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Vocal loading is the stress inflicted on the speech organs when speaking for long periods



Background
Of the working population, about 15% have professions where their voice is their primary tool.That includes professions such as teachers, sales personnel, actors and singers, and TV and radio reporters. Many of them, especially teachers, suffer from voice-related medical problems. In a larger scope, this involves millions of sick-leave days every year, for example, both in the US and the European Union. Still, research in vocal loading has often been treated as a minor subject.


Voice organ
Voiced speech is produced by air streaming from the lungs through the vocal cords, setting them into an oscillating movement. In every oscillation, the vocal folds are closed for a short period of time. When the folds reopen the pressure under the folds is released. These changes in pressure form the waves called (voiced) speech.


Loading on tissue in vocal folds
The fundamental frequency of speech for an average male is around 110Hz and for an average female around 220Hz. That means that for voiced sounds the vocal folds will hit together 110 or 220 times a second, respectively. Suppose then that a female is speaking continuously for an hour. Of this time perhaps five minutes is voiced speech. The folds will then hit together more than 30 thousand times an hour. It is intuitively clear that the vocal fold tissue will experience some tiring due to this large amount of hits.

Vocal loading also includes other kinds of strain on the speech organs. These include all kinds of muscular strain in the speech organs, similarly as usage of any other muscles will experience strain if used for an extended period of time. However, researchers' largest interest lies in stress exerted on the vocal folds.


Effect of speaking environment
Several studies in vocal loading show that the speaking environment does have a significant impact on vocal loading. Still, the exact details are debated. Most scientists agree on the effect of the following environmental properties:

Air humidity - dry air increases stress experienced in the vocal folds

Hydration - dehydration increases effects of stress inflicted on the vocal folds

Background noise - people tend to speak louder when background noise is present, even when it isn't necessary. Increasing speaking volume increases stress inflicted on the vocal folds

Pitch - the "normal" speaking style has close to optimal pitch. Using a higher or lower pitch than normal will also increase stress in the speech organs.

In addition, smoking and other types of air pollution might have a negative effect on voice production organs.


Symptoms
Objective evaluation or measurement of vocal loading is very difficult due to the tight coupling of the experienced psychological and physiological stress. However, there are some typical symptoms that can be objectively measured.

Firstly, the pitch range of the voice will decrease. Pitch range indicates the possible pitches that can be spoken. When a voice is loaded, the upper pitch limit will decrease and the lower pitch limit will rise. Similarly, the volume range will decrease.

Secondly, an increase in the hoarseness and strain of a voice can often be heard. Unfortunately, both properties are difficult to measure objectively, and only perceptual evaluations can be performed.


Voice care
Regularly, the question arises of how one should use one's voice to minimise tiring in the vocal organs. This is encompassed in the study of vocology, the science and practice of voice habilitation. Basically, a normal, relaxed way of speech is the optimal method for voice production, in both speech and singing. Any excess force used when speaking will increase tiring. The speaker should drink enough water and the air humidity level should be normal or higher. No background noise should be present or, if not possible, the voice should be amplified. Smoking is discouraged.

A test conducted by a Health Science group, Grant-Williams. This is a test on vocal loading- amount of tension on the vocal cords.

Vocal warm-up was studied in terms of changes in voice parameters during a 45-minute vocal session in the morning. The voices of a randomly chosen group of 40 female and 40 male young students were loaded by having them read a novel aloud.

The exposure groups (5 females and 5 males per cell) consisted of eight combinations of the following factors: (1) low (<> 65 ± 5%) relative humidity of ambient air; (2) low [<> 65 dB(SPL)] speech output level during vocal loading; (3) sitting or standing posture during vocal loading.

Two sets of voice samples were recorded: a resting sample before the loading session and a loading sample after the loading session.

The material recorded consisted of /pa:ppa/ words produced normally, as softly and as loudly as possible in this order by all subjects. The long /a/ vowel of the test word was inverse-filtered to obtain the glottal flow waveform.

Time domain parameters of the glottal flow [open quotient (OQ), closing quotient (CQ), speed quotient (SQ), fundamental frequency (F0)], amplitude domain parameters of the glottal flow [glottal flow (fAC) and its logarithm, minimum of the first derivative of the glottal flow (dpeak) and its logarithm, amplitude quotient (AQ), and a new parameter, CQAQ], intraoral pressure (p), and sound pressure level (SPL) values of the phonations were analyzed.

Voice range profiles (VRP) and the singer's formant (g/G, a/A, c1/c, e1/e, g1/g for females/males) of the loud phonation were also measured. Statistically significant differences between the preloading and postloading samples could be seen in many parameters, but the differences depended on gender and the type of phonation.

In females the values of CQ, AQ, and CQAQ decreased and the values of SQ and p increased in normal phonations; the values of fAC, dpeak, and SPL increased in soft phonations; the values of AQ and CQAQ decreased in loud phonations; the harmonic energy in the singer's formant region increased significantly at every pitch.

In males the values of OQ and AQ decreased and the values of dpeak, F0, p, and SPL increased in normal phonations; the values of fAC and p increased in soft phonations. The changes could be interpreted as signs of a shift toward hyperfunctional voice production. Low humidity was associated with more hyperfunctional changes than high humidity. High output was associated with more hyperfunctional changes than low output.

Sitting position was associated with an increasing loading at both margins of male VRP, whereas the case was the opposite for standing position.