Is it audible?
Sounds with a frequency of 20 Hz and lower are called infrasound. Low frequency sound is understood to mean sound, the frequency of which is below 125 Hz. Infrasound and low frequency sound are difficult to hear, but they cannot be called completely inaudible. Rather, one speaks of whether or not these are inaudible sound levels: one can hear it if the sound level is high enough.
Besides natural sources of infrasound and low frequency sound like volcanoes, thunder and earthquakes, there are also artificial sources: trains, air and road traffic and industry. Also ventilators, air-conditioning, compressors and loudspeakers are known sources of infrasound and low frequency sound.
Annoyance through infrasound and low frequency sound
Infrasound and low frequency sound may cause annoyance, even if they are inaudible. It has to do with the sensitivity for this type of acoustic vibrations of certain auditory nerve cells, the outside hair cells. Through this, inaudible levels of infrasound and low frequency sound can be felt as pressure in the ears.
Also not being able to identify or locate the source of a low frequency humming, can contribute to a feeling of discomfort. This is so, because sound with a low frequency can reach further than sound with a high frequency. That is why the sound source is difficult to recognize: one only hears the low frequency sound and may sometimes not be able to determine whether the perceived sound is produced by a train, truck or a technical installation. Also the direction from which a low frequency sound is coming, is difficult to determine, because the difference between what the right and left ear hear, is difficult to detect.
Sometimes sounds are heard, which are not there (tinnitus). Through unclear perception of the low frequency sound it is difficult to determine whether it concerns a “real” sound. There are also great differences in individual sensitivity, as a result of which people often have the feeling of being misunderstood. One person may hear the low frequency sound of infrasound indeed, whereas another person may not be able to hear it.
Standardisation of infrasound and low frequency sound
In the area of infrasound and low frequency sound, there is no specific regulation. Low frequency sound must comply with the same laws as the other sound. Infrasound is usually not taken into account. A possible solution is to keep the sound levels below the threshold of audibility (as described in the standard ISO 226 or ISO 389) of infrasound, although this is not always possible. When measuring a low frequency sound it is best to apply the G weighing, in accordance with the ISO 7196.
For complaints in connection with annoyance through infrasound or low frequency sound in your living environment, please contact the regional administration responsible for limiting noise pollution. An on-site investigation may help localising the source of noise.
Is it audible?