When does the whirring of fans sound pleasant? Two hearing researchers from Oldenburg are tracking the acoustic quality of ventilation.
Many people who have to work in heatwaves, such as in this summer, will have a fan in the office that at least provides a little cooling breeze. But the whirring of fans is not just ever-present in the summer: these rotating machines are also at work in computers, heat pumps and large industrial cooling systems – and can sometimes be a source of constant, irritating noise. Now two hearing researchers from the University of Oldenburg, Dr. Stephan Töpken and Prof. Dr. Steven van de Par, have developed a prognosis model which will allow manufacturers to optimise the sound of their fans. In the current issue of the "Journal of the Acoustical Society of America" they report that in particular the ratio between mid-frequency content and that from high frequencies is decisive in determining how pleasant or unpleasant the noise of a fan is perceived to be.
"The sound of a fan is an important selling point," says Töpken, a postdoctoral researcher in the acoustics division at the Department of Medical Physics and Acoustics. However, up to now, he explains, it has been difficult to assess on the basis of physical properties how pleasant a noise may sound subjectively or how loud it is perceived to be. "The technical parameters currently used do not reflect how people actually perceive fan noise," van de Par, the head of the acoustics division adds.
High frequencies sound unpleasant
The researchers performed hearing experiments with 40 test subjects for their study. The subjects were asked to assess and state their preferences on 30 noises including eleven original fan noise signals, 18 compiled by the researchers and a neutral whooshing sound. Töpken and van de Par divided the noises into several subcategories on the basis of certain physical parameters to find out which acoustic sound characteristics play a decisive role for the human auditory system. The result: higher frequencies in particular were perceived as unpleasant, and the ratio of high-frequency noises to that of middle-frequency noises should not be too high. By contrast, a high ratio of low-frequency noises does not have a negative impact on how a fan sounds.
The researchers explain that the parameters thus obtained can help manufacturers make an initial evaluation of the noise a fan makes on the basis of psychoacoustic indices without having to conduct complex hearing experiments. "This gives companies a tool which will make it easier for them to optimise fans," says van de Par. The paper was the cover topic of the June issue of the Journal of the Acoustical Society of America.