No
The echo is just longer!
Let's say the cab + room you captured is completely silent after 170ms. Idealized case! Then the IR which has a length of 170ms captures everything there is and the 500ms IR has 330ms of silence at the end. Therefore it would be a a waste of processing power to use such a long IR in this special case.
But rooms usually don't go silent that fast. Think of a church in where you clap your hands (= the impulse). The room will echo (= the response) to your clap much longer than 170ms, even longer than 500ms. So when you capture such a room and want to put it into a 170ms IR the echo will stop abruptly, which sounds unnatural. But the first 170ms of the 500ms IR sound exactly the same like the 170ms IR. Only the 500ms IR can still playback contents of the room's echo AFTER those 170ms, simply because there's more information stored in this longer IR.
But in the Axe we don't use IRs for rooms, we use it for cabs! Cabs are silent much faster than rooms. We can add the room with either the reverb block or the reverb tab in the cab block.
Now, why is UltraRes great and what problems can it cause:
+ With UltraRes we get quite long IRs, 170ms long. Cliff says that 100ms would be quite enough. What does this mean? It means that with UltraRes IRs you definitely have the whole response of the cabinet you made the IR of. Then in theory there's almost absolutely no difference between using the IR or using the miced cab, e.g. in a studio situation etc. With a shorter IR it's also possible to capture the whole response the cabinet gives. But if it's too short, it's possible that you'll hear a difference, especially in the lower frequencies, because there's more energy in low frequency tones (bigger amplitudes) which takes longer to fade away.
- The longer the IR, the more room you capture! Look at it this way: When you clap your hands in this church, it'll take a small amount of time until the room's echo will reach your ears, because the sound has to go from your hands to the walls, the ceiling or whatever and then back to you ears. When you capture a cab the cab's sound will also not be echoed immediately, it takes a little time until the echo reaches the mic. Because we know the speed of sound to be about 330m/s this can be measured. If e.g. the distance between the speaker to the closest wall (usually the floor) is 20cm and back to the mic it's 30cm (you Americans will have to translate it to your strange units
) there's a total distance of 50cm = 0,5m for the sound. With 330m/s this 0,5m distance takes the sound 0,5m/(330m/s) = 0.0015 seconds = 1,5ms. In this case there will be room content from the floor captured in the IR! But a room usually also has 4 walls and 1 ceiling. You can do the math for those if you know the distances.
The perfect case would be to have an IR that ONLY captures the cab, but no room content (or only content from the floor as just calculated). The problem is, if you capture a 170ms long IR in a bad room, the IR will sound shitty, because there's a lot of echo in it which stops abruptly. But if you record it in a good room which doesn't have much echo after 170ms and you fade out this last bit of echo that's still present at 170ms, your IR has the potential to sound awesome!
I've seen (heard) many free IRs with this problem! Even commercial ones. What needs to be done is to put a fade-out to the IR so the room doesn't stop abruptly! But if captured in a bad room with lots of echo it'll still sound really bad, because the fade-out just sounds weird. What else can be done? You can simply make the IR shorter, so there's less or even no room content in it, but then it's possible that the IR doesn't contain all of the response the cab gives.