Thank you very much Bjorn for taking the time to write an in-depth and educational reply.
Ten milliseconds is the figure I arrived at experimentally about twenty years ago, and is also the figure Earl Geddes uses. Acoustician David Griesinger comes up with this same figure, though in a different context (concert hall and lecture hall acoustics and psychoacoustics):
"Transients are not corrupted by reflections if the room is large enough - and 10ms of reflections free time is enough."
The graphs are confusing to me. In the second figure you posted, it looks to me like the Direct Sound arrives around 8 milliseconds, and that the strong onset of reflections is around 18 milliseconds, and that the part labelled RFZ only lasts for about 10 milliseconds. I don't expect you to troubleshoot my misunderstanding, but it's not clear to me that the RFZ gap is 18 milliseconds.
THIS is FASCINATING!! So if I understand correctly, we want a STRONG rather than gradual onset of diffuse reflected energy to clearly demarcate the termination of the ISD gap. Over in the world of concert hall acoustics the "scale" is different but I think the same general principle applies:
"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." - David Griesinger
Implied is a time gap separating the direct sound and the onset of reverberation... in a small room, the latter being presumably analogous to the onset of reflections at the termination of the ISD gap.
But you are the first one to point out to me the "a strong termination of the ISD gap" plays a critical role. THANK YOU!! IF it matters a lot that this strong onset of reflections be diffuse instead of specular, one implication is that you need a lot of channels to do multichannel right because just having a small number of rear-channel speakers mimics specular reflections, at least as far as the arrival directions go.
I may have more questions, but let me do some homework first and see if I can answer them for myself. [edit: My homework took an expensive turn; I just bought D'Antonio and Cox's book.)
There isn't to my knowledge an objective definition of early and later reflections. But normally we're talking about early reflections till 10-12 ms area and late after that.
Ten milliseconds is the figure I arrived at experimentally about twenty years ago, and is also the figure Earl Geddes uses. Acoustician David Griesinger comes up with this same figure, though in a different context (concert hall and lecture hall acoustics and psychoacoustics):
"Transients are not corrupted by reflections if the room is large enough - and 10ms of reflections free time is enough."
The RFZ or ISD (initial signal delay) gap of that graph is around 18 ms. Where you see the sudden rise in level, is where you have the termination of the ISD gap.
The graphs are confusing to me. In the second figure you posted, it looks to me like the Direct Sound arrives around 8 milliseconds, and that the strong onset of reflections is around 18 milliseconds, and that the part labelled RFZ only lasts for about 10 milliseconds. I don't expect you to troubleshoot my misunderstanding, but it's not clear to me that the RFZ gap is 18 milliseconds.
For example the termination of the ISD gap is very important. The level will decide how much liveliness of space you achieve and how well you can cover earlier arriving audible reflections. While treatment will attenuate specular reflections, it's common to still have some minor ones that's audible. A strong termination of the ISD gap tricks the brain to overlook these besides giving a boost of energy/liveliness which is very pleasing. The purpose of the ISD termination is also to remove the localization cues of the later arriving energies and to reinforce the localization cues of the direct energy.
THIS is FASCINATING!! So if I understand correctly, we want a STRONG rather than gradual onset of diffuse reflected energy to clearly demarcate the termination of the ISD gap. Over in the world of concert hall acoustics the "scale" is different but I think the same general principle applies:
"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." - David Griesinger
Implied is a time gap separating the direct sound and the onset of reverberation... in a small room, the latter being presumably analogous to the onset of reflections at the termination of the ISD gap.
But you are the first one to point out to me the "a strong termination of the ISD gap" plays a critical role. THANK YOU!! IF it matters a lot that this strong onset of reflections be diffuse instead of specular, one implication is that you need a lot of channels to do multichannel right because just having a small number of rear-channel speakers mimics specular reflections, at least as far as the arrival directions go.
I may have more questions, but let me do some homework first and see if I can answer them for myself. [edit: My homework took an expensive turn; I just bought D'Antonio and Cox's book.)
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