El reconocimiento del derecho de la libertad de contratación en la Constitución

Several authors have investigated the prevalence of voice problems in teachers [4, 35, 36] and all have agreed that vocal loading and vocal effort are two of the most important causes of voice dysfunctions [37, 2, 38]. Several studies have used

a portable voice analyzer to assess SPL, F0, and the phonation time: they were significantly higher in teaching situations compared with non-teaching situations [39, 40], thus highlighting a risky situation for teachers at work. Up to 50% of teachers reported having suffered adverse vocal symptoms at least once in their career [36].

In the existing literature, only few studies have been conducted on long-term voice monitoring during teachers’ working activity and such studies mainly inves- tigated the objective changes that took place in teachers’ voice parameters over a period of one working day. Rantala et al. [41] investigated voice changes during a working day in 33 primary and secondary school teachers using voice recording methods. The teachers recorded the first and the last lesson during a working day with a digital audiotape recorder and a head-mounted microphone located on the side of their mouth. The most evident change during a working day was an increase of 7.9 Hz in the mean value of F0 from the first to the last lesson. Laukkanen et al. [38] evaluated the vocal fatigue of 47 primary school teachers over one working day using a portable digital recorder and a microphone attached to the headset. They found by comparing the vocal data recorded at the beginning and at the end of the working day that teachers showed a higher SPL and a higher F0 at the end of the working activity. Hunter and Titze [42] used data from the National Center for Voice and Speech (NCVS) vocal data bank, collected by means of the NCVS Voice Dosimeter [43] over a period of two working weeks, to study the voice use of 57 teachers during occupational activities. They found that the F0 appeared to trend upwards throughout the working day, and that the teachers experienced a wide range of voicing time percentages (Dt%) (30% ± 11%). Increased F0 and SPL, following vocal loading, have been interpreted in different studies as an adaptation to vocal loading [41, 38, 43, 44].

Changes in voice production can be induced by environmental factors, such as the noise level. Södersten et al. [45] used a DAT recorder to measure voice SPL, F0, and phonation time on ten female preschool teachers during teaching situations and in a separately performed reading task without background noise: a 9 dB louder SPL and a higher mean F0 in the teaching situation compared with the reading task were found. The involuntary tendency of speakers to increase their voice level as the noise level increases in order to improve intelligibility of the speech signal is called Lombard effect [46–49]. Lane and Tranel [46] summarized a wide range of findings reported in the literature about this effect. Lazarus [47] found that the

1.1 Vocal loading 7

speech level rises as the noise level rises with a slope of 0.3–0.6 dB for each 1 dB of increase in the mean value of the A-weighted noise level distribution above 45 dB. Bottalico and Astolfi [48] and Sato and Bradley [49] studied the vocal parameters of primary school teachers in relation to activity noise levels; they found a growing rate of the speech level with the noise level of 0.7 dB/dB. To the best of the author knowledge, the relationship between the activity noise levels and voice parameters of secondary school teachers has not yet been studied. As far as the activity noise levels in secondary schools are concerned, only Shield et al. [50] measured the noise

levels during teaching activities. They found LA90 background noise levels (i.e.,

the A-weighted noise level that is exceeded by 90% of the sample) to be between 38 and 63 dB, with a mean value of 51 dB (standard deviation=6 dB). However, Shield et al. [50] and other authors [51–54], who reported the activity noise level in primary schools, only documented the activity noise conditions over one period during the school year. Therefore, it is not known whether the noise conditions remain unchanged during the course of the year, or whether prolonged exposition to high noise levels has any effect on the students’ behaviour, such as whether students make more noise or less noise.

Another important factor that should be taken into account to evaluate voice production under realistic communication conditions is the effect of room acoustics. Brunskog et al. [55] and Pelegrín-García et al. [56] found that the average voice level of speakers is closely related to the “room gain,” which represents the gain that is given to the speaker’s voice due to the reflections in the room. Room gain has been found to be closely correlated to reverberation time [57]. Brunskog et al. [58] found a variation in the voice power level at a rate of -13.5 dB per 1 dB of increase in room gain. A tendency to lower the voice level when the room gain increased was also found in a study conducted by Pelegrín-García et al. [56]. They investigated the vocal effort of 13 male subjects under four different acoustic room conditions, with a reverberation time that ranged from 0.04 to 5.38 s. They found that talkers tend to vary their voice power level at a rate of -3.6 dB per 1 dB of room gain. Bottalico and Astolfi [48] monitored the voice parameters of 41 primary school teachers over one working week, and found that SPL mean @ 1 m and reverberation time are related by a quadratic regression curve, which shows a minimum value in correspondence to an average mid-frequency reverberation time of 0.8 s in occupied conditions.

The above mentioned studies have shown that the teachers’ voice level depends on both noise and reverberation. Although noise and reverberation are simultaneously

present during teaching activities, studies that evaluate the simultaneous presence of both parameters and their combined effect on teachers’ voice use have not yet been carried out. Moreover, all the studies on teachers’ voice parameters have been conducted over a short period of time, i.e., no more than two weeks of working activities, or changes of teachers’ vocal use has been evaluated during the course of only one working day. There is a lack of longitudinal studies with repeated measures to assess changes in teachers’ vocal behaviour or noise conditions during teaching hours over a long period, such as one entire school year.