While humans are primarily visual creatures, the human brain has developed an uncanny ability to discern distance with hearing. The acuity of the ability to hear sounds and determine distance is as fast as 40 milliseconds. Duje Tadin, associate professor of brain and cognitive sciences at the University of Rochester, reported a set of experiments that defined the limits of humans hearing in the determination of distance.
The results of the experiments indicate that people have an unconscious mechanism that determines distance based on sound. The speed at which people respond to a sound that indicates distance is faster than the average speed that conscious action and the chemistry of nerve impulses for conscious thought would allow. The ability to judge distance by hearing alone may have developed as a protective mechanism in early man.
The researchers proved their theory with two experiments. In one experiment, participants wearing 3D glasses were exposed to visual images that were accompanied by a clicking sound. The sound either preceded the sound or was heard after the image was seen. Even though the images were the same distance from the participants, all of the participants claimed the image was at a greater distance if the click was heard after the image was seen.
The same experimental set up determined that the average time that a person could discern distance by hearing a sound was 42 milliseconds. This experiment involved seeing an image that was moving away from a person or toward them while the participants heard a sound. The participants could not consciously detect sound delays in either experiment but could judge distance accurately through hearing.