Why Does a Tuning Fork Sound Louder Near a Hole in Cardboard?

[yulia7]

Homework Statement

I need to do an experiment with a tuning fork and here are some questions that I need to answer. The problem is: I misplaced the tuning fork and can't answer the questions. Please help!
Tuning Fork
Take a tuning fork and
(a) strike it against the edge of a table. While it is vibrating, touch the bottom
end to a variety of surfaces. Explain what you hear.
(b) Hold the fork close to your ear while it is vibrating and slowly rotate the
tuning fork. Describe and explain what you hear.
(d) In a piece of cardboard or construction paper, cut a strip out about half an inch wide and approximately the length of a tine of the tuning fork. Strike the tuning fork again and hold it near the hole in the cardboard. The sound becomes louder. Why?

The Attempt at a Solution

 

[DaveC426913]

Well, where do you live? We'll see if there's someone close by who has a tuning fork they can lend you.

Or, try to answer the questions best you can from what you know. We may be able to help.

Either way, we are forbidden to do your homework for you.

 

[yulia7]

ok, its 11pm here, I doubt that I may find a fork anywhere now... but I will try to answer the questions. I definitely need help on explanation for the last question...please advise why it becomes louder, or suggest a link where I could find the answer please!