Vibrational Modes of a Metal Rod. If you pinch a long, slender aluminum rod near its midpoint with one hand and strike the rod with the other hand you can make the rod vibrate enough to "sing" emitting a clear, loud, ringing noise, Using this information, deteermine the resonant frequencies for a 90 cm alumminum rod. HINT: The speed of sound in alumminum is 5100 m/s. This is thee speed of the waves traveling in the rod that cause the sound.