When a sound reaches our ears, it doesn’t always arrive at the exact same moment for both sides. This slight difference in arrival time, known as interaural time difference (ITD), is a fundamental cue our auditory system uses to pinpoint where a sound originates in space. The brain constantly analyzes these minute temporal disparities to comprehend how humans perceive sound direction.
The Physics of Sound Arriving at Your Ears
Sound travels through the air at a finite speed, approximately 343 meters per second at 20 degrees Celsius. When a sound source is positioned anywhere other than directly in front of or behind a person, sound waves travel a slightly different distance to reach each ear. This path length difference means the sound will arrive at one ear before the other, creating the interaural time difference.
For instance, if a sound comes from your left side, the sound waves will reach your left ear first before traveling the additional distance around your head to your right ear. The maximum ITD a human can experience is around 600 to 700 microseconds, which occurs when a sound is directly to one side.
ITD is effective for localizing low-frequency sounds, those below 1000 to 1500 Hz. These low-frequency sounds have longer wavelengths, larger than the human head. Because of their longer wavelengths, these sounds can bend around the head (diffract) with minimal loss of intensity, making the time difference in their arrival the primary cue for their direction.
How Your Brain Calculates Sound Location
The brain acts as a time difference detector, performing rapid calculations to determine sound location. Specialized neural circuits, primarily in the brainstem’s superior olivary complex, process auditory signals from both ears simultaneously.
Neurons within these brainstem areas compare the arrival times of signals from the left and right ears. They are sensitive to time differences as small as 10 to 20 microseconds. By detecting these minute disparities in arrival times, the brain computes the sound’s horizontal angle, or azimuth, relative to the listener.
This mechanism is primarily responsible for localizing low-frequency sounds, where the phase difference between the sound waves at each ear provides reliable directional information.
Other Ways We Localize Sound
Beyond interaural time difference, another cue for sound localization is the interaural level difference (ILD), also known as interaural intensity difference. ILD refers to the difference in sound intensity or loudness between the two ears. This difference arises from the “head shadow” effect, where the listener’s head acts as an obstacle, blocking or attenuating sound waves as they travel to the far ear.
The head shadow effect is more pronounced for high-frequency sounds, those above 1500 Hz. These shorter wavelength sounds are more easily blocked by the head, resulting in a noticeable loudness difference between the ears. Consequently, ILD serves as the primary cue for localizing high-frequency sounds, complementing ITD’s role with low frequencies.
The brain integrates these primary cues with other supplementary information to create a comprehensive perception of sound in three-dimensional space. Pinna cues, which are spectral changes caused by the complex shape of the outer ear, provide information about sound elevation. Additionally, head movements play a role, allowing the brain to continuously update and refine its localization estimates by altering the ITD and ILD values. The auditory system combines all these cues—ITD, ILD, pinna cues, and head movements—to construct an accurate and detailed map of sound sources in the environment.