Location Services

The Haas effect also impacts where we perceive a sound to be coming from – the supposed location of the source is determined by the sound which arrives first, even though the sounds may be from two different physical locations. This holds true until the second sound is about 15dB louder than the first when the perception of direction changes.

Sound localization is a very complex mechanism performed by the human brain. It’s not only dependent on the directional cues received by the ears, but it is also intertwined with the other senses, especially vision and proprioception.

Our ability to determine a sound’s location and distance is called binaural hearing, and in addition to all the psychoacoustic effects discussed so far, it’s also heavily influenced by the physical shape of our heads, ears, and even torsos.

The outer ear or “pinna” functions as a directional sound collector that funnels sound waves into the ear canal. The head and the topography of our face and torso influence how sounds from any position other than a 0-degree angle are heard, as they create an acoustic “shadow.”

Our brains process the differences between the information that our two ears collect and interpret the results to determine where a sound is coming from, how far away it is, and whether it’s still or moving.

At lower frequencies, below about 2kHz, this is mostly determined by the inter-aural time difference; that is, the discrepancy in time between when the sound reaches each ear. Above 2kHz the information gathered comes from the inter-aural level difference; that is, the discrepancy in volume between the sound that each ear hears. This clever evolutionary adaptation is due to the relative lengths of sound waves at different frequencies.

For frequencies below 800 Hz, the dimensions of the head are smaller than the half wavelength of the sound waves so that the brain can determine phase delays between the ears. However, for frequencies above 1.6 kHz, the dimensions of the head are greater than the length of the sound waves, so a determination of direction based on phase alone is not possible at higher frequencies; instead, we rely on the level difference between the two ears. These binaural disparities are known as Duplex theory and play an important role for sound localization in the horizontal plane.

Finally, if the frequency drops below 80 Hz it becomes difficult to impossible to use either time difference or level difference to determine a sound’s lateral source because the phase difference between the ears becomes too small for a directional evaluation, hence the experience of sub-bass frequencies being difficult to localize.

Making Distinctions

While this phenomenon makes it easy to sense which side a sound is coming from, it’s harder to determine direction in the up/down and front/back planes due to our ears being placed at the same horizontal level as each other. Some types of owls have ears placed at different heights to allow for greater efficiency in finding prey when hunting at night, but humans have no such facility.

This can result in “cones of confusion” where we’re unsure as to the elevation of a sound source because all sounds that lie in the mid-sagittal plane have similar interaural differences; however, once again the shapes of our bodies help us out. Imagine a sound source is right in front of you. There’s a certain detour the torso reflection takes and hence a certain difference of this torso reflection in relation to the direct sound arriving at both ears.

It yields a slight comb filter pattern that will change if this source is elevated. The same is true if this source is now moved behind us; the torso reflection changes and our brains process the information discrepancies to help us locate the source.

In the final installment of this series, we’ll look a ground-breaking new technology that takes IEM mixing to a whole new dimension.