“Music is perpetual, and only the hearing is intermittent.” -Thoreau
A couple of weeks ago, my wife and I went to the circus together. Someday, I vowed, I’ll be strong, flexible, and stable enough to do the amazing hand-balancing tricks we saw.
And all the while, the six-year-old girl behind us screamed her piercing, high-pitched scream, cheering the performers on.
(This is totally appropriate behavior, IMO, and no children reading this should be discouraged from screaming at the circus.)
Now, one of us has better hearing than the other. And while one of us found the high pitched screaming to be a minor annoyance, the other was simply in agony. What’s going on here?
It’s going to take a little bit of biology and a little bit of physics to figure it out. First off: how does your ear work?
The sound waves — which are pressure waves — enter your ear and press up against the tympanic membrane, better known as your eardrum.
The vibrating eardrum causes the three little bones in there — the hammer, anvil, and stirrup, known collectively as the auditory bones — to vibrate as well. The last one, the stirrup, pushes against the cochlea, and this is where your hearing takes place. How? To get the simple answer, we need to know what the cochlea looks like on the inside?
Your cochlea is a spiral-shaped structure, like a snail’s shell, that’s filled with fluid and lined with tiny hairs known as cilia. The most sensitive cilia are the ones closest to the outside: only a tiny vibration is needed to set them in motion. These are also the most easily destroyed. So when you do things to damage your hearing like go to rock concerts without earplugs, listen to your headphones or stereos too loudly, fire a gun without protective gear, or have your “friend” scream in your ear, these sensitive cilia get destroyed.
The bad news? Once they’re destroyed, they pretty much never grow back. So while a newborn baby can hear up to about 20,000 Hz, very few adults can. At age 31, my hearing stops somewhere around 13,000 Hz. There are a few sites out there to test your hearing, so I’ve stolen some sound files to allow you to find where yourhearing, approximately, cuts out.
I can hear the 12,000 Hz tone but not the 14,000 Hz one. However, as many of you will notice, the closer the tone gets to your hearing threshold, especially if it’s loud, the more painful it gets to listen to!
In other words, people with better hearing are also more sensitive to these ultra-high-frequency sounds, and are more likely to find them painful!
So now, onto the physics. When you have something like a tuning fork (above), it emits sounds at just one frequency. In other words, if the fork is tuned to emit at 440 Hz, you hear a sound at 440 Hz and nothing else.
But what if you didn’t have a tuning fork. What if you had something like an open pipe, a string, or vocal cords? (Thanks, Nathan @5.)
If you just play the string (or hit the pipe, or make a note with your voice, etc.), you don’t just play at the frequency you’re trying to hit. You also get all of the overtones. If you sing an “A” at 440 Hz, you also make a sound at 880 Hz, and another one at 1320 Hz, and another one at 1760 Hz, and so on.
What does this mean for the screaming girl? If she screams at 5,000 Hz, then someone like me can hear the 5,000 Hz fundamental and the 10,000 Hz overtone, but that’s it. However, someone with better hearing than I have can also hear the 15,000 Hz overtone, and possibly even the 20,000 Hz one!
(A fun test of this is to see is how many of you can hear an old CRT television when it’s not turned on to a channel. I can’t, but if you can hear 15.6 kHz or higher, you probably can!)
So if you can hear these high-pitched sounds, you’ve got the bittersweet blessing of having excellent hearing, but also of being sensitive to painful sounds that people like me will never hear. At least now, when you’re the only one in a room who can (or can’t) hear something, you’ll know why!