We know pretty well how auditory processing works. But exactly how our perceptual system distinguishes between an aria in the auditory pathway and a spoken sentence remains unclear. Of course, musical and spoken forms of sound waves have different pitches, timbres, phonemes, and melodies. But the brain doesn’t process all of these at the same time.
A team of scientists from New York University, the Chinese University of Hong Kong, and the National Autonomous University of Mexico decided to find out what cues the brain relies on to quickly distinguish between singing and speaking. Experts believe the key clue is amplitude modulation (how quickly the volume, or “amplitude,” of a series of sounds changes over time).
Previous scientific studies have shown that the frequency of amplitude modulation in speech is fairly constant across languages, ranging from 4 to 5 Hz. That is, the sound wave rises and falls four to five times per second. Moreover, the frequency of amplitude modulation in music is constant across genres, at around 1−2 Hz. In other words, the pitch of our voice changes much more quickly when we speak than when we sing over a period of time.
In four experiments, 300 people listened to audio files and then had to say whether they sounded more like speech or music (singing). It turned out that audio clips with slower amplitude modulation rates and more regular rhythms were more likely to be rated as music, while the opposite pattern was more likely to be rated as speech. This means that our brains associate slower, more regular changes in amplitude with music, and faster, more irregular changes with speech.
Source: Ferra
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