When you hear a sudden noise, you instinctively turn your head in the direction of the sound. We use sound to gather information and avoid danger.
Have you ever wondered how our two ears precisely locate the source of a sound in three-dimensional space? If one ear is deaf, can you only hear the sound but not determine its direction?
To answer this question, we need to understand how humans determine the direction (left, right, front, back, up, down) and distance of a sound.
Binaural Spatial Hearing: The Power of Two Ears
Binaural spatial hearing is the primary mechanism for sound localization. Human sensitivity to sound varies with direction. Specifically, we are most sensitive to sounds on our left or right, followed by front and back, and lastly, up and down. In essence, we are better at distinguishing whether a sound is coming from the left or right, rather than above or below. Two key factors determine the direction of sound received by our ears: Interaural Time Difference (ITD) and Interaural Level Difference (ILD). Sir Rayleigh, a British physicist, first proposed the "binaural effect" theory in 1896. When the sound frequency is below 1500 Hertz, the sound reaches the ear closer to the source first, creating an ITD. For frequencies above 1500 Hertz, the phenomenon known as the "head shadow effect" occurs, where the sound is blocked by the head, resulting in an ILD. In the 1960s, scientists discovered that humans can discern time differences as short as 10 microseconds and level differences as small as 1 decibel.
Pinna Effect: One Ear Can Still Navigate
If the sound source is centered left to right but differs vertically, and the time and volume reaching both ears are the same, we cannot use time and level differences to distinguish the direction. How then do humans locate sources above and below? Enter the pinna effect. The shape of our earlobes affects sound perception, especially for low-frequency sounds. When external sounds reach our ears, the pinna reflects the sound, creating a set of brief delayed reflections. The irregular, elongated shape of our earlobes, with various protrusions and recesses, causes subtle differences in time and intensity for reflected sound from different directions. Our brain utilizes these differences to determine sound direction.
Also known as the monaural effect, the pinna effect allows us to discern the source of sound in three-dimensional space with just one ear. While human ears may seem similar, each person's earlobe shape is unique. From birth, our nervous system continually learns to use our distinctive ear shape to accurately judge sound direction. After years of learning and training, our brains become adept at detecting the subtle differences in reflected sound caused by these fixed earlobes.
The pinna effect has intriguing applications, such as its description in acoustic physics as the Head Related Transfer Function (HRTF). Altering the earlobe's shape changes the HRTF, leading to corresponding changes in perceived sound. Advanced audio technology now utilizes HRTF to manipulate sound's subtle differences in delay and intensity, delivering an immersive surround sound experience with just a pair of headphones. The pinna effect also guides our choice of headphones. In-ear earphones direct sound into the ear canal, unaffected by the pinna. On-ear headphones, however, emit sound waves that first pass through the pinna, mimicking the natural propagation of sound. Therefore, considering the pinna effect, over-ear headphones with a loose fit, minimizing changes to earlobe shape, contribute to a more natural and realistic soundstage.
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