Low frequency noise during work. Effects on performance and annoyance

Abstract

Aims. Low frequency noise (LFN) is defined as "a noise with a dominant frequency content of 20 to 200 Hz". Common sources of LFN in occupational environments are ventilation, heating and air-conditioning systems, computer network installations and compressors. The aims were to evaluate the influence of LFN on performance, annoyance, other subjective effects, cortisol levels and subjective stress. A further aim was to evaluate whether the frequency balance and modulation frequency in a LFN influenced a subject's perception of pleasantness. Methods. All studies were laboratory experiments. The experiment reported in papers I and II comprised 32 subjects who worked for 2 h with four performance tasks under high workload during exposure to LFN and a reference noise at an A-weighted sound pressure level of 40 dB. The experiment reported in paper III comprised 38 subjects who worked for 4 h with six performance tasks under low workload during exposure to LFN or reference noise at an A-weighted sound pressure level of 45 dB. The experiment reported in paper IV comprised 30 subjects who varied the level of the sound characteristic's frequency balance and modulation frequency in the LFN to make the noise more pleasant.Results. LFN impaired performance in tasks with high and moderate demands on cognitive processing when carried out under high workload and in tasks with moderate and low demands when these were performed under low workload. No difference between noise conditions was found in low demand tasks performed under high workload and tasks evaluating motivation performed under low workload. LFN was rated to have a greater impairment on the work capacity and be more annoying than reference noise; the difference between noises was significant under high workload. No difference between noise conditions was found in subjective symptoms, but annoyance and reported impairment of the work capacity due to LFN was related to several symptoms. This was less frequently found for reference noise. Subjects high-sensitive to LFN or to noise in general performed less well and reported higher annoyance due to LFN. The effects caused by LFN were most pronounced for subjects high-sensitive to LFN. Exposure to LFN during high workload resulted in elevated cortisol levels among subjects high-sensitive to noise in general, and a tendency towards the same result was found for subjects high-sensitive to LFN. No clear relationships between cortisol levels and subjective stress were found. The resulting pleasant LFN comprised less perceivable modulations and a lower content of frequencies below 500 Hz. The effect was less marked when the original LFN did not comprise modulations and, when the A-weighted sound pressure level could be altered, a steeper slope was preferred to a higher level. Conclusions. The experiments showed that exposure to LFN during work can impair performance, lead to subjective annoyance and increase cortisol levels, even at moderate sound pressure levels. The effects were influenced by workload and noise sensitivity. The performance effects are hypothesised to be mediated by impaired learning and reduced attention. To achieve a more pleasant LFN, the noise should contain no or little perceivable modulations and a lower relative content of low frequencies.