I suppose it's more or less like this.
Go into my diff and go look at 00:26:286 (1) -
Listen to it with the slider, and then listen to it after deleting the note. They sound exactly the same. It doesn't give enough of an effect for players to know they hit the note properly or not.
next, try swapping the same note to soft without any customs. If you listen to the note now, you can easily hear the soft effect and it gives a lot more feedback to the player so they know they actually hit the note. I think the main reason is that it's the actual hit-normal. If it was similar sounding hitsounds, it wouldn't be as bad since it'll still provide a little bit of feedback, both from the clap and the soft noise. Another way to compare this is to low volume on the heads. Lots of maps have been DQ'd in the past for not being able to hear hitsounds because they were too quiet. Comparing those to this hitsound, it definitely doesn't provide enough feedback in the same way.
There was some talk awhile back on this map and the hitsound volume was raised quite a bit. It's still a bit hard to tell hitsound from music, even after the volume change.
https://osu.ppy.sh/s/242856That's about as much reasoning I can deduce from this DQ. Me personally, I didn't even think about the hit-normal, but I'll be careful about this from now on. >_>
