The development of acoustic studies as related to the opera singer gained some steam through the consistency that can be observed in the tenor high voice. If anything had been made clear is that most of the great tenors display a clear strategy between the dominance of the first formant (F1) in the lower range and the dominance of the second formant (F2) in the higher range and a near equal balance on notes that lie at the border between F1 and F2 dominance—the area we call the passaggio (Fundamental Frequency depends upon vowels–i.e. the passaggio begins earlier for low-F1 vowels like [u] and [i] and later for [E] and [a], [o], [O] and [e] fall in between extremes). Beyond F1/F2 balance, the presence of the Singer’s Formant (SF), attributed for the “ring” or “squillo” (that which makes the voice present in the listener’s ears when the singer is accompanied with orchestral forces), is expected throughout the male range (not all singers achieve this. Even the great Pavarotti had little SF in his voice).
F1: [a] (or what passes for [a] in context) = 750 Hz (+/- 100Hz); F2: [a] (or what passes for [a] in context) = 1200Hz (+/- 100Hz)
F1: [o] (or what passes for [a] in context) = 400 Hz (+/- 100Hz); F2: [o] (or what passes for [o] in context) = 1000Hz (+/- 100Hz)
F1: [u] (or what passes for [u] in context) = 260 Hz (+/- 100Hz); F2: [u] (or what passes for [u] in context) = 900Hz (+/- 100Hz)
F1: [E] (or what passes for [E] in context) = 350 Hz (+/- 100Hz); F2: [E] (or what passes for [E] in context) = 1450Hz (+/- 100Hz)
F1: [i] (or what passes for [i] in context) = 280 Hz (+/- 100Hz); F2: [i] (or what passes for [i] in context) = 2100Hz (+/- 100Hz)
SF= c. 3000 Hz for female voices (c. 2800 Hz for males). The SF is broad ranging enough that it can catch harmonics as low as 2700 Hz in the female voice (as low as 2500 in lower males voices).
Female voices are much less analyzed for the simple reason that female voices show little consistency by the time that acoustic analysis became available to the average voice geek (pedagogy teachers and the like). This is fascinating. In a sense, the standards for male voices have remained consistent (though many professionals do not meet the standards). For female voices the standards have pretty much fallen off.
The balance of F1 and F2 is just as important in the females voice as it is in the male voice. The two formants determine accuracy of vowel shaping (modification) to fit the needs of the overtone series that is being produced on a given note. However, this dynamic between source-tone and vocal-tract filtering can only have meaning when a strong source tone is produced. This necessitates that the vocal folds close completely during close phase of phonation but that they are not “pressed” tightly together, for the propagation of air is what we hear as sound pressure. Since the discovery of the F1 limits, it began to become clear why women have issues in the lower passaggio. The range of F1 limits, from around c4 on the [i] vowel to f4# on the [a], is precisely the area of problem for female voices and not coincidentally, also for male voices. The difference is that the event occurs from the middle into the high range for male voices, whereas it is the low into the middle for female voices.
Given that the majority of opera singers are obsessed with the development of their important top notes, the lower middle range of female voices in particular (true of high males voices too) is often under-developed and that is certainly the case with women in modern operatic times. Instead of acknowledging the difficulties in the lower passaggio and work (as in the past) to even it out by developing substance and support, modern pedagogy “dictates” that the singer should go into some kind of “mixed” phonation in the passaggio. This word “mixed” unfortunately means different things to different people.
If mixed meant working the substance of the chest voice with the stretch of the folds that facilitates high overtones and “head” sensation, all the while maintaining efficient closure (fully closed but not pressed) the results would be most satisfying. Instead of working the two main muscle groups opposite each other to achieve a balance (taut and substantial vocal folds yield a much more efficient vibration. See here), the easier but vocally inefficient option is to loosen the closure of the vocal folds resulting in a weak source tone that yields little power under acoustic analysis.
It should also be understood that the acoustics of the voice is a comparative exercise. Having a strong F2 dominance in the upper range means that vowel modification is correct but it says very little about carrying power. In the best singers, appropriate F1 or F2 dominance is accompanied with an equal dominance in the SF (i.e. The strength of the SF should match that of the dominant lower formant [F1 or F2).
The average female voice shows very little in the way of balance between F1 and F2 in the lower and middle range up to e5 where both formants should still be consistently active. Beyond f5 (the fundamental frequency, F2 presence depends on vowel. F2 often falls between harmonics (overtones) on the [a] vowel and therefore F1 dominates until around b5 and then F2 of the [a] vowel takes over again. More importantly, when the source tone is strong and formants are appropriately managed the SF is also very active in the female voice, even in the high range. F1 and F2 of other vowels are unreliable as they fall most often between harmonics beyond f5.
When we listen to Lisa Della Casa and Orelia Dominguez, the result is precisely what is explained above. Voices that have a lot of body (substance–fold contact area) and and a lot of brilliance, with the vocal folds adducted completely but not “pressed”. For modern ears used to hollow, darkened voices, these voices might sound overly bright on first hearing. If however one can recalibrate the ear to become sensitive to the balance between low and high overtones, then one understands why these voices sound so visceral, powerfully present and neither shrill nor artificially darkened.
Lisa Della Casa, R. Strauss: Es gibt ein Reich (Ariadne auf Naxos)