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.
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.