Conrad Von Zabern's Thoughts on Singing High Notes
‘Another fault which is more obvious than the others is singing high notes with an unstintingly full and powerful voice. This is even more careless than what we have cited above, as will soon become evident. When this shouting is done by individuals with resonant and trumpet-like voices it disturbs and confuses the singing of the entire choir, just as if the voices of cattle were heard among the singers. In a certain eminent collegiate establishment I once heard singers with these trumpet-like voices singing with all their strength in the highest register as if they wished to break the windows of the choir, or at least to shake them. As I marvelled not a little at their coarseness, I was moved to make up this rhyme: In choir you bellow Like cows in the meadow I use this jingle in an informal fashion in my efforts and teaching regarding the art of good singing in order to ridicule all those presuming to sing loudly in the high register, to the end that they might recognize their careless crudeness and, after recognizing it, zealously desist from it. In order to recognize this error completely it must be realized that whoever wishes to sing well and clearly must employ his voice in three ways: resonantly and trumpet-like for low notes, moderately in the middle range and more delicately for the high notes-the more so the higher the chant ascends. Whosoever does otherwise is careless in his singing, and yet there are innumerable ecclesiastics who make this error a primary rule of their singing... Therefore, let him who wishes to sing flawlessly never again presume to sing with a full and strong voice in the upper register, for this disfigures the chant, pointlessly weighs down and fatigues the singer, makes him hoarse and consequently useless for singing. The human throat is delicate and easily injured when it is abused, as it is by loud singing in the upper register. The harm having been done, hoarseness soon ensues. Everyone has experienced this personally. But on the other hand, when one sings with a delicate tone in the upper register the voice then corresponds to the high- pitched sound of the small pipes of the organ, as well as to the upper range of the monochord. In this fashion one can also sing without fatigue and can sing higher than would ever be possible with full voice. Thus in chants of unusually high range one can help the choir dependably and well without either injury or hoarseness-something which is not possible when using full voice’
*Edit* perhaps a ‘strong voice’ is a better translation for the term ‘valida voce’ found in the last sentence.
‘Alia rusticitas prae ceteris notabilior est in acutis sive altioribus notis cantus plena arteria sive forti et valida voce cantare, quod prae aliis supra commemoratis nimis est indiscretum, ut mox infra satis patebit. Et quando hoc fit a personis tubales sive grossas voces habentibus, nimium perturbat et confundit totius chori cantum, sicut si quaedam bovinae voces inter cantantium voces audirentur. Et audivi tamen in quodam notabili collegio, quod cantores tubales voces habentes validissimis vocibus iuxta omnes vires suas in acutis sive in altioribus cantabant, quasi chori fenestras rumpere vellent cantando vel saltem movere, ut ego non parum de eorum admirarer ruditate utque moverer ad faciendum hunc rigmum: Ut boves in pratis, sic vos in choro boatis. Quo rigmo familiariter in actibus meis sive lectionibus de modo bene cantandi deridere volo omnes valida voce cantare praesumentes in acutis, ut amplius discant suam indiscretam rusticitatem agnoscere et post agnitionem ab hac studeant resilire. Pro hac ergo rusticitate plenius agnoscenda est sciendum, quod quilibet discrete et bene cantare volens debet sua voce uti trivarie, hoc modo scilicet: grossius sive tubalius in gravibus, id est inferioribus notis, et medio modo in mediis, et subtilius in acutis, id est altioribus notis, et hoc magis magisque, quo cantus altius vadit. Qui secus agit, indiscrete se habet in cantando, sit qualiscumque aut quantuscumque. Et sunt tamen innumerabiles ecclesiasticae personae hanc indiscretionem quasi pro regula servantes in cantando... Caveat ergo quisque irreprehensibiliter cantare volens, ne ultra in futurum plena et valida voce in acutis, id est altioribus notis, cantare  praesumat, quia hoc primo cantum ipsum deformat et secundo cantantem inaniter gravat et fatigat ac tertio ipsum cito facit raucum et per consequens ad cantandum ineptum. Arteria enim hominis delicata est et faciliter laeditur, quando violentatur, quod utique fit, quando in acutis valida voce cantatur; laesione autem facta mox sequitur raucedo, sicut quisque in se experitur. E contrario vero, cum quis voce subtiliata canit in acutis, vox debit am habet correspondentiam ad gracilem sonum tam parvarum cannarum organi quam superioris partis chordae ipsius monochordi. Item secundo sine fatigatione tunc canit. Item tertio haud dubium multo altius sic cantare poterit quam plena et valida voce ullo modo posset, ut sic in cantibus notabilis altitudinis possit chorum fideliter et bene iuvare sine gravamine et sine raucedinis incursione, quod valida voce non esset possibile.’
Conrad von Zabern, Die Musiktraktate Conrads von Zabern, ed. Karl-Werner Gümpel (München: Bayerische Akademie Der Wissenschaften, 1956). Translation by Joseph Dyer, "Singing with Proper Refinement": From De Modo Bene Cantandi (1474) by Conrad von Zabern,” Early Music 6, no. 2 (April 1978): 207–29, https://doi.org/10.1093/earlyj/6.2.207.
The image is from Francinus Gaffurius' 'Musica Practica,' which was printed in Milan, 1496.