Time delay, when is it less real?
I have no reason to question reports stating that the ear can hear 2usec interaural delay. It is true that a change in time delay of say 2usec is a very tiny acoustic difference, 0.024 inch!!! How can anyone hear that? Can one hold their head that steady?
Clearly such reports do no suggest that you leave the room, come back to the same sitting position within .024 inches. What they tell you is that a sudden change in acoustic distance (or time delay delay) can be heard.
Say you want to repaint a part of your wall. The color is say light blue. The place when the old and new paint meet requires a near perfect match, because “side by side” comparison “magnifies" the difference. In fact, the old paint serves as a reference point for comparison with the new paint. But say you decided to paint the whole room with the new paint. Without a reference, the new paint may look indistinguishable from the old paint…
I am pretty sure that anyone hearing 2usec or 100usec interaural differences are talking about a “sudden change in delay”. So the report is interesting, but does it have any value in the music world? Just because the ear can hear a sudden change, do we need to be concerned with it? Not without the existence of mechanisms that introduce such sudden changes while listening to music. I am not aware of any.
Some have proposed that interaural sensitivity should dictate faster impulse response, thus possibly faster sample rates. These is of course another confusion between apples and oranges, thus another fruit salad:
One can take a fast impulse or a slow impulse, and delay it in time. The impulse width is determined by the audio bandwidth. Imulse and bandwidth are one in the same! But you can take a 10usec, 1000usec or any signal (impulse or not) and delay it by 1nsec, sec or an hour. Impulse and delay are independent from each other. The interaural response is about delay, not about impulse width.
The confusion stems from the fact that indeed, it is easier to detect small time timing differences when listening to an impulse (short duration with a fast and distinct attack). But it is wrong to assume that by making the impulse narrower and narrower (faster and faster), such interaural audibility will become better and better. Why?
What happens when we run say a 2usec impulse through a device capable of handling no faster than 20usec impulse? Recalling that we can view impulse as a bandwidth issue, the question can be reworded:
What happens when you run a signal containing 1MHz energy through a device limiting the bandwidth to 100KHz? The answer is: only 100KHz is going to pass through.
We can now look at the answer in the time domain: 100KHz bandwidth? It is a 20usec impulse. The narrow impulse changed into a wider one.
So making the impulse narrower beyond some limit, will not yield faster and more distinct sudden sound. The impulse simply does not get narrower beyond some point. It does get weaker, because the bandwidth limitations imposed by the lowest bandwidth device in the chain removes some of the energy out of the signal. Weaker signal yes, narrower impulse, no.
Again, what is the limiting factor? It is the lowest bandwidth device in the chain (mic speaker or anything else). The proposals to go for 1MHz sampling does dont make sense, not even with 500KHz microphones, speakers, amplifiers, converters… There is still the ear which can not accommodate 500KHz. The ear will only react to the energy portion within the hearing bandwidth, and will filter anything else. The ear will react and interpret a supper fast impulse as a lower amplitude, wider impulse.
To be continued…
BR
Dan Lavry