The Science
of
Screams
尖
叫
聲
的
科
學
by Raphaella So
蘇韋霖
Human
ears are extremely sensitive
to signals for help. Shrieks or calls of distress
natural ly el icit immediate head turns in al l
di rections. This evolutionar ily advantageous
response is easy to comprehend. If someone
nearby screams, “fire”, the instant response would
be to run for your life and hopefully escape
danger. It turns out that human screams possess
a distinctive acoustic property that activates a
specific centre in the brain and generates the
fight-or-flight response to a perceived harmful
event or threat of survival.
Our brains respond dif ferently to var ious
frequencies in the audio spectrum. By determining
the fundamental frequency of someone’s voice,
we can judge their gender. Similarly, by paying
attention to the slow temporal fluctuations in
speech, we can decipher the tone, underlying
meaning or emotion of the deliverer. To enable
ourselves to respond quickly to distress signals,
it would be reasonable to be able to detect
d i s t i nct i ons bet ween no rma l speech and
unpleasant sounds such as screams and alarms.
It turns out that sounds with rapid modulation
between 15 – 300 Hz in frequency have this exact
distinction. In other words, the sound fluctuates
from loud to soft rapidly [1]. While conventionally
thought to carry no ecological significance, this
region is now identified as “roughness” and plays
an important role in recognising danger.
Also known as a modulation power spectrum,
roughness is the measure of the volume of sound
changing over shor t per iods of time. These
fluctuations are not found elsewhere in normal
speech. Compared to neutral vocalisations
such as singing or speak ing, the roughness
component is significantly stronger in screaming.
This finding also transcends spoken languages,
even when some languages appear to sound
more aggressive than others. Meanwhile, artificial
alarms such as sirens make use of this special
audio niche. Their roughness indices mimic those
of natural screams, thereby allowing sirens to alert
for danger.
