None of them are designing for listening to playback in the home. Those that are, and who have based their approach on validated research, Toole is not inconsistent with.
Read the recommendations in his books. They are not recommending "thin absorption on part of the side walls", nor "bare sidewalls".
However, he is right to point out that acoustic consultants coming out of the pro audio space are not well equipped for consulting in the home audio realm, although they think they are. He wisely advises home audio enthusiasts not to listen to them or adopt their studio/pro audio solutions.
cheers
I will go out onto the proverbial shaky limb and say that
the single most important thing to avoid is the standing waves that set up between two large parallel walls or between the floor and the ceiling. The room needs to be irregularly shaped, with no parallel walls. If this is accomplished, the need for sound absorption is greatly diminished, and it won't make much difference whether the absorption material/object is fixed to the walls, suspends from the ceiling, or sits somewhere in the middle of the room.
The first standing wave for two parallel surfaces occurs where 1/4 of the wavelength is equal to the distance. The next one is where 3/4 of the wavelength is equal to the distance, then where 5/4 of the wavelength is equal to the distance, etc. In other words, the wavelength (WL) of standing waves that set up between two parallel surfaces separated by distance d are given by the expression n/4 x WL = d, where n is any odd integer. Rearranging this expression, the wavelengths at which standing waves will set up between two parallel surfaces separated by distance d are given by the expression WL = 4d/n.
The "to be avoided" rule for room dimension ratios is the same as for the interior dimensions of speaker enclosures. What is to be avoided is the situation where standing waves of the same length will be associated with two (or more) of the dimensions. This obviously will occur if two of the dimensions are the same, but more generally it will occur if the expression 4d/n is the same for any two dimensions, where n is any
odd integer and
generally is not the same for both dimensions. If we let d1 and d2 represent any two room dimensions, and n1 and n2 represent any two odd integers, then we want to avoid satisfying the equation "4 x d1/n1 = 4 x d2/n2". Manifestly, the '4' cancels out of both sides, leaving us with "d1/d2 = n1/n2".
Thus, the dimensional ratios that are to be avoided are the ratios of odd integers. This includes the cases where one of the odd integers is 1, but also the cases where one of them is 3, or where one of them is 5, etc.
The bad ratio that you are most likely to encounter with room dimensions (ignoring the floor-to-ceiling dimension) is 3:5. Fortune smiles upon you if the room available to you has a vaulted ceiling. If so, you will most likely have at least one pair of parallel walls, probably two pairs, and will need to deal with them. But most people aren't blessed with a vaulted ceiling, and will more likely have to contend with a low drop-down ceiling built within a basement corner. In this situation, you absolutely, positively want to avoid the situation where the standing waves associated with the floor-to-ceiling distance also occur in association with the distance between either pair of parallel walls. And it can potentially occur with both pairs of parallel walls, even if the two room dimensions are different.
If the ceiling height is H, then you want for neither of the other two dimensions to be equal to H multiplied by any of these ratios: 5/3, 7/3, 7/5, 9/5, 11/5, 13/5, 9/7, 11/7, 13/7, 15/7, 17/7, or 19/7. If you study these ratios for a moment you will realize that the ratios you get with a small denominator, e.g. 3, leave you with wide gaps that are usable, but that when you proceed to use larger denominators, these gaps start filling in. What this means is that while it is possible to avoid compounded standing waves (standing waves associated with more than one room dimension) at low frequency, that it isn't possible to avoid this at higher frequency so long as there are parallel walls.
Absorptive panels on the walls can be helpful for wavelengths where the panel thickness is at least 1/4 of the wavelength. Since wall panels are rarely more than a couple inches, they are typically only effective for treble and upper midrange. Why are the pyramidal cones you see in anechoic chambers so long from base to tip? Because to be effective, the height of the pyramidal cone from base to tip needs to be at least 1/4 of a wavelength.