Hearing With Two Ears: How Your Brain Works Out Where a Sound Is Coming From

Close your eyes in a park and you can still point towards a barking dog, a passing bicycle or a friend calling your name. That ability feels effortless, but it is the result of fast, precise calculations. Your brain receives two slightly different versions of every sound, one from each ear, and treats the differences between them as a map.

How do two ears tell the brain where a sound is?

Imagine a sound coming from your right. It reaches your right ear first and then has to travel around your head to reach the left. The delay is far too short to notice consciously, yet specialised nerve cells in the brainstem detect it. The bigger the delay, the further the sound is to one side.

The head also gets in the way. It casts a kind of sound shadow, so the ear on the far side receives a slightly softer version. This loudness difference is strongest for higher pitches, whose short waves are easily blocked by the head. Lower pitches, with their longer waves, bend around the head more easily, so for those sounds the brain leans more on timing.

Put together, these two clues, timing and loudness, give a fairly precise sense of left and right. The brain does not wait for you to think about it; the comparison happens automatically, in the lower parts of the brain, before the sound even reaches awareness.

What about sounds in front, behind or above?

Timing and loudness have a blind spot. A sound directly in front of you and one directly behind you reach both ears at the same moment and at the same level. The same is true for sounds straight above.

To solve this, the brain uses two more tools. The first is the shape of the outer ear, whose folds colour the tone of a sound slightly differently depending on its direction, as described in the outer ear and ear canal. The second is movement. Turning your head even a little changes the timing and loudness at each ear, and the brain uses those changes to settle any doubt. That is why people instinctively tilt or turn their heads when a sound is hard to place.

Why do two ears help in noise?

Locating sound is only part of the story. Two ears also make listening easier in busy places.

• Fuller sound. The brain combines input from both sides, so a sound heard with two ears seems a little louder and richer than the same sound heard with one.
• Separating voices. When the voice you want and the noise you don't come from different directions, each ear receives a different mix. The brain can use that difference to focus on the voice and push the noise into the background.
• A better side. At any moment, one ear may have a clearer view of the speaker than the other, for example when someone is beside you at a table. Having two ears means there is usually a better-placed one to rely on.

Even with two ears, noisy rooms can still be hard work, for reasons explored in hearing but not understanding.

What changes when one ear hears less?

If one ear hears noticeably less than the other, the brain receives an unbalanced picture. Sounds may seem to come from the side of the stronger ear. Finding a ringing phone or a voice calling from another room can become a puzzle. Following speech on the weaker side, at a dinner table or in a car, may take more effort, and noisy places can feel more tiring than before.

People often adapt without realising it: choosing a seat with the stronger ear towards the conversation, or turning the head to catch speech. These habits are useful clues to mention to a professional. A difference between the ears is something a doctor will want to look into, and when one ear hears less than the other covers this in more detail.

Key takeaways

• The brain locates sound by comparing when and how loudly it arrives at each ear.
• Timing helps most with lower pitches; loudness differences help most with higher ones.
• Front, back and above are sorted out using the outer ear's shape and small head movements.
• Two ears make sounds fuller and help separate a voice from background noise.
• A difference between the ears can make locating sounds and listening in noise harder.