Is an iPod more dangerous for your ears than a NASCAR race car?

Anyone who’s ever been to the track knows that racetracks are loud. Quite frankly, it’s one of the things many people (including me) like about actually being at the track as opposed to watching on television. But could that noise be doing your hearing permanent damage? Is it any worse than any of the other sounds we encounter on a daily basis?

Let’s go to the data…

What is Sound?

Think about a guitar string vibrating and the molecules that surround it. When the string moves, it pushes molecules out of the way.  Those molecules bump into other molecules and so on and so on. When the guitar string moves back and forth, the air molecules do, too. When they move back and forth 440 times each second, we call that a note (concert A, to be specific).

We model the waves as compressions and rarefactions — or squishes and non-squishes. When I picture a sound wave, it looks like this:BSPEED_PressureWave

The dark areas are where many air molecules are bunched up together. The light areas are where there aren’t many air molecules. The wave on top has a higher frequency than the wave on the bottom because there are more light and dark areas in the same length than there are in the wave on the bottom.

Vocal cords work the same way. They vibrate and the set the air near them into motion.  The air is alternately compressed and rarefied.  Each molecule bumps into the next until the vibration reach your ear.

To get a louder sound on a guitar, you pluck the string harder. It moves more, so the molecules move more. Similarly, when you shout, your vocal cords vibrate with a larger amplitude and the noise that comes out is louder.

When I speak to you, the sound moves along the air between my mouth and your ear. (Note that the air molecules don’t actually travel between my mouth and your ear. They just move back and forth quickly.)

BSPEED_EarAnatomy

The outside is shaped to collect and focus sound waves so they pass through the ear canal (the external auditory canal in the picture) and hit the eardrum.

The eardrum (called the tympanic membrane in the picture) separates the outer and middle ears.  It is a tightly stretched membrane, much like a drum head. When the vibrating molecules reach it, it vibrates with the same frequency as the air molecules. The eardrum causes the hammer, anvil and stirrup bones to vibrate in the same pattern, which amplifies the sound.

If you ever have a chance to look at one of your speakers without the covering on it, it’s worth doing. Find something with really strong bass. If you look carefully, you can actually see the speaker move. It’s converting electrical signals into motion of air molecules and those sounds travel through your ear.

The video below shows a speaker filmed with a high speed camera. As the video plays, they’re increasing the frequency of the sound being sent to the speaker. This is pretty much what your ear drum does as well.

This is pretty much exactly how your eardrum works. You can think of it as the speaker encodes the sound and your ears decode it. The louder the sound, the further the speaker cone must move.

As you might expect, there’s a limit to how far a speaker cone can move and, of course, YouTube is full of videos showing you what happens if you put too much power into a speaker.

Why is that at all relevant aside from the fact that it’s fun to blow things up?

Because the exact same thing can happen to your eardrum. Very loud sounds force the eardrum further than it was designed to bend, and then it breaks.

Anyone who’s had a burst eardrum knows that it is not just inconvenient, it is downright painful. Sometimes it heals, and sometimes it requires surgical intervention. But bursting an eardrum isn’t the most common problem.

As the vibrations travel into the ear, the inner ear coverts the pressure (sound) waves into electrical nerve outputs.  The cochlea contains over 20,000 strands of hair-like receptor cells called hair cells.  Different lengths allow different cells to detect different frequencies.

BSPEED_Sound_HairCells

And if you’d like to actually see a hair cell in action, responding to Bill Haley’s “Rock Around the Clock”, you can.

Here’s the problem. Loud sounds kill hair cells. Sharks and chickens can regenerate hair cells. People cannot. Once they’re gone, they’re gone. Racetracks are loud, so you want to take all the precautions you can to prevent permanent hearing damage. There are two primary considerations: sound intensity and length of exposure.

BSPEED_Sound_Flowchart

You can damage your hearing from one really loud sound, but you can also permanently damage your hearing by repeated exposure to lower volume sounds.

Sound Intensity

The loudness of a sound is proportional to its intensity – how much power per area the sound wave contains. The further you are from a sound source, the more the power has spread out over and the quieter the sound seems.

BSPEED_Sound_Intensity

It’s one of those distance-squared things, so if you move twice as far away, you get four times less intensity. Remember this – it’s going to be really important later.

Ears have an amazing dynamic range. The human ear can hear from 10-12 Watts per square meter (the threshold of hearing) to 1013 Watts per square meter (the threshold of pain). That’s from o.ooooooooooo1 Watts per square meter to 10000000000000 Watts per square meter.

It’s pretty hard to talk about things when they span such a great range, so we use a logarithmic scale and a unit called deciBels (dBs). The original unit was the Bel, which was named after Alexander Graham Bell and was developed as a way to measure the sound level on the brand new concept of telephony. A deciBel is a tenth of a Bel, but no one uses the Bel as a unit anymore.

Logarithmic scales work very differently than straight scales.  To show this, let’s plot sound intensity vs. deciBels to see how they compare.

BSPEED_Sound_dBLogScale

Whoops. That isn’t very helpful, is it? You can see the three biggest values and everything is too small to even show up. Let’s blow the scale up.

BSPEED_Sound_dBLogScale2That isn’t very useful, either. The three highest value are off the chart and I can barely see 130 to 70. The last four are so small they’re about worthless.

This is why deciBels are useful. Let’s look at just 20dB range. I’ve normalized the data relative to 80 dB, so 80 dB is 100% intensity and all the other bars are measured relative to that.

BSPEED_Sound_20dB

Let’s say you change the volume of your radio from 60 dB to 70 dB, then from 70 dB to 80 dB. How does the sound intensity change?

  • When you go from 60 to 70, you increase the sound intensity by ten times.
  • When you go from 70 dB to 80 dB you make the sound intensity another ten times more. 80 dB is a hundred times more than 60 dB.
  • 90dB is a thousand times more than 60dB. Not shown, but you get the idea, right?

Although the logarithmic scale makes things easier to use, its nonlinearity makes you have to think a little harder about how to understand measurements. The size of the change depends on the value you’re looking at.

  • Going from 80dB to 79dB decreases the sound intensity from 100% to 79%.
  • Going from 79dB to 78dB decreases the sound intensity from 79% to 63%.
  • Going from 80dB to 77dB decreases the sound intensity from 100% to 50%.

Note that the loudness you perceive is not the same thing as sound intensity. That’s a whole ‘nother topic.

There’s one more thing we need to consider: how long you subject your ears to a sound. One loud sound for a few minutes might not have any effect, but listening to the same sound for eight hours could start to kill hair cells. So here’s a summary of what we’re worried about when it comes to sound permanently damaging our hearing:

How Loud?

Remember that how loud something is (i.e how it will affect your ears) depends both on how loud the source is and how close you are to it.  Here are some representative sounds.

BSPEED_Sound_Levels

The NASCAR values in orange come from peer-reviewed scientific articles and my own measurements. The yellow is the maximum volume I measured for my iPod (Gen 5, I believe).  These are average values. It makes a difference if a car is idling or if the driver is gunning the engine. It makes a difference if you’re sitting in the front row or high in the stands or standing on pit road.

The upper line on the graph is the threshold of pain. This is the point at which sound starts to hurt. It is significantly higher than the sound levels that can cause permanent hearing damage.

The second line comes from the Occupational Safety and Health Administration (OSHA) recommendations for occupational noise exposure. As the sound intensity increases, the time you should experience that sound decreases.

BSPEED_Sound_OSHATimeIntensity

The louder the sound, the shorter time you want to hear it.

  • OSHA doesn’t recommend listening to a 90dB sound for longer than eight hours.
  • When you get to a 105 dB sound (like a single Sprint Cup car), you should limit exposure to less than an hour.
  • A 110 dB sound shouldn’t be listened to for more than half hour
  • A 115 dB sound shouldn’t be experienced for more than fifteen minutes

The noise you experience at a NASCAR race averages around 96-100dB and it’s usually only for three or four hours. The actually noise you hear will vary – when the cars are further from your seat, they’re not as loud.

Peak measurements of 140 dB were found at Bristol. Those measurements were spikes and didn’t last very long. Bristol is a small track that is ringed all the way around and filled with people. At tracks like Daytona or Atlanta, the stands don’t go all the way around the track and the track is much larger. The sound isn’t going to get as loud at a track like that.

Now if you’re the driver, you’re spending four or five hours sitting right next to the source of the noise. Drivers are much more careful now about protecting their ears than they were back in the day. If you visit the track and spot any of the older drivers, you’ll notice most of them wear hearing aids.

Even the recently retired Jeff Burton has noted that he wishes he would have paid more attention to protecting his hearing earlier in  his career because he’s noticing hearing loss now. (Some of that, unfortunately, is just part of getting older, but if you compare a race car driver with someone the same age who worked in an office, you’ll find a significant difference in their hearing levels.

The same thing goes for folks who work in the garage, which means not only the crews, but the reporters, the track personnel, etc. who spend just as much time there as the crews. One estimate I read suggested that, at worst case, you can get permanent hearing damage in just six minutes on pit road without ear protection.

Smack dab in the range of NASCAR noises is the iPod. You might be surprised that a little iPod can be louder than a NASCAR race car, but remember the importance of proximity (its one of those distance squared things, so twice as close is four times louder). When you’re using an iPod, the sound source as close as you can get it to your eardrums, whereas you are not (I hope) putting your ear right next to the exhaust of a NASCAR race car.

But audiologists are increasingly seeing iPod (and similar device)-induced hearing loss so significant that the people needed hearing aids. And these are very young people, mind you!

An expert noted that, although older devices (Walkman, anyone?) posed the same problems, those ran on batteries, so you could only use those for a few hours before the batteries gave out. I can listen to my iPod for eight to ten hours straight without it wearing down.

How to Keep Your Ears Safe

Earplugs and headphones. Seriously. You’ve noticed during the National Anthem that most of the drivers’ kids on pit road are wearing over-the-ear headphones. Kids’ hearing is much more fragile than adults. Kudos to the Kenseths and the Gordons, especially, for always making sure their kids ears are covered.

You’re thinking – I’m good. I have my scanner and those are over-the-ear headphones. Yes, but if you’re blasting the volume, you’ve got the same issue as an iPod. The experts recommend turning the volume all the way down, then turning it up until you can just hear it. Your ears acclimate to sounds.

If you’re at the track and there are car noises, you need to be wearing earplugs. Those squishy disposable foam earplugs work amazingly well, provided they fit right (some people have bigger ear canals than others!) and provided you use them.

Remember: a dead hair cell never comes back. You’ve lost part of your hearing that science has no way of restoring yet, save for hearing aids (which have their own issues). Save those hair cells. Earplugs. Always.


Also published on Medium.

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