Originally Posted by ophic

thats not quite right. 4 bikes would not make 240dB.

1 bike, say 60dB
add a second bike which makes an identical noise - you've now got 2 bikes at 60dB each, so you've doubled the sound intensity - thats now 63dB, as 3dB represents "twice as loud". (I always thought it was 6dB but that could be something else - its been a few years since i was at college).
Add another 2 identically sounding bikes, you've doubled it again - so 66dB.

Now these bikes would not normally be making identical noises in sync with one another. Sound is a wave - 2 identical waves in phase would produce double the sound intensity, but entirely out-of-phase would produce silence. So in fact with 4 bikes all producing 60dB - you're probably not gonna get to much more than 61dB there.