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(Intro)

Take a moment and listen carefully to all the noises around you.  If you hear your neighbor's music or the hum of a nearby highway or the clatter of a passing train, that's a sound that could be killing you.  Okay, not right away and not all by itself but the sounds we hear every day really do have effects on our health, not just our ears, but our hearts and even our brains.  So what we hear as sound is actually pressure waves traveling through the air around us, and like any wave, sound carries energy.  

We usually talk about that energy in a unit called the decibel, but what decibels actually are is a little complicated.  Every decibel is actually one-tenth of a bel, named for the inventor of the telephone, Alexander Graham Bell, and for weird historical reasons, the bel exists mainly in your physics textbook and decibels rule the rest of the world.  The decibel scale is not linear.  It's logarithmic, as the number of decibels increases, the amount of sound energy goes up exponentially rather than at a constant rate.  That means twice as many decibels isn't twice as noisy.  It's the difference between a loud conversation and the deck of an aircraft carrier.  

Because of the way our brains process sound, ten times more sound energy means we hear something as being only twice as loud.  So yeah, decibels are weird but so are our brains.  Our ears also aren't equally sensitive to all frequencies, so you might hear two sounds with the same power at totally different volumes, so to correct for that and to make loudness values more intuitive, scientists use the A-weighted decibel scale.  

Today we're gonna be focusing entirely on sounds that we can actually hear, but we're also constantly hit with frequencies above and below that range.  Scientists know less about how these other kinds of sound affect us, but it's still out there and still has energy, so we probably interact with it on some level.  For example, there is some evidence that infrasound, below the range of our hearing, might be part of what leads people to think they've seen stuff like ghosts.

Now, we all know that loud, abrupt noises like firecrackers can damage our hearing, but exposure to low-level noise is still dangerous, even if it's not enough to make you go like, ow, my ears, and it's not your eardrum that gets hurt.  It's your hairs.  That is, the tiny hair-like sensors called hair cells inside your ear.  The process that turns pressure waves in the air into sounds in your brain is not simple.

It starts with your eardrum, which vibrates as each wave hits it.  Those vibrations shake three tiny bones inside your ear, which in turn shake the cochlea, a pocket of fluid shaped kind of like a snail's shell, and now comes the key part.  Embedded inside the cochlea are around 16,000 tiny hair cells.  As they move with the fluid, some hair cells generate electrical signals that are carried by the auditory nerve to your brain.  That's how you hear.  

The louder the noise, the more those hair cells move and the stronger the signals they produce, but if there's too much movement, as in a sound that is too loud, the hair cell can become damaged.  We can lose 30-50% of them before noticing our hearing start to decline.  Hair cells can bounce back to some degree, but too much damage will destroy them.  They don't regrow and no treatment currently in use can bring them back.  

Sometimes that hearing loss manifests itself as just things being harder to hear.  

Other times you can get tinnitus, which sounds like a ringing in the ears that just won't go away.  15-20% of people will get at least mild tinnitus in their lives, often as a result of old age, but persistent loud noises can bring on tinnitus more strongly and earlier in your life, like for example, if you're touring in a rock band ever. 

Fortunately, the CDC and the World Health Organization have figured out what sound levels are safe and it's not too bad.  Basically, anything below about 85 decibels is safe for an eight hour workday.  As we've explained, decibels aren't terribly intuitive, but a good rule of thumb is that if you've got to raise your voice to talk to someone an arm's length away, your environment is louder than 85 decibels.  A hockey game at 95 decibels is safe for only 47 minutes.  100 decibel nightclub just 15, and a 120 decibel drum set is okay for just nine seconds a day.  So being a drummer, definitionally, is an unsafe work environment, so bless them all.

Of course, these values are averaged across lots of people in many different circumstances, but every time you cross the threshold, you're taking a chance with your ear hairs.  One risky behavior many of us do every day is just listening to music through our headphones.  At max volume, an iPhone puts out around 100 decibels which, again, is safe for only a few minutes a day.  To listen safely eight hours a day, you want to keep that slider around 70% or less.  

Of course, we can't always avoid loud noises so easily.  Many critical jobs take place in loud environments or use noisy equipments, like at factories, mines, and farms, and farmers and factory workers need jobs and everybody needs the stuff they make, so it's not like just avoiding that noise is an option and losing your hearing can have a big effect on your life, not just your ears.  Like, one study in 1999 found that people with some hearing loss and no hearing aids were 50% more likely to be depressed than those who used aids to restore some hearing.

The sensory loss and social isolation caused by impaired hearing may also be a risk factor for dementia and other cognitive disorders.  The thing is, even if you keep your music down and work somewhere quiet, noise exposure is probably still affecting your life in ways you don't even realize.  Most people who live in cities are frequently exposed to noise levels above 50 decibels, and even a background level of 60 is not unusual.  Cars, trains, and planes are the most common sources of these sounds. 

50 or 60 decibels has no effect on your hearing, but it does lead to a range of psychological responses, like getting annoyed.  Don't dismiss annoyance as a health consequence, by the way.  Like, we've all been there.  Noise annoyance has been linked with a range of physical, mental, and behavioral responses from tight shoulders to depression.  Annoyance is especially common when our expectations for the noise level are out of sync with what's actually happenings.  For example, you probably wouldn't think twice about people chatting in the park, but if it's happening in the library, that can drive you up the wall.  

Some studies have shown that up to 40% of office workers are irritated by noise of 55 to 60 decibels, while for industrial workers, the level needs to be higher than 85.  Noise at 85 decibels has a thousand times more power than 55, so this is clearly more than just a physical effect. 

Sound with a lot of information content, like people talking, can have an outsized effect on your ability to focus, learn, and generally do, like, brain stuff good.  Children may be especially sensitive which isn't ideal, considering most formal learning happens in childhood.  For example, in the early 1990s, the German city of Munich moved their major airport to a new location.  A major study, published in 2002, tested students near the old and new sites on their reading, memory, and attention skills.  

The researchers found that two years after the airport had moved, students at the old, now-quiet site, were doing measurably better while kids at the new airport were doing worse than before, and the overall difference was less than 10 decibels.  That's why the World Health Organization recommends classrooms restrict background noise to below 35 decibels, about the volume of a loud whisper.

But probably the most pervasive effect of background noise is sleep disturbance.  Sound levels as low as 33 decibels have been shown to have negative effects, while volumes above 55 decibels can lead to substantial changes in mood and even sleep deprivation.  This is not an isolated problem either.  A 2003 study conducted in the Netherlands found that a quarter of the population reported being highly disturbed by outside noise at least once in the last year.  About half of those reports came from road traffic, a situation many of us can sympathize with.

Being woken up once in a while at night or annoyed every so often during the day might seem like kind of a minor problem, but these feelings can lead to stress.  This might be because evolution shaped hearing as the body's critical early warning system.  Sound works whether it's day or night and even if you're facing the wrong direction.  It's also processed by the brain much faster than vision is, and works at a subconscious level.  Over millions of years, we've come to associate a loud noise with something like 'You're about to be eaten by a lion' which is a stressful feeling, and when your body is stressed, it produces higher amounts of hormones like cortisol, adrenaline, and epinephrine, which affect your heart rate and your blood pressure.  Too much of these stress hormones for too long is one way to get what doctors call hypertension, better known as high blood pressure, but the effects go far beyond that including messing with your immune system and even contributing to diabetes, so if you have a super loud car, please don't drive it down my street at night.

Maybe the scariest outcomes though are cardiovascular disease and heart attacks.  Remember at the start when I mentioned that the sounds around you might literally be killing you?  Well, in 2014, the European Environment Agency estimated that noise-related health problems like cardiovascular disease are responsible for 10,000 deaths each year in Europe, and the World Health Organization calculates that in all, European citizens lose more than a million healthy years every single year due to traffic noise alone, and if you do get ill, you might end up in the hospital, and even there, you may not get the peace and quiet you need.

The heart rate monitor by your bed, the telephone at the nurse's station down the hall, and a thousand other sounds can add up to an environment that's 15-20 decibels louder than what the World Health Organization recommends, and it's not just you that might be affected.  Doctors and nurses work there, too, and it turns out that can be a pretty loud working environment, so even healthcare comes with occupational noise risks, and those people are not the ones that you want to be experiencing noise-related cognitive difficulties, so what can be done about all this?

The first thing is to simply be aware of the noise around you.  Apps that use your smartphone's microphone do a pretty good job of estimating the volume of ambient noise.  Even smartwatches are getting in on the game, alerting you to how loud your current surroundings are.  The rest of us will have to keep an ear out the old fashioned way.  When you're headed to a concert or a sporting event, consider packing a pair of those tiny foam earplugs.  If you have to shout for your neighbor to hear you, it is probably a good idea to put them in.  That may or may not make it easier to have a conversation, but it is definitely safer for your ears.

Next time you're flying or on the train, think about using noise-canceling headphones.  They'll quiet the world around you, which means you can also keep the volume on your music lower.  Ultimately, though, unless you plan to become a hermit, you're going to experience some loud noises and that's okay, but loud environments can affect your state of mind and overall wellbeing, so it's always a good idea to be responsible.

One day, your ears, your heart, and your brain will all thank you.  We spent a ton of time in this episode talking about how decibels are such a tricky unit to handle.  If you're interested in understanding more about the basics of how we measure sound and energy, you might like Brilliant.org's course on waves and light.  It'll teach you all about the waves that carry light and sound from noise-cancelling headphones to earthquakes to distant stars.  Like all of Brilliant's 60+ courses, it's designed to be hands-on with interactive quizzes and guided problems, each of which comes with a full explanation.  All of Brilliant's science, engineering, computer science, and math courses are there to help you sharpen your scientific thinking skills and the first 200 people to sign up at brilliant.org/scishow will get 20% off their annual premium subscription, so it's even easier to check it out, and also, it supports the show, so thanks.

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