Subglottal pressure (Psub) is one of the most influential physiological parameters controlling the voice quality. It refers to the overpressure of air in the respiratory system. Its main function is to vary vocal loudness, high pressure producing loud voice. Vocal loudness is commonly measured in terms of sound pressure level (SPL), measured at a specified distance to the microphone, as shown in the figure below. 

Figure 1. SPL as a function of subglottal pressure. 

Vocal loudness is an important parameter for emotional expressivity in both speech and singing. It is commonly used to mark the musical phrase structure, adding emphasis to important events. From all parameters than can contribute to the perception of vocal loudness, i.e., subglottal pressure, glottal adduction and formant frequency tuning, subglottal pressure is the most influential one. Increase of subglottal pressure is typically associated with a decrease of the steepness of the spectrum slope. 

Figure 2. Illustration of the effect of subglottal pressure on the slope of the spectrum envelope. 

Subglottal pressure needs to be varied with pitch: high pitches, produced with stretched vocal folds, need higher pressures than low pitches, which are produced with lax vocal folds. Subglottal pressure ranges vary between voices and also between voice classifications and musical styles.  

Figure 3. Illustration of the relationship between subglottal pressure, pitch and phrasing. The graph shows an ascending arpeggio of a C major tonic chord, followed by a descending arpeggio of a dominant 7th chord. Note that subglottal pressure is raised with pitch and also, the first note belonging to the second chord, receives the highest pressure for reasons of phrasing. 
Audio 1. Ascending arpeggio of a C major tonic chord, followed by a descending arpeggio of a dominant 7th chord.

Subglottal pressure is controlled by the respiratory system. Expanding the lungs causes a pressure drop (inhalation) while compressing them produces an overpressure (exhalation). The subglottal pressure is controlled by both muscles and the elasticity of the thoracic-pulmonary unit. Typically, it varies with lung volume; high volumes, resulting from a deep inhalation, produce high pressures and often also loud voice. However, irrespective of lung volume, subglottal pressure can be lowered by activating inspiratory muscles, thus counteracting the expiratory force caused by elasticity. For example, singing high notes in pianissimo at the beginning of a long musical phrase will require contraction of inhalatory muscles: the diaphragm and the external intercostals. 

Another physiological parameter of major importance to voice quality is glottal adduction, the strength of the force bringing the vocal folds together. When weak, the vocal folds will vibrate also for a low subglottal pressure and the voice becomes breathy. When strong, a higher subglottal pressure is needed for bringing the folds into vibration and the voice becomes pressed. Habitual use of pressed phonation is often associated with vocal problems.  

The mobility of the vocal folds can be measured at the lowest subglottal pressure that causes vocal fold vibration. It is referred to as the phonation threshold pressure (PTP). Typically it rises with vocal fatigue, when the vocal folds are more stiff, and decreases with higher levels of hydration. Sex steroid hormones also may alter this threshold, making it higher in the presence of a vocal fold edema. 

Further readings: 

Björklund S. & Sundberg J. (2015). Relationship Between Subglottal Pressure and Sound Pressure Level in Untrained Voices. Journal of Voice, 30(1): 15-20. 

Chang, A. & Karnell, MP. (2004). Perceived Phonatory Effort and Phonation Threshold Pressure Across a Prolonged Voice Loading Task: A Study of Vocal Fatigue. Journal of Voice, 18(4): 454-466. 

Chan, RW. & Titze, IR. (2006). Dependence of phonation threshold pressure on vocal tract acoustics and vocal fold tissue mechanics. The Journal of the Acoustical Society of America, 119(4): 2351–2362. 

Enflo, L. & Sundber, J. (2009). Vocal fold collision threshold pressure: An alternative to phonation threshold pressure? Logopedics Phoniatrics Vocology, 34: 210-217. 

Enflo, L., Sundberg, J. & McAllister, A. (2013). Collision and Phonation Threshold Pressures Before and After Loud, Prolonged Vocalization in Trained and Untrained Voices. Journal of Voice, 27(5): 527-530. 

Enflo, L., Sundberg, J., Romedahl, C. & McAllister, A. (2013). Effects on Vocal Fold Collision and Phonation Threshold Pressure of Resonance Tube Phonation with Tube End in Water. Journal of Speech, Language and Hearing Research, 1530-1538.  

Fisher, KV. & Swank, PR. (1997). Estimating Phonation Threshold Pressure. Journal of Speech, Language and Hearing Research, 40: 1122-1129. 

Hertergard, S., Gauffin, J. & Lindestad, P-A. (1995). A Comparison of Subglottal and Intraoral Pressure Measurements during Phonation. Journal of Voice, 9(2): 149-155. 

Jiang, J., O’Mara, T., Conley, D. and Handson, D. (2009). Phonation threshold pressure measurements during phonation by airflow interruption. Laryngoscope, 109 (3): 425:432. 

Lã, FMB. & Sundberg, J. (2012). Pregnancy and the singing voice: reports from a case study. Journal of Voice, 26(4): 431-439. 

Motel T., Fisher KV. & Leydon, C. (2003). Vocal warm-up increases phonation threshold pressure in soprano singers at high pitch. Journal of Voice, 17: 160–167. 

Solomon, N.P. & DiMattia, M.S. (2000). Effects of a Vocally Fatiguing Task and Systemic Hydration on Phonation Threshold Pressure. Journal of Voice, 14(3): 341-362. 

Sundberg, J. (2018). Flow Glottogram and Subglottal Pressure Relationship in Singers and Untrained Voices. Journal of Voice, 32(1): 23-31. 

Titze, IR. (1988). The physics of small-amplitude oscillation of the vocal folds. Journal of the Acoustical Society of America, 83(4): 1536-1552. 

Titze, IR. (2009). Phonation Threshold Pressure Measurement with a Semi-Occluded Vocal Tract. Journal of Speech, Language and Hearing Research, 52: 1062–1072. 

Titze, I. R. (2021). Simulation of Vocal Loudness Regulation with Lung Pressure, Vocal Fold Adduction, and Source-Airway Interaction. Journal of Voice. In press. 

Verdolini-Marston K., Titze, IR. & Druker, DG. (1990). Changes in phonation threshold pressure with induced conditions of hydration. Journal of Voice, 4: 142-151. 

Verdolini, K., Titze, IR. & Fennell, A. (1994). Dependence of phonatory effort on hydration level. Journal of Speech and Hearing Research, 37: 1001-1007.