Localization is one of the most important aspects of our hearing. Its importance isn't just that we know where sounds is coming from. Much more important than that is that it is the basis for our being able to hear in noise.
There are two main factors that allow you to determine where a sound is coming from.
- First, your outer ear, or pinna, is shaped somewhat like a cup and facing forward. That helps you hear things in front of you better than things in back of you, so it's a crude form of localization.
- Second, and much more effective, is that you have two ears, on opposite sides of your head. Because sounds around you sound slightly different in those two ears, your brain, which is "listening" to both ears, can "triangulate" and determine where various sounds are coming from.
But, how does that help us hear in noise? The details are pretty amazing. There are two primary differences in how a given sound is heard by each ear.
- If the sound is coming from the side, then the ear closer to the sound's source will hear it a little louder than the ear that is farther away. The difference may be tiny, but your brain can use that tiny difference to help determine where the sound is coming from.
- The other difference in how a sound is heard by both ears is based on the fact that sound travels in waves. Because your ears are several inches apart, the compression part of each wave from a single sound source arrives at a slightly later time at the second ear than at the first (assuming the sound isn't directly in front of you or behind you). This time difference yields a phase shift in the waves as perceived by the two ears. Again, it's a tiny difference, but your brain uses that phase shift to tremedous advantage.
Sound waves are not really like the "up and down" waves the ocean; they are more like the waves of compression and rarification you might see in a slinkey. These compression fronts arriving at your ears are what vibrate your ear drum and start the amazing process of hearing.
Sound waves travel at about 750 miles per hour in air, so we're talking about timing differences between the two ears of only about 1 millisecond. That's not nearly enough time difference for you to detect it yourself, but the brain is an amazing thing. It not only detects the volume and phase shift from the two ears, but it then communicates with both of your cochleas to get them to be more (or less) sensitive to certain types of sounds or sounds from certain directions. The net result is that you can focus in on what someone is saying even though background noise might otherwise completely overwhelm your ability to understand their words.
You may think that your hearing nerve just transports sounds from your ear to the brain. In fact, there are more nerve fibers going DOWN to the cochleas than UP to the brain. All those nerve fibers going DOWN are the communication channel for your brain to work with the cochleas on focusing in on just what you want to hear and suppressing the background noise that would otherwise prevent you from hearing what you want to hear.
The amazing thing is that it all happens without your even thinking about it. You don't have to consciously focus, it just happens.
Unfortunately, if you have a hearing loss, your loss often not only damages your ability to hear, but it can damage your ability to localize sound and know where it's coming from. That means that the worse your hearing is, the less you can suppress background noise. It's a double whammy!
It's also one of the reasons that hearing aids don't help as much in noise. Hearing aids do a wonderful job of amplifying sounds so you can hear them better, but they can't do as well as good ears in noise. They have some tricks up their sleeve, though:
Normal ears can do a much better job of noise suppression because they are working with the brain and using the subtle differences in volume and phase from two ears. If you have trouble hearing in noise (who doesn't) and you're considering hearing aids, then you should be sure to consider directional microphones. They don't work as well as normal ears, but they are dramatically better at noise management than regular hearing aids.
- For a long time, hearing aids have used compression to keep sounds from getting too loud, but that only helps a little with noise that's interfering with something you want to hear.
- Some newer aids (with directional microphones) do an amazingly good job of suppressing background noise, but the are doing that alone.