Question 14.AP.6: WHY DOES THE PROFESSOR SOUND LIKE DONALD DUCK? A professor p...
WHY DOES THE PROFESSOR SOUND LIKE DONALD DUCK?
A professor performs a demonstration in which he breathes helium and then speaks with a comical voice. One student explains, “The velocity of sound in helium is higher than in air, so the fundamental frequency of the standing waves in the mouth is increased.” Another student says, “No, the fundamental frequency is determined by the vocal folds and cannot be changed. Only the quality of the voice has changed.” Which student is correct?
The second student is correct. The fundamental frequency of the complex tone from the voice is determined by the vibration of the vocal folds and is not changed by substituting a different gas in the mouth. The introduction of the helium into the mouth results in harmonics of higher frequencies being excited more than in the normal voice, but the fundamental frequency of the voice is the same, only the quality has changed. The unusual inclusion of the higher frequency harmonics results in a common description of this effect as a “high-pitched” voice, but that description is incorrect. (It is really a “quacky” timbre.)