Humans are traditionally considered quite insensitive to infrasound (sound with frequencies below 20 Hz) and low frequency sound (sound with frequencies in the range 20-200 Hz). Nevertheless, cases are occasionally seen, in which persons claim to be disturbed by infrasound or low frequency noise. Often, there is no obvious noise source, and often noise measurements show very low levels. Information of such cases is uncoordinated, and it is found, e.g., in complaints at various environmental authorities and in case reports in the daily press. Additionally, the Department of Acoustics receives numerous phone calls and written enquiries for help. With the purpose of establishing a more systematic knowledge about these cases, a questionnaire study was conducted with participation of approximately 200 annoyed persons. The persons typically describe the sound as deep and humming or rumbling, like coming from a distant idling engine of a truck or pump. Nearly all respondents report of a sensory perception of a sound. In general, they report that they perceive the sound with their ears, but many mention also a perception of vibrations, either in their body or of external objects. The sound disturbs and irritates during most activities, and many consider its mere presence as a torment to them. Many of the respondents report on secondary effects, such as insomnia, headache and palpitation, which they associate with the sound mainly because it occurs at the same place as the sound. Only a small fraction of the persons has had the problems solved through noise abatement of a known noise source. In a majority of the cases, only a single or few persons can hear the sound, but there are also examples, where it is claimed to be audible to everybody. Typically, measurements have shown that existing limits (and normal hearing thresholds) are not exceeded. The investigation leaves the key question: Are the troubles induced by an external sound or not, and, if they are, which frequencies and levels are involved? With the purpose of answering these questions, a new project has been initiated, in which a number of complainants are carefully examined (see project "Explanations of annoyance from low frequency noise at very low levels"). The questionnaire has been initiated and conducted with the assistance of the mediation network AKUNET.
|Period||31/12/02 → 31/12/02|