Audio and informatics researchers are perhaps quite familiar with retrieval systems that try to analyse recordings to identify when an important word or phrase was spoken, or when a song was played. But I once did some collaboration with a company who did laughter and question detection, two audio informatics problems I hadn’t heard of before. I asked them about it. The company was developing audio analytics software to assist Call Centres. Call Centres wanted to keep track of the unusual or problematic calls, and in particular, any laughter when someone is calling tech support would be worth investigating. And I suppose all sorts of unusual sounds should indicate that something about the call is worth noting. Which brings me to the subject of this blog entry.
Screams occupy an important evolutionary niche, since they are used as a warning and alert signal, and hence are intended to be a sound which we strongly and quickly focus on. A 2015 study by Arnal et al. showed that screams contain a strong modulation component, typically within the 30 to 150 Hz range. This sort of modulation is sometimes called roughness. Arnal showed that roughness occurs in both natural and artificial alarm sounds, and that adding roughness to a sound can make it be perceived as more alarming or fearful.
This new study suggests that a peculiar set of features may be appropriate for detecting screams. And like most fields of research, if you dig deep enough, you find that quite a few people have already scratched the surface.
I did a quick search of AES and IEEE papers and found ten that had ‘scream’ in the title, not counting those referring to systems or algorithms given the acronym SCREAM. This is actually very few, indicating that the field is underdeveloped. One of them, is really about screams and growls in death metal music, which though interesting in its own right, is quite different. Most of the rest all seem to mostly just ‘applying my favourite machine learning technique to scream data’. This is an issue with a lot of papers, deserving of a blog entry in future.
But one of the most detailed analyses of screams was conducted by audio forensics researcher and consultant Durand Begault. In 2008 he published ‘Forensic Analysis of the Audibility of Female Screams’ In it, he notes “the local frequency modulation (‘warble’ or ‘vibrato’)” that was later focused on in Arnal’s paper.
Begault also has some interesting discussion of investigations of scream audibility for a court case. He was asked to determine whether a woman screaming in one location could be heard by potential witnesses in a nearby community. He tested this on site by playing back prerecorded screams at the site of the incident. The test screams were generated by asking female subjects ‘to scream as loudly as possible, as if you had just been surprised by something very scary.’ Thirty screams were recorded, ranging from 123 to 102 decibels. The end result was that these screams could easily be heard more than 100 meters away, even with background noise and obstructions.
This is certainly not the only audio analysis and processing that has found its way into the courtroom. One high profile case was in February 2012. Neighborhood watch coordinator George Zimmerman shot and killed black teenager Trayvon Martin in Sanford, Florida. In Zimmerman’s trial for second degree murder, experts offered analysis of a scream heard in the background of a 911 phone call that also captured the sound of the gunshot that killed Martin. If the screamer was Zimmerman, it would strengthen the case that he acted in self-defense, but if it was Martin, it would imply that Zimmerman was the aggressor. But FBI audio analysis experts testified in the case about the difficulties in identifying the speaker, or even his age, from the screams , and news outlets also called on experts who noted the lack of robust ‘screamer identification’ technologies.
The issue of scream audibility thus begs the question, ‘how loud is a scream.’ We know they can be attention-grabbing, ear –piercing shrieks. The loudest scream Begault recorded was 123 dB, and he stated that scream “frequency content seems almost tailored to frequencies of maximal sensitivity on an equal-loudness contour.”
And apparently, one can get a lot louder with a scream than a shout. According to the Guinness Book of World Records, the loudest shout was 121.7 dBA by Annalisa Flanagan, shouting the word ‘Quiet!’. And the loudest shout ever recorded is 129 dB (C-Weighted), by Jill Drake. Not surprisingly, both Jill and Annalisa are teachers, who seem to have found a very effective way to deal with unruly classrooms.
Interestingly, one might have a false conception of the diversity of screaming sounds if one’s understanding is based on films. The Wilhelm Scream, a sound sample that has been used in over 300 films. This overuse perhaps gives a certain familiarity to the listener, and lessens the alarming nature of the sound.
For more on the Wilhelm scream, see the blog entry ‘Swinging microphones and slashing lightsabres’. But here’s a short video on the sound, which includes a few more examples of its use than were given in the previous blog entry.