(Those that don't understand cricket, please ignore this context, I will get to the physics...)
England are playing Pakistan at Lords and a decision has once again been overturned based on evidence from the 'snickometer'. (see over 1.4 ) It's always bothered me slightly that there seems to be a possibility that what this detects is not the ball hitting the bat (or glove in this case) but could just be the two passing very close.
When two objects collide in air, e.g. a ball hitting a bat, there is typically a noise and a deviation in the motion of one or both of the objects. In cricket, these collisions can be very fine as the ball passes the edge of the bat and detecting them is important to the game itself. The primary detection devices for the ball hitting the bat are the eyes and ears of the umpire, but in recent years the teams have been able to ask for decisions made in this way to be reviewed by an umpire with technological assistance. One of these pieces of technology, the 'snickometer', or 'Snicko' detects and enhances the sound of the ball supposedly hitting the bat. My doubt is whether this is fully accurate or whether Skicko fails to distinguish between contact and a very-near-miss.
My thoughts run something like:
- The sound we wish to detect is caused by the vibration in the surface of a bat and/or ball when they collide.
- Sound, as a pressure wave, could also be caused by the compression of air as the ball approaches very closely to the edge of the bat, and rarefaction as it leaves the vicinity of the bat.
- How much do these sounds differ and how can they be differentiated by a sound detection device?
The first thing that springs to mind is that the vibrations in the case of the bat and ball colliding are plural, and thus will show for longer in the detection device. A perfectly spherical ball passing a bat would only cause a single rise-fall-return change in pressure. However, a cricket ball is not a perfect sphere, it has a raised, stitched seam; this creates miniature ridges that would vary the compression between the ball and bat-edge as it passed.
So, is my knowledge and understanding sound? (pun unintended but left anyway) If it is, how would we, or how does Snicko, differentiate between 'ball hitting bat' and 'ball passing very closely past bat'?