Originally Posted by kbarnes701
I regard all reflections as 'bad'. I cannot see any way that they can help the SQ in a modern m/ch HT. They may have a positive role to play in a 2 ch stereo system -
there are multiple "perspectives" within an acoustical space of which to view from.
take Non-Environment control rooms for example. a NE response (in the time-domain/ETC measured at the listening position) will show the direct signal and then drop off to the ambient noise floor. the speaker-to-room
response is effectively anechoic - no indirect energy from the speakers (via the room / reflections) is allowed to impede the listening position. the direct signal is all that is heard/processed/measured at the listening position (for "accuracy").
now, some people will assume this "effectively anechoic" response as measured at the listening position must mean that the room itself is dead/anechoic - which can be quite uncomfortable to be in! but this isn't true at all as the front wall and floor are reflective.
so, the speaker "sees" an anechoic room (speaker-room response), but the human/listener does not. when the human speaks, moves about, etc ... there are
indirect room reflections. the difference is that in this case the human is the "source" and "receiver" - and this "response" is NOT anechoic as energy reflects from the floor and front wall. but when the speaker is the "source" and the listening position (mic) is the "receiver", then the response is anechoic.
so you can indeed construct the room such that the speaker-to-listener response is anechoic and accurate ... while the room itself does not "sound dead" to the listener. it's all about perspective
it's not just the time-arrival but the gain of the energy. you'll find Binary Amplitude Diffusers (RPG BAD, for example) common in many home theaters. simply using absorption to attenuate the high-gain, sparse, early arriving signals can quickly lead to a dead room (from the human's perspective) - especially when confronted with many rows/seating positions (thus, many "reflection points"). using BAD can provide mid-LF absorption while also offering HF spatial dispersion. spreading the energy out in many directions (diffusing) by nature lowers the gain of the energy that reflects specularly to the listening position. thus, you can have some energy/reflection but it is low enough in gain as to not be destructive. it also provides a sense of "reflected energy" to the humans as they talk and move about the room and restrict the "deadness".
in a 2ch there are multiple approaches. if one is concerned with accuracy with respect to the direct signal (and minimizing the masking/skewing effect their room imparts on what is heard/processed at the listening position), then the high-gain (sparse) indirect reflections within the haas interval are indeed attenuated below human detection threshold such that they are not destructive to intelligibility, localization, and imaging.
but that doesnt mean they must be eliminated
(far too commonly misinterpreted here).
other 2ch time-domain responses do not "absorb" these early sidewall reflections but instead simply delay them in time by redirecting them to the rear of the room where they can be diffused and re-introduced
to the listening position (later in time, outside of the haas interval!) as a laterally-arriving
, exponentially decaying diffused sound-field. this is accomplished via 1dimensional reflection phase grating diffusers installed on the rear wall/rear side-walls to provide this "passive" surround sound / lateral
returns for envelopment --- with the wells oriented vertically to provide spatial dispersion in the horizontal/lateral plane.
eliminating the sparse
reflections also greatly reduces the 3d spatial polar lobing which translates into the frequency response as the "comb-filter" interference pattern. the sparse comb-filtering is detrimental - but we can use diffusers to "break-up" these sparse reflections into MANY reflections dispersed spatially and temporally in time ... which makes the comb-filtering more complex and dense
- greatly minimizing the frequency response anomalies while providing spaciousness (the "well-mixing" of the reflections/acoustical energy via the diffusers - of which happens naturally in larger, more "spacious" rooms).
again, care must be taken not to lump all reflections together. gain, time-arrival, spectral content, ingress angle, sparse/diffused, etc are all characteristics of acoustical energy. we don't refer to lakes, glaciers, icecubes, rivers, oceans, mist, fog, steam, rain as simply "water" ... yet we often see all forms of acoustical energy lumped together as simply "reflections". some 2ch models go to great lengths to attenuate these destructive early arriving signals while at the same time striving to preserve the acoustical energy such that it can be reintroduced to the listening position at a later time.Edited by localhost127 - 4/26/13 at 8:00am