Journal of Voice: Predicted Singers Vocal Fold Lengths and Voice Classification A Study of X-Ray Morphological Measures
This is the first of, what I hope, will become a regular blog feature should these be of interest to the community. I shall pick a piece of research relevant to singers from a peer reviewed scientific journal and break it down into easily understandable information that will hopefully be of interest.
Please note that these are not thorough critical analysis but are intended to provide an overview of some relevant literature with the aim of stimulating community enthusiasm in the science behind singing.
This month's entry comes from the Journal of Voice. This study claims to show that there are consistent anatomical laryngeal differences between singers of different voice classifications, which are of relevance to pitch range and timbre of the voice.
This may initially seem very obvious. However, it is very difficult to prove due to the difficulties in collecting any consistent data. For example, every pitch with involve different lengths of vocal cords and vocal cords when open are longer than when closed. These are just 2 of the many possible variables that affect vocal fold length. However, the most difficult problem is actually in the collection of data. The position of the vocal folds makes actually getting any data very difficult. People have tried using scopes, ultrasound and even lasers. This study makes use of xrays films.
What type of study is this?
This study was a retrospective analysis. This means that certain data had already been collected and the authors decided to use this to draw a conclusion to their question.
What did they aim to find?
The authors aimed to find out whether there were consistent anatomical differences between singers of different voice classification.
What did they find?
They found that the lengths of the vocal folds were statistically significantly correlated to vocal classification. Subjects with longer vocal folds were more likely to be basses and those with shorter folds more likely to be soprano.
This study does have a significant number of problems. It is very difficult to measure vocal fold length on an x-ray. Indeed, of the 132 cases they had, they were only able to make estimates of vocal fold length in 29 individuals, clearly not enough to draw conclusions from. The authors recognized this and therefore used other anatomical landmarks to estimate vocal fold length. In other words, they measured the distance between 2 fixed points that they expected were correlated to vocal fold length, such as the width of the windpipe below the vocal folds. This, however, as a soft structure, can presumably vary in size even when measured in identical individuals. Whilst this may have been an acceptable compromise given the difficulties, it is clear that in order to make a firm conclusion, absolute lengths of vocal cords would be required. The authors did use the accurate data they had for the 29 individuals and plot these against the estimated lengths using their own anatomical landmarks. It would appear that the landmarks they were using to estimate vocal fold length and actual length are correlated. They therefore identified the problem and took steps to increase reliability of the data. However, despite this, making use of a magnetic resonance scanner would have given more reliable measurements but that was beyond the scope of the study and is not without its own limitations.
A further problem with this study is that the larynx obviously takes different positions when under different stresses. Whilst it was stated that these films were taken whilst not phonating, it is unclear as to whether the subjects were doing anything that may have altered laryngeal positioning such as raising or tilting. However, it is unlikely given that the same procedure should have been followed for all films.
Another problem arrives with the vocal classification which could be considered subjective. All students whose vocal classification had changed were not included in the study, however it was not made clear if more than one teacher had identified students vocal fach. In the scientific world, not stating this information means it was carried out. Whilst this may seem pedantic, it wouldn't surprise me if classification opinions differed between some instructors.
What are the implications?
Whilst vocal fold length and classification appear to be correlated, vocal classification should not be made based on vocal fold length due to measurement problems and the number of other variables that affect vocal fach.
Why would this happen?
Longer fold lengths should allow for a greater amount of airflow than shorter folds given the same note. This should allow for, when singers singing the same note, the individual with longer folds to be able to produce a stronger note.
Despite its problems, this study does add to our knowledge of vocal physiology. Measurement problems were adequately dealt with given the scope of this research. It does appear to show that longer vocal folds equates to a deeper vocal fach. However, it does not take into account other variables in vocal fach, such as the connective tissue and manipulations of the supraglottic (area above the vocal folds) tract which would alter the formant frequencies. The study did acknowledge this but it failed to address these issues and therefore this paper did not to fully answer it's initial question. This is a major flaw. Furthermore, it failed to even acknowledge vocal fold mass as a contributor to vocal fach.
Despite this paper not actually answering it's question, this research does add to evidence suggesting vocal fold lengths may influence vocal fach.
F Roers, D Murbe, J Sundberg. Predicted Singers Vocal Fold Lengths and Voice Classification A Study of X-Ray Morphological Measures. Journal of Voice. Volume 23. Issue 4. p.408-413.