oh yippee. i'm going to jump in.
this came up a week or so ago on here. and the next day i was driving the family minivan to take my youngest for her 1 year checkup. in the van was my wife, one year old and 3 year old daughters and myself. sadly i can't remember if we had the radio on... but if we did it was my 3 year old's cd and the music was faded to the rear and very soft (those young ears don't like loud music for some reason).
while driving we came up on a v-twin cruiser (don't remember the make) that had straight pipes. it was pretty loud. so i decided to pay a little extra attention rather than spouting off that you can't hear them when they're behind you. again these were straight loud pipes. i could hear the bike very well when it was in front of me. but as i passed i noticed something... as soon as the bike got out of my peripheral vision i could hardly hear the bike. by the time the bike was in my side mirror there was no sound i could hear from it. and this is with me taking the time to pay attention. and our van was not loud. i don't have screaming kids - or wife.
now if this were a car with the stereo loud or kids screaming... forget about it. this is only one bike that i've had the chance to pay attention to since this came up here. i'll keep looking and listening. but as of right now, this doesn't hold any water for me. all it does is annoy neighbors and cause people to want to ban bikes from being allowed in their neighborhoods. personally, i like being about to come in from work at 10:00 at night and know that i won't wake my kids or neighbors.