Quote:
Deviations in frequency response depend on the frequency, volume, and (believe it or not) room acoustics unless you're using headphones. Auditory memory is good for less than one second, though there's more to it than that.
--Ethan
The audibility of a response deviation depends on both the magnitude and the Q (bandwidth) of the deviation. The relationship is: the lower Q (the more spectrum affected) the less deviation is required to be audible, and the inverse, the higher Q the more deviation required to become audible. Positive response deviations are somewhat more audible than negative ones, especially at higher Q. To the extreme, a very high Q notch that is 30dB deep can be inaudible unless it lands on a specific musical note. An identical Q peak, but with 30dB gain is more likely to become audible.
I'm not sure the term "auditory memory" is correct for what we're going for, which I think is the ability to discern differences in two sample signals when compared with brief dead gaps between them, like ABX testing, in which it's well known that the switching time between A,B, and X must be kept extremely short for subjects to be able to resolve small differences. Even a .250ms gap hurts resolution significantly. I'm sure there's a paper...just don't quite know what to search for. "Auditory memory" turns up a bunch of stuff relating to memory based on audio stimulus.
The most well publicized audible group delay figures come from a 1978 paper by Blauert and Laws, which put the audibility threshold at 1KHz at 2ms, with the minimum threshold at 2KHz and 1ms. A 2005 AES paper by Finnegan, Moore and Stone used impulses and found the mid-band threshold at 1.6ms. But the audibility of GD depends not only on how much delay there is, but what the signal is, and whether or not the group delay curve is constant or changing. Much of the work done on the effects of group delay was motivated by early digital recorders with multi-pole analog anti-aliasing and reconstruction filters, which had by definition a fair amount of group delay. In the early digital days, mixing was done in the analog domain, so it was possible to pass through many iterations of anti-alias and reconstruction filters before finally ending up with audio out of a CD player. Today with over-sampling and high bit rates, the initial anti-aliasing filter requirements aren't as severe, and mostly realized digitally. Mixing occurs in the digital domain too, so the chances of filtering and re-filtering to accumulate audible group delay have diminished.
We could probably safely set the threshold of audibility for group delay at 1ms for any frequency group, though the extremes would obviously have higher audibility thresholds.
As to inter-channel delay (differential time delay between two channels), we could probably base that figure on the Minimum Audible Angle, the smallest detectable difference in a sources position on a horizontal plane. That's a very frequency dependent figure, ranging from about 1 degree at 1KHz and down, to 18 degrees at high frequencies (ref: "Hearing", ed. Brian C. J. Moore, Ch. 9, "Spacial Hearing and Related Phenomenon" by D. Wesley Grantham). Assuming a source straight ahead, a 1 degree angle shift right or left calculates out to a 9us inter aural time difference, whereas 18 degrees is a 160us difference. Probably throw that out, and stay with 9us as the audible inter-channel delay threshold.
h34r
of ms. But this is based on speaker building and drivers, not within a signal domain where "flat" is less of a problem. I also remember (reading, hearing at a presentation, who knows) that as frequency goes up, higher group delay will present more problems - that we're supposed to be more sensitive to 10ms of GD at say, 5khz, than we are at 50hz. But I'm not sure if that applies or not (I don't have a citation for this one).
