Semimusical Blocks of Wood

What it shows:

Drop a piece of wood on the floor and listen to the sound it makes. It may sound like noise, but it also makes a "semimusical" sound which is so poor in quality that one would be hard pressed to call it musical. Yet it is not pure noise because the sound contains a series of regular impulses that have a pitch. This may be demonstrated by dropping wood bars (one by one) onto the floor — a musical scale or tune is easily recognized.

How it works:

The tuned wood (oak) bars are 6½" long and 1" wide with thicknesses between 7/8" down to 7/16". Applying the formula for the frequency of transverse vibrations of a rectangular bar with free ends [T. Rossing, The Science of Sound 2nd ed, (Addison-Wesley, 1990) p. 259], it turns out that the frequency is directly proportional to the thickness of the bar, if you keep all other parameters constant. The bars were tuned with respect to each other and not to frequencies of a specific musical scale. Even though the bars were cut from the same board and all have similar grain, the density apparently varied by quite a bit and the thickness of each one had to be adjusted from the calculated thickness to obtain the desired frequency.

There are a total of 14 bars: three C's and G's, and two A's, F's, E's, and D's. One can simply play the entire scale by dropping eight of the bars in succession, or play a tune such as Twinkle Twinkle Little Star.

Setting it up:

The bars should be dropped onto something hard, like the cement floor. In hall E (which is carpeted), lay out some flat cinder blocks to drop on. To preferentially excite the transvers mode of vibration, drop the bars onto their flat side.


It's an informative demonstration and is sure to elicit smiles from the audience if you play a tune.