We can make many different sounds with our voices, communicating not only with what is said, but also in which context it is said, and who is saying it. This large variability makes the voice a rich channel for communication, but it also presents us with challenges when we try to assess the status of a voice using quantitative measurements, rather than by listening. When voices run into trouble, even more variability can be expected. Voice production is usually described as three processes in sequence: respiration – breathing; phonation – the vibration of the vocal folds; and articulation – changing the shape of the vocal tract, which modifies the sound into vowels and consonants. For brevity, let’s look only at some aspects of phonation that can be expected to be clinically relevant. 

Types of Phonation

The healthy vocal folds are a curious contraption that can vibrate in a number of ways, depending on how the folds are positioned and tensioned, and how their oscillation is driven by the lung pressure. Phoneticians, who study speech sounds, often use the term ‘phonation types’ [1], and so will we, but here we’ll keep them a bit simpler. One may discern four different vibratory regimes, or types of phonation, that can be exercised in speech without the speaker being regarded as having a voice pathology or deviant voice. The first three [2] are modal voice (a.k.a. chest voice, or M1), falsetto (head voice, or M2), and creaky voice (vocal fry, or M0, of which there seems to be several subtypes [3]). While M1 is predominant in normal speech, episodes of M0 and/or M2 are common, especially in voicing offset from M1 or in elevated phonation frequency (fo) for M2. In untrained voices, the M1-M2 transition is often abrupt and obvious, and can even be embarrassing, when it is unintended; it is the ‘voice break’, which classical singers train very hard to conceal. The M1-M0 transition is usually also rapid, but it tends to sound like an octave shift that does not perturb the perceived pitch contour as much as the M1-M2 transition does. Indeed, speech technologists are now looking at how to synthesise M0 appropriately, for improved naturalness of computer speech. (There is also an M3, but only in high-pitched singing.)

Breathy Voice

But wait – that’s only three regimes? Findings from electroglottography (EGG) [4] suggest that soft (breathy) voice, in which the vocal folds are vibrating without actually touching, deserves to be thought of as a regime of its own (the phoneticians do it), to which an ‘M’ category remains to be assigned – so let’s temporarily use “MX” here. If you listen to the EGG signal in very soft voicing, the onset of VF contacting is usually quite abrupt, similar to a register change, but it has little or no effect on the sound we hear. It is only at a slightly higher vocal effort that the voice timbre starts to change. In normal speakers, MX appears mostly at the offset of phonation, or in intimate or otherwise subdued voice; and more so in some individuals than in others. In vocal performance, MX is rarely used, but clinicians and voice users are quite familiar with ‘breathy voice’. 

When a voice is impaired, its owner loses some control of these habitual phonation types, and may even resort to novel coping strategies, such as trying to phonate with other body parts than the vocal folds. The clinician, therefore, would like to document the patient’s ranges in both conventional and unconventional types of phonation. For instance, if a patient after treatment can phonate in M1 over a larger range than before, that is probably a desired outcome. So how might that be shown, through measurements of physical entities? 

Mapping of metrics

After decades of research, there are now more voice metrics in the literature than one can shake a stick at. The search for a clinicians’ “holy grail” metric, however, is probably futile, because pathologies are very diverse, and no single metric is likely to function as a universal marker. Most metrics vary not just with vocal status, but considerably more with sound pressure level (SPL), fo and phonation type. This can be true even across the limited range of habitual speech, of which the softer half tends to include the contacting threshold. So in order to detect any effects of an intervention, we need to account also for those underlying variations. The good news is that, although individual voices are as different as faces, we now know that such variations tend to be quite systematic within a given voice, from accumulated experience with voice maps [5]. Voice maps are similar to voice range profiles (VRPs), but focus on how various metrics change over an elicited range, rather than on determining the informant’s extremes in SPL and fo. For clarity of example, the figure below shows a ‘clean’ voice map of a highly trained baritone doing soft-loud-soft exercises on the vowel /a/ while staying in normal modal voice. The map took 5½ minutes to record, and shows data averaged from over some 46,000 phonatory cycles. Six of the voice map’s layers are shown here.