Influence of room acoustics on noise exposure from mobile air purifiers in educational institutions

Status:

completed 07/2022

Aims:

To reduce the risk of infection from SARS-CoV-2, more and more mobile air purifiers are being used in indoor areas where ventilation is inadequate. However, especially in educational institutions such as schools and day-care centres, strict regulations are in place concerning noise exposure to protect public health. This project investigated which kind of room acoustic configuration is required to meet the current national regulation 'Technical Rules for Workplaces ASR A3.7 on Noise', whenever mobile air cleaners are in use and also to ensure that human speech is as intelligible as possible.

Activities/Methods:

To carry out the study, two air purifiers were purchased, both of them complying with the existing funding guidelines of the German federal states and also with the recommendations and requirements published by the German Social Accident Insurance (DGUV). To measure noise emission levels, various practical tests were carried out under the latest specific methods, e.g. the expert recommendation VDI EE 4300-14, and the results were then compared with the details provided by the manufacturers (who remained anonymous). Based on the measured values, sound propagation and room acoustic simulations were carried out to determine the sound pressure levels in the room and the resulting intelligibility of speech. The simulations were performed for a typical classroom and the room of a group in a day-care centre, where comparative measurements were also taken. The determining factors were the spatial positioning of a single air purifier, the simultaneous use of several air purifiers and different versions of room acoustic configurations.

Results:

In the study, both air purifiers have 3 to 5 dB higher sound power levels than specified by the manufacturers. To ensure that noise emission is measured correctly, it is essential to use low-noise measuring equipment, especially at low performance stages of the air purifiers.

To achieve the necessary minimum air exchange of at least 4 times per hour, it was not possible to stay within the maximum background noise level of 35 dB L_Aeq, as required in ASR A3.7. This applies both to the simulations and to the measurements conducted in a day-care centre. When using the room acoustic configuration that meets the indoor acoustics requirements for educational facilities according to ASR A3.7, it is possible to reduce the noise level caused by the air purifier in a room, but it is still not possible to stay within the limits specified for background noise. In such a room, however, the intelligibility of speech clearly benefits from good room acoustics. By using two mobile air purifiers at a low performance stage, it is possible to obtain a high level of speech intelligibility provided that there is a good room acoustic configuration. In the future, however, closer attention will need to be paid to the effects on people’s efforts in speaking and listening and also the resulting non-auditory noise impact in such a scenario.

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