Our study highlights several issues. First of all, the sound levels were very high. The overall equivalent sound level (LAeq) was 60.3 dB(A) during phase 1. The maximum average per minute reached up to 76.5 dB(A). In phase 1, the thresholds of 52 and 55 dB(A) were respectively exceeded in 97.2 and 84.9% of the time whereas 66.8 and 43.9% of the time in phase 2. Second, the sound levels depended on the activity in the EMDC (number of working people and number of calls per minute). And, finally, the visual noise indicator proved itself very useful in reducing the noise level.
Noise Level assessment
The recommended limit for background noise level is 52 dB(A) for undisturbed intellectual work . The values found were greater [2, 4, 5] regardless of the considered time slot.
However, ambient noise is not the only sound perceived by the EMDs, additional noise comes from headsets too. A 20 dB(A) margin is necessary for intelligible and quality conversation. The sound level perceived directly by the EMDs could be much higher than the measured ambient noise level and may exceed the 80 dB(A) threshold requiring hearing loss preventive actions .
The ambient noise level within an EMDC seems comparable to that found in other call centres within the tertiary sector. It exceeded recommended limits almost all the time and sometimes even exceeded legal limits requiring preventive measures. This is worrisome. In EMDC, quick decisions engaging prognosis and even survival have to be made. This requires utmost attention and focus. This is hardly compatible with a very high level of ambient noise and causes stress and exhaustion. Ambient noise can also make radio transmission less audible, leading to inaccurate assessment with possibly unfortunate repercussions.
Factors Involved in Variation of Sound Level
The difference in sound level between the day and night time slots seemed logical and mostly due to a decrease in activity. Our data suggested that the average number of calls per minute declined from 2.5 by day to 1 during night time slots in phase 1 (p < 10− 3). Accordingly, the increase in activity logically leads to an increase in the sound level. In the same way, the more the people, the louder the sound. This could have several reasons. First, the mere presence of a higher number of people will obviously entail a greater noise nuisance. Finally, within EMDC, there are additional parasite noises arising particularly from operators’ interactions. There are indeed numerous exchanges between staff members ranging from simple verbal instructions without using communication headsets, to discussions mostly unrelated to work.
This partly explains the rise of the additional noise disturbances with the increase in the number of people present in the closed space of the EMDC. When a large number of employees were present in the centre, the noise level was lower when the workload was high rather than low. The reason is that an increased workload does not allow any slackness and forces the staff into focussing on work only.
A reduction of this parasite noises could be achieved by raising awareness and changing behaviours (such as forbidding off-microphone interactions) or by bringing in adjustments on work premises (less noisy material, installation or improvement of acoustic treatment solutions, enhanced office space layout …) or workstations (dual headsets, sound level controller, daily exposure time limitation).
Impact Assessment of the Noise Indicator
The visual noise indicator allowed a significant reduction of nearly 3 dB(A) in ambient noise and a 10% decrease in the proportion of time above the thresholds. This user-friendly device could lead to improved working conditions, therefore contributing to optimal call handling and better quality of work.
Bias and Limitations
This study has limitations. The use of a single sound level meter could inaccurately reflect the individual exposure to noise. Implementing different sound level meters could enhance accuracy in the measurments.
The sound level meter only gave equivalent continuous sound level for each minute. There are several fluctuating indoor and outdoor noise sources responsible for intense, punctual and significant short-term sound level increases in the EMDC: ambulances with sirens and helicopter landing and taking-off. They can exceed 120 dB(A) and be responsible of acoustics shocks. They are sources of stress but also of at least temporary hearing disorders. The most stressful and troublesome noises for the operators did not appear in our measures due to the time-averaged data processing. .
Another limitation is the short duration of recording (3 days for each phase). In addition to a possible problem regarding the representativeness of the recorded period, a possible "novelty" effect could not be ruled out. The staff’s attention, considerable at the beginning of the device installation, could have diminished over time and led to a return to the previous sound level.