Hi, a couple of things I would like to comment here.
Regarding flutter echo caused by parallel walls (which you didn't mention but, to me, is worth mentioning), this can be broken up with as little of an inch of non-parallel-ness. A slightly bowed sheet of thin ply on one wall. Vertical drapes turned slightly open. Pictures hanging on the walls with a slight downward tilt. Very easy to eradicate while still having parallel walls. Getting more formal, acoustical diffuser and absorber panels also do the job, of course.
Regarding room modes in the bass, it is a myth that parallel walls are the problem and non-parallel walls or ceiling are the solution. In the horizontal plane, room symmetry is too important at all non-bass frequencies, for non-parallel walls to be acceptable. Toole wrote, "A recurring fantasy about rooms is that if one avoids parallel surfaces, room modes cannot exist. Sadly, it is incorrect. Among the few studies of this topic, Geddes (1982) provides some of the most useful insights. He found that “room shape has no significant effect on the spatial variations of the pressure response.... The spatial standard deviations of the p2 response is very nearly uniform for all the data cases [the five room shapes evaluated in the computer model]."...." and "....for modes in rectangular and non-rectangular spaces...it is clear that both shapes exhibit regions of high sound level and nodal lines where sound levels are very low. The real difference is that in rectangular rooms, the patterns can be predicted using simple calculations.” Regarding vaulted ceilings being better than flat ceilings, not many homes have rock-solid floor and ceiling, so one or the other is going to let bass through and destroy the theoretical modal behaviour (which still exists for vaulted ceilings too). It is a theory that has been taken too far by lovers of theories.
Regarding room proportions, a room is not a reverberation chamber. Thus, analysis based on it being one is flawed, to the point of being misleading. One could argue it has some relevance between 40 and 120 Hz in small rooms (i.e. home hifi rooms), but researches in recent times refutes its usefulness. e.g. Fazenda et al (J. Audio Eng. Society, 2005) found that, "it follows that descriptions of room quality according to metrics relying on modal distribution or magnitude pressure response are seriously undermined by their lack of generality, and the fact that they do not correlate with a subjective percept on any kind of continuous scale."
Toole wrote that the 'irrefutable logic of the facts of room modes' has led to the appearance in pro audio text books of things like ideal, and undesirable, room proportions, and their widespread recitation by acoustical consultants. The fact that an acoustical consultant says so, is not, apparently, enough to make it true. Toole concluded that, "the acoustical performance of rooms cannot be generalized on the basis of their dimensional ratios and that reliably hearing superiority of a “good” one may not be possible....It is difficult to understand how this concept of an optimum room got so much traction in the field of listening room acoustics, and why it has endured."
You might need to revise that "1/4 wavelength" rule. The wavelength of 250 Hz is 54 inches, demanding a 13 inch thick panel by your rule, yet my 4-inch panels have an absorption factor of 1.00. In fact, at 125 Hz their factor is 0.60 -- and that is one way, so maybe 0.8-ish on a wall -- so are still highly effective, although your rule would require 26 inches of panel thickness.
But your main point, that typical 2-inch panels are no help in the bass, is well made.
cheers
Well, good for you.
For the record I provided a precise, carefully written, fully correct mathematical description of standing waves that set up between parallel surfaces, and of
the proper mathematical understanding of the conditions by which two or more of the dimensions of a room with parallel surfaces will promote standing waves with the same frequency. It was textbook-quality stuff if I do say so myself, and of strong, fundamental relevance to the avoidance of standing waves in listening rooms. But rather than display any appreciation for my having gone to the trouble to do that, Newman chose to dismiss it and refute it through a hasty assertion that this sort of fundamental, theoretical understanding of the nature of standing waves has no relevance to real listening. To support this endeavor he used some quotes from a certain book, which quotes are not nearly as directly opposed to what I wrote as he would have us believe.
Newman, you wrote this:
"Regarding room modes in the bass, it is a myth that parallel walls are the problem and non-parallel walls or ceiling are the solution. In the horizontal plane, room symmetry is too important at all non-bass frequencies, for non-parallel walls to be acceptable."
I made a concerted effort to understand what that second sentence was intended to mean exactly. I eventually abandoned that effort and moved on to the bigger reason that I take exception to what you wrote there. You insinuated, rather strongly in fact, that what I had written amounts to a myth. The fundamentally correct understanding of standing waves in resonant cavities and in musical instruments is decidedly not a myth, but if what I wrote is a myth, then all of it must be.
Your statement further insinuates that the supposed myth is disproved by the quote you provided from a certain book. The book quote is concerned specifically with low frequency standing waves and with a phenomenon that is specific to wavelengths that are very long in relation to the interior dimensions. The explanation that I had written was not expressly concerned with very long wavelengths. By inference, your intent was to argue that only the long wavelengths matter. But you never said that exactly, and I don't think there is any question that you did not realize that the only way that what you wrote would truly make sense would be if this is what you were asserting, i.e., that so far as standing waves are concerned, the only wavelengths that matter are the very long ones.
Certainly it is possible for standing waves to set up along a path that is more complex than the simple rectilinear path between two opposing parallel surfaces. Perhaps this occurs to a significant degree inside of a home, but I am in favor of a simpler understanding. When the irregularities of a room are small in relation to wavelength, reflection occurs and reverberation ensues even though casual observation may not discern the reflective surfaces. What does not appear to the eye to be acoustically reflective is reflective when the dimensions of the irregularities are compared to long wavelengths. In any case, if an observed instance of reverberation is not believed to be adequately explained by way of the long-established understanding of the phenomenon of reverberation, then what is called for is further elaboration and specialization of the long-established theory, not some hasty and poorly-considered insinuation that the established theory has no relevance in explaining the observed phenomenon. I haven't read Toole's book, but based on the quote you provided I would be inclined to direct the same comment to him.
Regarding room proportions, a room is not a reverberation chamber. Thus, analysis based on it being one is flawed, to the point of being misleading.
Sheesh. In effect you are saying that it is misleading to analyze reverberation in a room because a room isn't a reverberation chamber. Watch me fall out of my chair. Let me see if I follow logically. Premise: a room is not a place where reverberation occurs. Implication: Analysis of reverberation in a room is flawed. Well, I suppose it does actually make sense, but could it be more obvious that all this really amounts to is the unsubstantiated assertion that reverberation doesn't happen in a room? Why do people write stuff like this? Write some preposterous assertion and then attach the obvious implication and act as though this is somehow intelligent and profound? It's just dumb.
Implicitly the reason you put the absorptive panels on your walls is not related to room reverberation (perhaps you were only wanting to deal with first reflections, I dunno). In most ordinary rooms in most ordinary homes, if you take every bit of the furniture out of the room and take all the drapes and curtains off the walls and then stand somewhere in the middle of the room and talk at a moderately loud level,
room reverberation is inescapably apparent and it is most decidedly true that it is associated in a direct and obvious way with the distance between the parallel walls. You might as well argue that the sun doesn't ever rise before 7 AM because you've never gotten up early enough to have noticed it any earlier than that.
Regarding vaulted ceilings being better than flat ceilings, not many homes have rock-solid floor and ceiling, so one or the other is going to let bass through and destroy the theoretical modal behaviour (which still exists for vaulted ceilings too). It is a theory that has been taken too far by lovers of theories.
You are saying that when the ceiling is flat that either the ceiling or the floor will not behave as a reflective surface and that as such the flat ceiling is not any problem. You are further saying that if the reasoning by which the flat ceiling is thought to be a problem were correct, the vaulted ceiling would be just as big a problem. At face value this makes no sense, because
the reasoning by which the flat ceiling is deemed a problem is based fundamentally on the premise that reverberation occurs between two parallel surfaces. Furthermore, if ceilings and floors weren't reflective, why would it be as easy as it is to detect and measure floor bounce and ceiling reflections?
And setting aside the question of reverberation, isn't the mere fact that first reflections from a flat ceiling are directed into the main listening area reason enough to prefer a vaulted ceiling? I've listened to music in countless rooms with flat ceilings, and in enough rooms with vaulted ceilings to know with absolute certainty that the difference is stark. Hell, the difference is unmistakable even with casual conversation. But you assert that vaulted ceilings aren't better, and even though this assertion of yours falls squarely under the heading of "theories that probably aren't worth taking seriously", you thought it appropriate to write, "It is a theory that has been taken too far by lovers of theories."
You might need to revise that "1/4 wavelength" rule. The wavelength of 250 Hz is 54 inches, demanding a 13 inch thick panel by your rule, yet my 4-inch panels have an absorption factor of 1.00. In fact, at 125 Hz their factor is 0.60 -- and that is one way, so maybe 0.8-ish on a wall -- so are still highly effective, although your rule would require 26 inches of panel thickness.
For something like this it is highly preferable for you to refer to studies that show the relationship between wavelength, panel thickness, and the effectiveness of the absorption. Please include a cursory explanation of what the "absorption factor" is, i.e., does an absorption factor of 1 mean that 100% of the acoustic energy impinging on the panel is absorbed? Please note also that I bothered to justify the 1/4 wavelength rule by way of comparison to anechoic chambers, where the accepted rule is that the height of the pyramidal cones needs to be at least 1/4 wavelength to be fully effective. A fully genuine response from you would offer an explanation as to why the rule for anechoic chambers does not apply to home listening rooms.
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