Human hearing is an amazing sense. Our ear/brain apparatus can accommodate a range of dynamic levels that extends from the quietest sounds (the sound of our own heartbeat and nervous system in an anechoic chamber) to the roar of an F-16 blasting off the deck of an aircraft carrier with full afterburners (not recommended without hearing protection). This range is billions of multiples in terms of the energy delivered to our eardrums. No other sense comes close. And the frequency response, while typically 20 Hz to 20 kHz may just depend on higher ultrasonic frequencies to fully contain a musical or natural sound or the interplay of ultrasonic partials or overtones.
But what about directionality? How is that some clever signal processing can change our perception of where a sound comes from? The Headphones[xi]™ tracks that I posted to the FTP site a couple of days ago clearly demonstrate the dramatic difference between a traditional stereo mix played through headphones and one that has been “processed” to expand the listening experience to involve a “modeled” physical space.
So today I present a short primer on “directional hearing”. I’ll continue with an explanation of how production techniques and post processors can maximize immersive listening…even through a set of headphones.
There are three primary factors that contribute to directional hearing. The first is the difference in amplitude delivered to our brains by our two ears. Sound amplitude diminishes by a formula called the “inverse square rule” (I wrote about this some months back and included a graphic that illustrates the concept…to reread that post, you can click here.) Because our ears are located some 5 inches apart and are not “coincident” (meaning they don’t occupy the same point in space), any sounds that are not directly in front or behind us will be heard as different amplitudes by our ears. See figure 1 below:
Figure 1 – A bird’s eye view of an off axis sound source and the factors that cause us to experience the sound to the left of center.
In addition to the amplitude difference between our left and right ears for a sound in front and to the left of center, there is a timing difference between our ears as well. Sound travels at about 1160 feet per second at sea level. For a sound coming from the front left, it takes slightly less time for the sound to reach our left ear than it does to reach our right ear. It might be a very small difference but our ears and brain can detect the difference and make us experience the sound in the right place.
Finally, there is a timbral difference between our ears. The pathway to the left ear is not blocked by our thick head and is received filtered by the outer ear or pinna. The other side loses some of the high frequency information because the right ear is “shadowed” by our head. There are subtle timbral differences between the direct sound and filtered sound that contribute to our ability to hear directionally.
These three factors contribute to directional hearing depending on the frequencies being heard. Our directional hearing is better in the middle frequency range than at the lower ranges. And different amplitudes also change our perceptions.