Time lag, wave length and tone - all these factors play important parts for the brain when determining the direction of sound.
In the following description, they are treated under separate headings, but when a person registers a sound, all three factors interact, helping to determine the direction from which the sound originates.
Time lag is of particular importance when determining so-called impulse sounds like a click or a bang.
If a sound comes in at an angle to the right of the face, the direction of the sound waves means that the sound will not reach both ears at the same time.
The time lag is due to the fact that the distance from the source of the sound to the left ear is a little longer than it is to the right ear. Therefore, the sound waves must travel a little longer before reaching the left ear which is farthest away.
The brain registers the time lag and informs us that the direction of the sound is a little to the right of the face.
When sounds are light treble sounds (over 1 kHz), the wavelength plays an essential role for the brain in determining the sound direction. These sounds all have a limited wavelength of less than 30 centimetres.
When a person hears sounds of limited wavelengths, the head functions as a screen. If the sound comes from a direction to the right of the face, the head will prevent the sound waves from reaching the left ear. Deep base sounds, on the other hand, have a larger wavelength, and the head will not prevent the sound waves from reaching both ears.
The tone of the sound
If the direction of the sound waves is not from the sides, but rather from above, below or immediately in front of the face, there is no time lag between the ears. In situations such as this, the outer ear is important as it helps determine the tone of the sound.
Experience has taught us that the tone can help determine the direction of the sound waves. People riding a motorbike wearing a helmet, for example, often find it difficult to hear where an ambulance is coming from, as the helmet reduces the outer ear's ability to determine the tone of the sound.