What causes the sound to become louder in a tuning fork experiment?

[bhoul]

Homework Statement

This is the final part of an experiment involving a tuning fork - the instructions say to cut a hole in a piece of cardboard (half an inch wide by the length of the tine) And then it just asks to strike the tuning fork and move it over the opening. We need to describe what happens and explain why it happens.

The Attempt at a Solution

So when I did this, the sound became louder as it was moved over the opening. I noticed that the sound would be louder if you held it anywhere close to the surface of the cardboard, but it was loudest directly over the opening.

I'm not really sure what causes the sound to become louder though. The only concept that I knew beforehand that involved sound waves traveling through an opening like this is diffraction. Part of me thinks that could be the explanation, but I haven't been able to figure out if diffraction applies directly to this scenario. My thinking is that the diffraction causes the sound wave to spread out thus creating vibrations across a greater surface area. Am I on the right track with this idea that diffraction is the cause, or should I look for a more relevant reason?

 

[haruspex]

I would guess the cardboard is resonating in sympathy and acting as a sounding board. E.g. it may be that the two edges of the slot are vibrating towards and away from the fork. If so, seems to me that they would need to be 180^o out of phase, which suggests the fork should not be directly over the slot but offset a little to one side (so that the distances from the tines to the slot edges are a half wavelength different). But I'm not at all confident of that analysis.
Are you able to repeat the experiment and collect more precise data? Maybe touch one side of the slot to detect/inhibit vibrations.