I have a question about something in that paper:
"Since in small rooms, there is no Dc, no well mixed sound field, hence, no reverberation but merely a series of early reflected energy, the measurement of RT60 becomes meaningless in such environments."
My understanding apparently has gaps and/or misconceptions. Can you explain to me why there is no Dc ("critical distance", I presume) in a small room? (I think I understand the rest of that statement, and that I have been using the term "reverberation" in a way which is not technically correct since small rooms always have discrete reflections.)
Thanks!
Dc is where the sound level of the sound source and reverberation/reflective sound is equal. Imagine a speaker in a large room. Up close the level from the speaker will be higher vs the combined reflective sound. Move further away and at a point the level is approximately the same. If the room is naked with hard surfaces this will happen closer to the speaker because the level of the reflections will be high and higher compared to a treated room.
A small room is specular and modal, meaning the level varies with frequencies. The reason is the combination of close boundaries and the complex impedance of surfaces and furnitures. So while you may have a Dc at some spesific frequencies at a certain point in the room, it will not be so for a wider frequency area.
RTxx is the measurement of the decay time of
a well mixed reverberant sound field well beyond Dc. And a well mixed reverberant sound field isn't something we have in a small room unless is anechoich. It implies that the sound field is uniform and stochastic (randomely distributed) with no particular identifiable direction. Specular and modal is the opposite of this.
The result of using the Sabine equations to small room goes badly when treating it accordingly. The random distribution of absorptive materials, like Harman has done to a great deal, leads to a very poor result compared to controlling specific energy return with surgical placed treatment. How low in frequency the treatment is effective will also be completely different in these two rooms and the random incidence measurements of acoustic products are very misleading when used in small rooms.
Termination of ISD gap
The intensity of the ISD termination has a significant impact on the sense of liveliness of the space, while the laterally arriving semi-reverberant soundfield add to the sense of space of the room. Using reflective hard panels and the old "Haas trigger" will give a higher termination and precendence effect but also at the cost of accuracy compared to diffusers. In LEDE room accuracy was the goal and the Haas kicker was abandoned when diffusers were developed. The goal of the LEDE room was an environment that would reveal flaws and inaccuracies in micing techniques, recordings, and mixing choices. In that type of room there are indications that the energy of behind should be at something like 10-15dB lower than the direct signal.
However, that doesn't mean you can't choose something else in a listening room. Playing with Haas kicker is fun and can actually make it sound better than original.
Another goal of the LEDE room was resemble the best concert halls and that's why it's important that the diffuse tail energies arrives laterally. Schroeder wrote the following:
"..research, based upon a subjective evaluation of the acoustics of 20 major European concert halls, has shown that many modern halls have poor acoustics because their ceilings are low relative to their widths. Such halls do not provide the listener with enough laterally traveling sound waves - as opposed to sound traveling in front/back direction and arriving at the listener's head in his "median" plane (the symmetry plane through his head). Such median plan sound, of course gives rise to two very similar acoustic signals at the listener's ears, and it is thought that the resulting excessive "binaural similarity" is responsible for the poor acoustical quality."
The easiest way to achieve a strong termination of the ISD gap is to use speaker with controlled/narrow directivity, focusing more of the energy towards the diffusers rather than spraying the sound in other directions. Similar to adjusting a water hose to a narrower beam. That's one of the reasons why I personally like large horn speakers with a uniform directivity. As a side note. Most horns are not constant though, and I find discussion of horns at forums very difficult because so few have heard good horn designs and refer them as colored, resonant, etc.
Take note that using active surrounds to achieve a high level of return is something completely else. More sources leads to more comb filtering and polar lobing. A much more chaotic response and a step away from clarity, intelligibility, localization and correct tonality. Not my cup of tea.