Hi all,
Dr. Toole replied to my email, and has gone into considerable detail about Bruno's situation and the non-linear nature of the ear with high-volume playback.
Floyd Toole responds:
"The following comments assume that no clipping or distortion is being generated within the power amp. Several comments in the forum correctly note that a 4 ohms load impedance is close to the current limiting spec for lots of receivers, so this is a very real possibility. BTW, when listening to music clean clipping is hard to hear until it exceeds about 6 dB, which allows us to get away with undersized amps for non-critical applications. However, when it does become audible virtually all aspects of sound quality are seriously degraded. Also, some amps don't "clean clip", Power supply voltages sag, causing several problems, or they can revert to a protective mode that does strange things.
Let's look at what room acoustics can do. Bruno's addition of the rug obviously reduced the overall reverberation time in the room, making it more acceptable (ideally the reverberation time should be less than 0.5 s at mid frequencies). However, being on the floor, it can do nothing to change reflections in the horizontal plane - i.e. among the walls. Our ears are in the horizontal plane, and therefore are more sensitive to these reflections than to those in the vertical plane, where the new rug would have it dominant effect..
Now, to get into imaging we need to be aware that imaging will be most "clear", most "sharp" when the direct sound is the dominant factor. The more reflections in the horizontal plane that are audible, the more decorrelated, or confused, are the sounds arriving at the two ears. A certain amount of this is highly desirable, adding a pleasant sense of ambiance, or "air" around the instruments, called in the scientific literature "ASW - Apparent Source Width". However, too much of this results in what Bruno describes: "the sound became more "blurry". Why does this occur when the volume is turned up? Because more of the lower level reflections become audible; they have been elevated above the audible hearing threshold. Grunt's comment about the highs being annoying in a live room are part of this. Most high frequency sounds are transient in nature and they draw attention to themselves, making the reflections more distracting, and the soundstage more "blurry".
Also, as Alan points out, the ears become increasingly non-linear at high sound levels and we are less able to discriminate fine details, both spectral and spatial. In the reverse direction, as you turn the volume down, the soundstage gets progressively more simplified, until almost all sense of "air", space and envelopment is gone. This is why we recommend that serious listening - foreground listening - be done at a moderately high level; but not too high, obviously.
So, how can this situation be improved in Bruno's room?
Nowhere in these discussions did I learn where Bruno's head is with respect to the rear wall. If there is a bare wall close behind the listener, all bets are off. The first thing to do is to place a broadband absorber behind the head - something fibrous (a cushion, fiberglass, etc. that is not less than 4 inches thick). Reflections from the back wall corrupt the perception of the front soundstage. We tend to get tied into knots worrying about where we put the loudspeakers, and then sit down in a chair backed up against a rear wall. This is not logical, right?
If this is not an issue, I suggest more furniture, or more acoustical treatment, mostly on the side walls. Either or both will be beneficial. In general, surface irregularities - bookcases, display cases, tilted pictures, etc - are useful, as are purchased "diffusers", so long as they are moderately thick (ignore the "toy" diffusers that are only 2-3 inches thick - they only work at tweeter frequencies). A certain amount of absorbing material may be needed if the room is too live, but don't overdo it; excessively dead rooms are not pleasant places in which to spend time. Scattering the sound using furniture or irregular surfaces makes whatever absorption that is in the room work harder, and the sound energy is preserved.
Of course, all of this and more is explained in my book: "Sound Reproduction", Focal Press 2008."
Good luck,
Floyd
