Ideas about reflections have been bouncing around for the past few pages. Some of these ideas have been absorbed, some diffused, and some reflected specularly...
(Sorry, couldn't resist.)
At the risk of further degrading the signal-to-noise ratio, here are some thoughts on the subject:
Imo the floor-bounce dip is most audible when it is the only significant early reflection. In most rooms, other early reflections arrive with enough intensity and with their dips at different frequencies such that they subjectively fill in the floor bounce dip. That being said, ime designs which mitigate or avoid the floor-bounce dip tend to sound more natural in the midbass (cello) region.
If anyone is up for a sighted listening test with all biases fully in play, try this: Indoors, preferably on a hard floor, have someone stand a few feet away and talk to you. Focus on the timbre of their voice. Then go outside and stand on a sidewalk out in the open. Have them talk to you again from the same distance, again focusing on the timbre of their voice. The floor-bounce dip was present in both situations, but other reflections were also present inside the room. Was there a difference in the timbre of their voice? If so, that difference was caused by the presence of reflections other than the floor bounce. Did those other reflections "warm up" the timbre of their voice? If so, then arguably those other reflections were filling in the floor-bounce dip.
Note that the floor bounce does not significantly affect imaging, as its arrival time is the same for both ears, or close enough. In contrast early sidewall reflections effect imaging, broadening the "apparent source width" in the direction of the sidewalls (and thereby widening the soundstage), but arguably degrading image precision and image depth. I am not an advocate of strong early sidewall reflections, but concede that this is a matter of personal preference.
Here are a few imo relevant quotes about reflections. First from psychoacoustics researcher David Griesinger ("GREE-zin-GURR"), whose work is mostly focused on large venues but the underling psychoacoustic principles are still applicable to home audio, we get the idea that early reflections have downsides:
"The earlier a reflection arrives the more it contributes to masking the direct sound."
"When presence is lacking the earliest reflections are the most responsible."
"Transients are not corrupted by reflections if the room is large enough [i.e. if the early-reflection-free time interval is long enough] - and 10 milliseconds of reflections free time is enough." [Sound travels about 13.5 inches per millisecond, so 10 milliseconds corresponds to the time it takes for sound to travel about 11 feet.]
Psychoacoustics researcher Earl Geddes, with the focus specifically on home audio:
"At mid and high frequencies, there are several factors that must be considered in the loudspeaker design that will present some formidable problems. At these frequencies
there are enough modes that the sound field and wave motion basically acts like a ray or beam of sound moving from the source and reflecting off of walls etc. – this is called the geometric or ray acoustics region. It is well know that reflections can be good and bad and it is critical that a loudspeaker designer understand the difference. One must look to psychoacoustics to determine what is required of the loudspeaker. There are a couple of characteristics of the ear that need to be understood in order to understand what is important in small room acoustics at mid to high frequencies. These are mostly related to how the human ear perceives reflections and diffraction. The perception of reflections and diffraction is a highly complex topic, but there are a few principles that are most important here:
"The earlier and the greater in level the first room reflections are, the worse they are. This aspect of sound perception is controversial. Some believe that all reflections are good because they increase the listeners' feeling of space – they increase the spaciousness of the sound. While it is certainly true that all reflections add to spaciousness, the very early ones (< 10 ms.) do so at the sake of imaging and coloration. There is no contention that reflections > 20 ms are positive and perceived as early reverberation and acoustic spaciousness within the space. In small rooms, the first reflections from an arbitrary source, mainly omni-directional, will never occur later than 10-20 ms (basically this is the definition of a small room), hence the first reflections in small rooms must be thought of as a serious problem that causes coloration and image blurring. These reflections must be considered in the [loudspeaker] design and should be also be considered in the room as well.
"Reflections become less of a problem as coloration and image shift at lower frequencies. Below about 500 Hz. early reflections are not as much of an issue. The ear has a longer integration time at lower frequencies and it has a poorer ability to localize resulting in a lower sensitivity to early reflections. Image localization is strongly weighted towards the higher frequencies.
"A reflected signal that arrives at the opposite ear from the direct sound is less perceptible as coloration and image shift than if both signals arrive at the same ear. This is because of head shadowing above about 500 Hz and the fact that our ears can process signals between them. When the two signals arrive at the same ear, the signals are physically merged in space even before they enter the ear and no amount of auditory processing can separate them. When these signals arrive at different ears, the auditory processing system can diminish the adverse effects of these early reflections through cognitive processing between the ears...
"From an acoustics reproduction standpoint then, the loudspeaker system design must help to provide as much delay as possible in the early reflections and allow for speaker placement and orientation such that the earliest reflections occur at opposite ears rather than the same ear. This needs to be done above about 500 Hz. Below 500 Hz other factors, such as room characteristics and our hearing mechanism, may dictate an entirely different approach.
"An obvious question always comes up – “Why not just make all the reflections and modes go away? Doesn’t this solve many of these problems?” That approach is (unfortunately) used in a great many situations, but it is far less than ideal. Without real room reverberation the perception of the playback is dead, lifeless, in acoustics parlance it lacks spaciousness or ambiance – the feeling of being engulfed in an acoustically spatial environment. If a non-echoic space were desirable then an anechoic chamber would be the ideal listening room, but as anyone who has ever listened to speakers in this kind of space will tell you, it really isn’t a good listening environment. One is always aware that they are listening to speakers – the room adds nothing. It’s something like listening to headphones, which admittedly some people like.
"To achieve good spaciousness in a room requires a multiplicity of lateral reflections (vertical reflections don’t really contribute much) arriving from many directions, i.e. a
diffuse sound field." (Link to Earl's paper:
http://www.gedlee.com/Papers/Philosophy.pdf)
Now back to Griesinger:
"Envelopment is perceived when the ear and brain can detect TWO separate streams:
A
foreground stream of direct sound.
And a
background stream of reverberation.
Both streams must be present if sound is perceived as enveloping."
"Presence depends in the ability of the ear and brain to detect the direct sound as separate from the reflections."
So here is my interpretation and distillation of the desired sequence of events in the listening room:
1. A strong and clear first-arrival sound; followed by
2. A relatively reflection-free time interval of at least ballpark 10 milliseconds; more is better, and is quite practical in the size room this thread is about; followed by
3. A lot of spectrally-correct reflections arriving from many directions, and in particular from the sides.
The above-mentioned relatively reflection-free time interval is necessary for the ear/brain system to separate the first-arrival sound from the reflection stream, which is desirable both for "presence" (immediacy, clarity, and dynamic contrast) and "envelopment" (spaciousness, sense of immersion). In my experience those later-arriving reflections can also enhance timbre and richness, and they function as "carriers" for the ambience cues on the recording (which is another topic that I won't go into here).
All of this of course assumes the reflections are "spectrally correct", which is in part a loudspeaker radiation pattern issue, and in part a caution against the overuse of absorption in the playback room.
In my opinion.
I generally find the Mantas to have more sense of envelopement and soundstage than regular speakers, and they create less room reflections, so this would support your case. I also generally enjoy well dampened room more, also with traditional speakers. What I however also find (with both the Manta and regular speakers) is that in most rooms I rather not dampen the sidewalls, I find the added reflections to sound better and with a wider soundstage. 
tektondesign.com
www.subwoofermania.com
