Having more than one sound source may create roughness. The degree of roughness depends on the difference in frequency between the two sources. Mathmatically related sound intervals can sound pleasant, but non-related intervals interfere with each other, hence roughness.
Diplophonia is the most common type of roughness. Two sound sources in the larynx leads to diplophonia. It is possible but less common to have three or more sound sources.
There are several types of unevenness between the vocal cords that lead to multiple sound sources:
- asymmetric stiffness
- asymmetric mass
- asymmetric length
It is also possible to have diplophonia from an irregular pattern of vibration even when the vocal cords are not asymmetric. This typically occurs at low tension and 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.