Registers are differences in voice quality which can be sustained over some range of pitches and loudness. The differences between the various registers are created by many factors, including the balance between the activity of the CT (cricothyroid) and TA (thyroarytenoid) muscles, the balance between adduction and abduction (closing/opening) of the vocal folds, the amount of the vocal folds that is in vibration, and the shape of the vocal tract. (Recall that the TA muscle shortens the folds, whereas the CT lengthens them.) We will explore how a listener hears the differences between registers, and how singers create them.

Perception and production of registers
The sound of the human voice, no matter what the pitch, is essentially a lot of small puffs of air, many per second, separated by the closure (or partial closure) of the vocal folds between each ‘puff’. However, if there are enough puffs of air per second, we cannot hear the individual puffs, but instead perceive a continuous sound. The cutoff frequency below which we can start to hear the individual pulses of sound energy averages around 70 puffs of air per second (or 70 Hz), but can vary between about 60-80 Hz depending on the listener. If the pitch we are listening to is below 70 Hz, we tend to perceive it as a ‘vocal fry’, also known as pulse register.

Unlike pulse register, perception of the chest, head and falsetto registers does not depend on a cutoff frequency, or on fundamental limits in our ability to hear rapid changes in sound waveforms. Instead, these registers are largely perceived by us based on variations in vocal quality, or timbre.

Chest register is perceived when the timbre is richer or heavier; this quality is produced when the singer contracts both the CT and TA muscles at the same time, but the TA is more active, thus tending to shorten the folds and produce a lower pitch range. The Fo and lower overtones are stronger than higher overtones in chest voice, and a large amount of the vocal fold tissue is in vibration. In addition, the vocal folds are usually closed through more than half of each cycle of vibration.

Head register is perceived when the timbre is lighter or thinner. Both the CT and TA muscles are contracted, but the CT muscle predominates, and so the range of pitch for head voice is higher, since the folds are lengthened, thinned and stretched. A smaller portion of the folds is in vibration in head voice; only the outer layers of the cover vibrate. The Fo and all overtones are weaker than in chest voice, and the folds are open for a larger portion (more than half) of each vibrational cycle than in chest voice.

Falsetto is the term commonly used to describe the fluty, often breathy tone produced in the female pitch range by adult males. Females are also able to produce this timbre, and it is sometimes referred to as flute register in females. In this register, the TA muscle relaxes completely, and so the length of the folds depends solely on the degree of contraction in the CT muscle. Since the TA muscle is lax, it also does not cause the vocal fold cover to stiffen or thicken. Only the outer layers of the cover vibrate, and almost all of the sound energy is in the Fo. There are very few higher overtones, and they are quite weak compared to the Fo. The folds are open for a very large portion of each cycle, usually over 70%.

For review, here is a table of the four registers and their distinguishing characteristics: