Why Do Alarms Make us Uneasy and Disturbed?
The ideal sound frequency for alarms is between 40 to 80 Hz and this has been used for a long time. Now neuro-scientists at University of Geneva (UNIGE) and Geneva University Hospitals (HUG), Switzerland have found the range of sound frequencies that are unpleasant. Alarm sounds, whether artificial (such as a car horn) or natural (human screams) are characterised by repetitive sound fluctuations, which are usually situated in frequencies of between40 and 80 Hz.
A study was conducted among 16
participants to find out which frequencies were perceived as rough and which
were perceived as smooth. The upper limit of sound roughness was seen at 130
Hz. They were made to hear sounds in the 40 to 250 Hz range. Above that limit, frequencies are heard as forming one continous sound.
Scientists found that frequencies considered intolerable were in the 40 and 80
Hz range. “That’s why alarms use these rapid repetitive frequencies to maximise
the chances that they are detected and gain our attention,” says the
researcher. In fact, when the repetitions are spaced less than about 25
milliseconds apart, the brain cannot anticipate them and therefore suppress
them. It is constantly on alert and attentive to the stimulus. The researchers then attempted
to find out what actually happens in the brain: why are these harsh sounds so
unbearable? “We used an intracranial EEG, which records brain activity inside
the brain itself in response to sounds,” explains Pierre Mégevand, a neurologist
and researcher in the Department of Basic Neurosciences in the UNIGE Faculty of
Medicine and at HUG.
When the sound is perceived as continuous (above 130 Hz), the auditory cortex in the upper temporal lobe is activated. “This is the conventional circuit for hearing,” says Mégevand. But when sounds are perceived as harsh (especially between 40 and 80 Hz), they induce a persistent response that additionally recruits a large number of cortical and sub-cortical regions that are not part of the conventional auditory system. “These sounds solicit the amygdala, hippocampus and insula in particular, all areas related to salience, aversion and pain. This explains why participants experienced them as being un- bearable,” says Arnal, who was surprised to learn that these regions were involved in processing sounds.
This is the first time that sounds between 40 and 80 Hz have been shown to mobilise these neural networks, although the frequencies have been used for a long time in alarm systems. “We now understand at last why the brain can’t ignore these sounds,” says Arnal. “Something particular happens at these frequencies, and there are also many illnesses that show atypical brain responses to sounds at 40 Hz. These include Alzheimer’s, autism and schizophrenia.” The neuroscientists will now investigate the networks stimulated by these frequencies to see whether it could be possible to detect these ill- nesses early by soliciting the circuit activated by the sounds.