How fast is sound?
There are different methods of doing this activity.  The one I use involves PVC tubing and tuning forks.  Each tuning fork is stamped with its frequency at the factory, so the "f" variable is known.
All that is left for us is to find the wavelength...


Above, you see a tuning fork bumping against air particles... each successive bump sends out a compressional wave that propagates throughout the room.  (Here I only showed them going sideways)  Regions where the particles are bunched together, it is called a compression and where there is nothing in between, it's a rarefaction.

One wavelength is the distance from compression center to compression center.

We calculate the wavelength by observing the effect of placing a tuning fork over a piece of PVC tubing with water in it.  As you release water, the column of air becomes longer.  The surface of the water is a node.  When a compression reaches the mouth of the tube, the tube/air column resonate.  This distance equals one quarter of a wave....

Since v = l*f, we can use v = (4)(obseved distance)(fork frequency).

Students always get pretty good results.  Here are a few working: