# Real-Time Visual Feedback in Singing Pedagogy: Current Trends and Future Directions
## 2 The Respiratory Subsystem
### Subglottal Pressure  
Breathing patterns influence lung volume, which affects the elasticity of the breathing apparatus and, hence, is significant to \(p_{\text{sub}}\). Along with adduction, extension and tension of the vocal folds, \(p_{\text{sub}}\) is also a key physiological parameter for controlling voice quality. Subglottal pressure is the main tool for controlling sound pressure level (SPL), which is significant to perceived vocal loudness [18; 32]. Besides affecting SPL and loudness, \(p_{\text{sub}}\) also has a strong influence on vocal fold contact time and closing speed [20; 33]. Acoustically speaking, increasing \(p_{\text{sub}}\) will decrease the tilt of the voice source spectrum, thus enhancing the higher frequency partials more than the lower [18; 32; 34]. Further, it needs to be increased with increasing\(f_{\text{o}}\). Also, \(f_{\text{o}}\) is slightly affected by \(p_{\text{sub}}\). An increase in \(p_{\text{sub}}\) tends to slightly increase \(f_{\text{o}}\)[18]. Thus, singers need to learn to produce the correct pressure for a given pitch and loudness before the tone starts [2]. This pre-planned fine-tuning is quite essential for vocal control. One way of automatizing this ability is to practice regularly staccato and arpeggio exercises [35]. In addition, exaggerated \(p_{\text{sub}}\) may lead to stronger vocal folds collisions, which, when habitual, often produce voice disorders [33]. Figure 3: Audio, lung volume (LV), ribcage (RC) and abdominal wall (AW) signals from a phrase of “Summertime” ([PERSON]), as sung by a female jazz singer and recorded by an omnidirectional microphone and the RespTrack System. Respiratory signals are displayed by the software Sopran (by [PERSON], www.tolvan.com, accessed on 6 September 2021): (1) inhalation; (2) phonation; (3) abdominal wall contraction, see text. Subglottal pressure peaks can be estimated from the intraoral pressure during the occlusion of the consonant /p/, as this consonant is produced with an open glottis and a closed mouth so the lungs-to-lips airway is open [36]. The intraoral pressure can be captured with a small tube inserted into the corner of the mouth. If attached to a pressure meter, e.g., the PG-100E unit (Glottal Enterprises, Syracuse, NY, USA), it can be visualized while singing syllables /p + vowel/ (Figure 4). Intraoral pressure peaks can be monitored in real-time using an oscilloscope or by means of the RespTrack recorder software. Providing real-time visual feedback of \(p_{\text{sub}}\) can be helpful not only for vocal health, but also for controlling vocal expressiveness. For example, \(p_{\text{sub}}\) is crucial to musical phrasing. Figure 5 shows audio and corresponding pressures for the first six bars of the aria, \"O mio babbino caro\", from the opera _Giunni Schicchi_ by [PERSON]. The excerpt was sung by a female soprano, substituting the lyrics with the syllable /pa/. The left red box highlights the first three notes of the first bar. Although they have the same pitch (Ab\({}_{3}\)), the \(p_{\text{sub}}\) peaks increase in value throughout this note sequence. This is required to produce a crescendo. The right red box illustrates another example of the use of \(p_{\text{sub}}\) for the purpose of musical phrasing; the highest pressure was not produced for the highest pitch in the phrase (Ab\({}_{5}\)), which occurs in an unstressed position of the bar, but in the following note (Eb\({}_{5}\)), which occurs in a stressed position in the bar (the first beat). Figure 4: (1) Subglottal pressures for individual notes of an arpeggio, recorded by a PG-100E unit (Glottal Enterprises, USA) and displayed by the software Sopran. (2) Plastic tube placed in the corner of the mouth. (3) PG-100E pressure meter unit (Glottal Enterprises, New York, NY, USA). Figure 5: Recording of the first 6 bars of the aria, “O mio babbino caro”, from the opera _Giunni Schicchi_ by [PERSON], sung by a female soprano on the syllable /pa/. Intraoral pressures were recorded with the PG-100E unit and audio with an omnidirectional microphone.