There are several potential types of unevenness between the vocal cords that may lead to diplophonia:
- asymmetric tension
- asymmetric mass
- asymmetric length
- asymmetric stiffness
It is also possible to have diplophonia from an irregular pattern of vibration even when the vocal cords are not asymmetric between each other. This is what occurs with vocal fry, which produces a regularly irregular vibration. The vocal folds vibrate at a regular rate and then a beat is skipped, perhaps every third beat, every eighth beat or some other regular interval. This ends up generating two pitches simultaneously.
Let’s explore the different types of asymmetries.