What we have learned about extreme metal production from HSDP
Here we describe laryngeal kinematics present during extreme heavy metal singing (EHMS). The aim was to explain how EHMS can be practiced without causing true vocal fold (TVF) trauma. The following EHMS vocal qualities known as growl (Gr), scream (Sc), and high & low pitch false vocal folds scratched voice (HPFSV & LPFSV) were investigated in a single subject. Glottis area was observed with distal chip scope (DCS) trans-nasal flexible fiberscopy, trans-oral HSDP, and acoustics.
Gr and Sc showed that TVF edges touched only occasionally and glottis stayed open for most of the production time while the false vocal folds (FVF) were active. HPFSV and LPFSV were produced with simultaneous steady and symmetric vibration of the TVF and FVF with the TVF adducted and the FVF partially adducted. The sound spectrographs of Gr and Sc showed a spectrum dominated by noise. It was not possible to distinguish the harmonic structure of the TVF-generated sound, proving the virtual absence of the glottic activity in the production of these aggressive sound qualities.
We observed different sets of movements (vibrations) from the supraglottic structures (SGS), generated by FVF, arytenoid caps (AC), and aryepiglottic folds (ARF). These combined vibrations were responsible for creating multi-periodic oscillations that characterize Gr and Sc. We noticed that the activity of SGS during EHMS was different from the sphincter-like activity that we find in dysphonic pathologic phonations that engage SGS.
It is our opinion that the brief duration of the glottic closure during Gr and Sc is to produce the safe range of the subglottic pressure (Ps) and to allow for the generation of high airflow required to set the SGS structures in motion. These factors, in our opinion, allow the EHM singers to produce aggressive voice qualities while avoiding phonotrauma to the TVF.