Formation of Waves in a Tuning Fork

Striking a tuning fork against a rubber pad sets its two metal prongs vibrating back and forth about their rest (mean) positions. As a prong swings outward, it pushes against the layer of air immediately in front of it, forcing the air molecules there to crowd closer together than usual; this locally raises both the density and the pressure of the air in that region, and this crowded, high-pressure region is called a compression. That compressed layer of air then itself pushes against the next layer further out, passing the compression onward, so that a wave of compression steadily advances away from the fork through the surrounding air. When the prong instead swings back inward, past its mean position, the air molecules immediately next to it are left with more room than usual, and both the density and pressure there drop below normal; this thinned-out, low-pressure region is called a rarefaction (or elongation). By repeating this outward-inward motion many times per second, a struck tuning fork generates a continuous, alternating train of compressions and rarefactions …