http://www.linkwitzlab.com/rooms.htm
To put it simply, treating the frequencies above the Schroeder frequency is a matter of adding damping material like rugs, bookcases etc. It can often be achieved without too much intrusion into the style of your home decor, although there are also more visually imposing solutions from the world of studio design. The exception to the idea that dealing with these higher frequencies would be quite easy is if like me you prefer a modern minimalist interior. Such rooms have a hard acoustic and softening the acoustic without changing the style of the interior is a challenge.
Conversely, adding rugs and the like to reduce peaks below the Schroeder frequency is pointless. What you are dealing with here is big resonant peaks that can be tamed in only three ways. The first is so-called bass traps. Unfortunately at the frequencies we are talking about these bass traps are necessarily large and ugly. The second is multiple subwoofers. Main speakers have to be located for best mid range response and imaging, and that is not necessarily best for bass reponse. So separating bass response from the main speakers allows you to locate the bass speakers at the best spot for them. Traditionally HT subwoofers were used alone, but it is now increasingly understood that using multiples smoothens their response because their response peaks and dips do not coincide. See here for some explanation: http://www.acousticfrontiers.com/20101029using-multiple-subwoofers-to-improve-bass-the-welti-devanti... You get twice as many peaks, but of much smaller amplitude, and that sounds a lot better. To be sure, this is still not an argument in favour of stereo subs. It remains true that at these low frequencies sound is not directional, so dual subs are mostly still connected as mono subs for a somewhat smoother response compared  to stereo subs.
Finally, remaining peaks may be equalized by dsp eq units like the Antimode 8033 for subwoofers. The results can be quite stunning with a far tighter and more tuneful bass. The limitation of room eq is that it works best in only one listening position. The smaller the room and the higher the frequency that has to be equalized, the more localized the good result. Using two or more subwoofers gives a good result over a much larger listening area than with a single sub.
It should also be obvious that a larger listening room is highly beneficial. Its Schroeder frequency will be much lower. And the lower the room mode frequency, the less obtrusive it feels. Moreover, equalizing a lower fequency works over a much larger listening area than equalizing a higher frequency.
So, to be honest, good bass reproduction in a small listening room of, say, 10x14 feet is not really feasible. There is no space for bass traps, room modes are at too high frequencies (and their upper harmonics even more so), and equalizing them only works for a very small listening position. In my view, the simplest solution for small rooms is to just use little monitor speakers without too much bass output. The brain is pretty good at imagining there is bass when there really isn't much of it.

REW is indeed very good. And to see what is going on the waterfall graphs are most instructive. They show that peaks are not just peaks but are also an issue in the time domain, by lingering on and giving you that feeling of ’slow’ bass.
For those of you who only use a computer as a source (as I do in my home office), equalization is very easy. Just ask REW to create an equalization graph of the range below about 200 Hz and download that file into the Equalizer Apo software on your computer (aided by the Peace interface). Make sure that you turn off the default shelf filter that boosts the bass, if you want a flat reponse.
In my case I was using my Harbeth P3ESR mini monitors as desktop speakers (with the tweeters raised to ear height by IsoAcoustics desk stands). Unfortunately the proximity of the desktop still created a little bass hump that I could equalize easily for a rather tighter and cleaner sound. If you use more sources than just your computer, you will have to download the equalization file into e.g. a miniDSP and fit that somewhere in the signal chain.

For a list of BBC research documents on room treatment, see: https://www.gearslutz.com/board/bass-traps-acoustic-panels-foam-etc/476018-bbc-documents.html

And for those of you with decent French there is a wealth of information here: http://www.homecinema-fr.com/forum/acoustique/

Thanks very much. This is interesting. You can clearly see that things get dirtier below the Schroeder frequency, even with your orange tree (wish I could have them in our climate). If and when you use a computer as a source, you could use it to equalize the low frequency response. If you like that, you could get a miniDSP to do the same with other sources.
I also like the clean decor, but yes hard surfaces like glass windows also make for a dirtier response. No equalization will help you there, only soft surfaces etc. There are some minimalist looking slotted panels etc that apparently dampen mid and high frequencies (often used in modern meeting rooms).