Putting the "psycho" in acoustics Study: Why Aztec death whistles sound like human screams The basic mechanism relies on the Venturi effect, producing a unique rough and piercing sound. Jennifer Ouellette Nov 20, 2024 2:37 pm | 36 The skull-shaped body of the Aztec death whistle may represent Mictlantecuhtli, the Aztec Lord of the Underworld. Credit: Sascha Frhholz The skull-shaped body of the Aztec death whistle may represent Mictlantecuhtli, the Aztec Lord of the Underworld. Credit: Sascha Frhholz Story textSizeSmallStandardLargeWidth *StandardWideLinksStandardOrange* Subscribers only Learn moreArchaeologists have discovered numerous ceramic or clay whistles at Aztec sites, dubbed "death whistles" because of their distinctive skull shapes. A new paper published in the journal Communications Psychology examines the acoustical elements of the unique shrieking sounds produced by those whistles, as well as how human listeners are emotionally affected by the sounds. The findings support the hypothesis that such whistles may have been used in Aztec religious rituals or perhaps as mythological symbols.Archaeologists unearthed the first Aztec death whistles, also known as ehecachichtlis, in 1999 while excavating the Tlatelolco site in Mexico City. They found the body of a sacrificial victim, a 20-year-old male who had been beheaded, at the base of the main stairway of a temple dedicated to the wind god Ehecatl. The skeleton was clutching two ceramic skull-shaped whistles, one in each hand, along with other artifacts. More skull whistles were subsequently found, and they've found their way into popular culture. For instance, in Ghostbusters: Afterlife (2021), Egon Spengler had such a whistle in his secret laboratory collection.Scholars have puzzled over the purpose of the skull whistles, although given the dearth of concrete evidence, most suggestions are highly speculative. One hypothesis is that it was used in battle, with hundreds of warriors blowing their whistles simultaneously as a battle cry. Music archaeologist Arnd Adje Both has dismissed that idea, suggesting instead that the whistle's purpose was more likely tied to ceremonial or religious practices, like human sacrifice. Yet another hypothesis proposes that the whistles were intended as symbols of a deity. The skull shape, for instance, might allude to the Aztec god of the underworld, Mictlantecuhtli.Aztec death whistles don't fit into any existing Western classification for wind instruments; they seem to be a unique kind of "air spring" whistle, based on CT scans of some of the artifacts. Sascha Frhholz, a cognitive and affective neuroscientist at the University of Zrich, and several colleagues wanted to learn more about the physical mechanisms behind the whistle's distinctive sound, as well as how humans perceive said sounda field known as psychoacoustics. The whistles have a very unique construction, and we dont know of any comparable musical instrument from other pre-Columbian cultures or from other historical and contemporary contexts, said Frhholz.A symbolic sound? Human sacrifice with original skull whistle (small red box and enlarged rotated view in lower right) discovered 198789 at the Ehecatl-Quetzalcoatl temple in Mexico City. Credit: Salvador Guillien Arroyo, Proyecto Tlatelolco For their acoustic analysis, Frhholz et al. obtained sound recordings from two Aztec skull whistles excavated from Tlatelolco, as well as from three noise whistles (part of Aztec fire snake incense ladles). They took CT scans of whistles in the collection of the Ethnological Museum in Berlin, enabling them to create both 3D digital reconstructions and physical clay replicas. They were also able to acquire three additional artisanal clay whistles for experimental purposes.Human participants then blew into the replicas with low-, medium-, and high-intensity air pressure, and the ensuing sounds were recorded. Those recordings were compared to existing databases of a broad range of sounds: animals, natural soundscapes, water sounds, urban noise, synthetic sounds (as for computers, pinball machines, printers, etc.), and various ancient instruments, among other samples. Finally, a group of 70 human listeners rated a random selection of sounds from a collection of over 2,500 samples.The CT scans showed that skull whistles have an internal tube-like air duct with a constricted passage, a counter pressure chamber, a collision chamber, and a bell cavity. The unusual construction suggests that the basic principle at play is the Venturi effect, in which air (or a generic fluid) speeds up as it flows through a constricted passage, thereby reducing the pressure. "At high playing intensities and air speeds, this leads to acoustic distortions and to a rough and piercing sound character that seems uniquely produced by the skull whistles," the authors wrote. (e) Digitalization and 3D reconstruction of the skull whistle replicas using CT scans of the replicas. (f) 3D models of an original skull whistle demonstrate the air flow dynamics, construction similarity, and sound generation process. Credit: Sascha Frhholz et al., 2024 That is consistent with the rough piercing sounds of the recordings of original skull whistles, per the authors. The spectral signal contains features of pink noise, along with high-pitched frequencies. There were only minor differences between recordings of the original skull whistles and the replicas. The whistle sound is most similar to natural sounds and electronic music effects, and least similar to other instruments like Mexican flutes. Animal, human, and synthetic sounds fall somewhere in between. Finally, the whistle sounds corresponded to a distinct pitch of the modulation power spectrum (MPS) with psychoacoustic significance, associated with primate screams, terrifying music, and the like.Perhaps, then, it is not surprising that human listeners consistently rated skull whistle sounds as having negative emotional quality, as well as sounding largely unnatural, scary, or aversive. This was further bolstered by a follow-up experiment in which 32 participants listened to skull whistles and other sounds while undergoing an fMRI. Per Frhholz et al., there was a strong response in brain regions associated with the affective neural system, as well as regions that associate sounds with symbolic meaning. So the death whistles combine basic psycho-affective influences with more complex mental processes involving symbolism.This is consistent with the tradition of many ancient cultures to capture natural sounds in musical instruments, and could explain the ritual dimension of the death whistle sound for mimicking mythological entities, said Frhholz. Unfortunately, we could not perform our psychological and neuroscientific experiments with humans from ancient Aztec cultures. But the basic mechanisms of affective response to scary sounds are common to humans from all historical contexts."Communications Psychology, 2024. DOI: 10.1038/s44271-024-00157-7 (About DOIs).Jennifer OuelletteSenior WriterJennifer OuelletteSenior Writer Jennifer is a senior reporter at Ars Technica with a particular focus on where science meets culture, covering everything from physics and related interdisciplinary topics to her favorite films and TV series. Jennifer lives in Baltimore with her spouse, physicist Sean M. Carroll, and their two cats, Ariel and Caliban. 36 Comments