Why are screams so chilling?

Screams are one of the most fundamental of human noises, something we're all programmed to do right from the second we're born, no matter what culture we're part of. Yet, until now, no-one has ever thought to investigate what it is that makes a scream so alarming. Could it be the sheer volume? The high pitch? Now scientists have discovered a scream-specific signal the brain is sensitive to. James Farr spoke to discoverer David Poeppel to hear what it is...

David - What we found is that screams occupy a very reserved niche of the acoustic soundscape of all the possible sounds you might entertain or generate. So, it's not just that it's speech that's very loud or song that's very loud or something like that; it's actually reserved, it has a special acoustic quality which has a weird technical definition that's actually called 'roughness'. And 'roughness' is actually the rate of modulation of the sound's loudness if you will.

James - That makes sense really. What makes us jump to attention is how quickly the sound goes from loud to quiet and back to loud again. If it does this once per second, then we say that the volume modulates at a rate of 1 hertz. So, how rough is say, normal human speech and how does this compare to the rate of modulation in a scream?

David - If you just measured or recorded our conversation right now, that turns out to be between 4 and 5 hertz - cross linguistically by the way whether I'm doing an analysis of mandarin Chinese or of German or of English. Now, if you take a sound and you modulate its amplitude much, much more rapidly like 30 to 150 hertz as what we found in our measurements, that's what gives it this quality of roughness. The more of that roughness modulation a sound has, the more screamy it sounds and the more scary you rate it in fact.

James - Now we know the key property of the scream itself but how can we find out if human screams are the only sound that has this effect on us? By measuring the local activity in all the different regions of the brain using an MRI scanner, you can see that the more rough a sound is, the more an area called the amygdala, aka 'the emotional hub' of the brain, becomes activated. You can then measure the roughness of a whole host of different sounds and predict exactly which ones will have you jumping out of your seat, just like a scream would.

David - It turns out that no other auditory signals really have this particular modulation with one crucial exception. And that turns out to be other alarm sounds. So, if you look at car alarms, police sirens, irritating alarm clocks, these are the kind of auditory signals that also have this roughness modulation. So it's interesting and it means that sound designers who make this kind of stimuli by trial and error in some sense, happened upon exactly this kind of sound quality. And that's because it's very salient and very attention grabbing. So, they're just super effective at getting you to be afraid, runaway, scream bloody murder, and what have you.

(Alarm recording)

James - Just checking you haven't switched off there. It's all very well and good, this new knowledge, but what exactly is the relevance of it?

David - We can take any old signal like me saying some not fantastically interesting sentence to you and I can modulate it in that way and make it sound like a scream.

James - It would be absolutely criminal if I didn't give this a go. Let's take the everyday sound of a yawn...and see what we can do.

That should do it. Hold on to your seats everyone...

Wow! I don't know about you, but I'm truly quaking in my boots. Still, I don't quite get what I can achieve with this though - other than using it in the next Naked Scientists Halloween Special of course.

David - Now, that means of course you can use it to make more effectively tuned alarm signals, more specific sounds, sirens, but you can also of course use it for all kinds of entertainment fun. You could make scarier movies, you can optimise this stuff by correlating it with other visual information and so on.