How Auditory Illusions Work: Exploring the Science of Hearing
The human mind is an amazingly complicated computer, capable of interpreting data given to it, extrapolating from that data to accurately predict the most likely outcome and making decisions on the fly based on that data. In most cases, even incomplete data provided by sensory perception is enough for the brain to accurately calculate missing gaps or excess information that is not meant to be there. Although sometimes, for some reason, this is not the case.
The brain either fails to perceive something that is actually there, incorrectly interprets sensory information or even falsely perceives something that does not exist. Amongst these misinterpretations, known as illusions, falls the category of auditory illusions. An auditory illusion is a distortion in the sense of hearing, revealing how the brain actually organizes and interprets aural stimulation. Auditory illusions are the aural equivalent of optical illusions, where the listener hears either sounds which are not present in the stimulus, or “impossible” sounds.
Auditory illusions highlight areas where the human ear and brain, as organic survival tools, differentiate from perfect audio receptors; this shows that it is possible for a human being to hear something that is not there and be able to react to the sound they supposedly heard. However, that does not prevent people from being fooled by auditory illusions.
Examples of Auditory Illusions
There are a multitude of examples out in the world of auditory illusions. Many auditory illusions, particularly of music and of speech, result from hearing sound patterns that are highly probable, even though they are heard incorrectly. This is due to the influence of our knowledge and experience of many sounds we have heard.
1. Binaural Beats
The third beat is perceived when two pure-tone sine waves with frequencies lower than 1500Hz and a difference of less than 40Hz between them are presented to one ear each. The third beat, known as the binaural beat, has a perceived pitch that correlates to the difference between the two presented tones.
2. Speech Perception and Multimodal Input
Contrary to popular belief, speech perception is not an auditory process. Speech perception is multimodal, meaning it combines input from different sensory inputs to enhance detection. The brain perceives speech as multiple sensory inputs all working together and more often than not, we do not separate visual input and auditory input when we are listening to a person speak. In this case, it combines visual perception with auditory perception to reduce the likelihood of hearing something ambiguous. This particular phenomenon is an excellent example of how the brain integrates sensory input.
3. The "Laurel" vs. "Yanni" Phenomenon
One of the most popular internet sensations in 2018 was a low-quality looping sound clip that had most people hearing either “laurel” or “Yanni”. People generally pay attention to three frequencies when listening to speech and the lowest of these three is key for hearing the letters l and r, which are the consonants that make up the word Laurel.
4. Front-Back Confusion
Inspired by owls and their amazing ability to find their prey by hearing alone, my team decided to test how good humans are at discerning sounds. We were surprised to find just how bad we are. As our research shows, our hearing is fooled in dramatic ways. Our experiment was simple. We tapped two metal spoons together in different positions around a blindfolded person. It was easy for them to distinguish between sounds coming from the left or right, but when we clapped the spoons directly in front of them, they almost always heard the sound as coming from behind them.
You can try this yourself - it makes for a great trick for your next dinner party. All you need is two spoons and a blindfold. You need to ask the participants to keep their head still and you should avoid giving away where you are. When you click the spoons in front of their face or down near their lap, they will typically hear it behind them. We have repeated this in many environments - in a lab, an office, a lecture theatre, a soundproof room and even out on a rural lawn. We also tested participants while standing or sitting down and we tried different kinds of sounds, including pure tones, explosions and a ringing bell. The result was always the same.
We have now also confirmed this under controlled laboratory conditions where a computer selects locations at random and a researcher taps the spoons there. With only two ears to rely on, our hearing infers direction from subtle timing and intensity cues. But these cues can be misleading. They will be similar for sounds directly in front and behind us. However, this only means we should be bad at telling where the sound is, known as “front-back confusion”. It does not explain why we hear the sound so convincingly behind us.
One reason for the illusion could be that the sounds we used are very brief. Many sounds in the real world last longer. Moving your head - and with it your ears - would break the illusion because each ear would hear the sound differently. Perhaps that is why we rarely notice this illusion in everyday situations. Our discovery highlights a limitation in how we interpret the soundscape around us.
Does our hearing rely on expectations about where sounds “should” come from? Perhaps our hearing prefers locations outside our field of view when we can’t see anything. We want to test this possibility in future research. We can test how the illusion changes after people have worn a blindfold for a longer time. This last point is important. Hearing where things are in the world matters for all of us - such as when crossing the road. But those with impaired vision must rely on their hearing. People who lost their sight only recently could be especially at risk.
Other Factors Influencing Auditory Illusions
Humans are fairly susceptible to illusions, despite an innate ability to process complex stimuli. Confirmation bias is believed to be largely responsible for the inaccurate judgments that people make when evaluating information, given that humans typically interpret and recall information that appeals to their own biases. Spatial information is processed with greater detail and accuracy in vision than in hearing.
In order to prevent hearing echo created by perceiving multiple sounds coming from different spaces, the human auditory system relates the sounds as being from one source. Sounds that are found in words are called embedded sounds, and these sounds are the cause of some auditory illusions. Composers have long been using the spatial components of music to alter the overall sound experienced by the listener. One of the more common methods of sound synthesis is the use of combination tones.