2015-09-16

Safe Sound Levels to Protect from Hearing Loss

Safe Sound Levels to Protect from Hearing Loss

Sadly, our ears are not built for loud noise, when exposed to it loud enough or long enough, the nerves that sense sound begin to lose their function.  If that happens enough, we lose hearing.

Hearing is one of the only senses where overuse leads to permanent loss.  Think about vision.  A bright enough light can cause sudden loss of vision, but a blink and some time almost always allows vision to recover fully.   It takes a very unnatural amount of light to lead to permanent blindness, a level none of us are likely to experience.  The same goes for taste, touch, and even smell.

Why do the nerves of hearing lose function is overstressed, and at what level of loudness does that happen?

Why the nerves of hearing can lose function permanently
Hearing a sound happens when vibrations in the air are translated into nerve signals, that are then transformed into the experience of hearing.   That sentence lists 4 distinct steps that have to happen if we are to hear anything:

1.  Air vibrating.  If there is no vibration in the ear, there is no sound, and nothing to hear.  It is interesting that everything we, or any animal, hears is the result of air shaking regularly, vibrating.

2.  Translation of air vibrating to nerve signals.  This is the step that explains how hearing ability can be lost from loud sounds.   The translation occurs along the pathway of ear to nerve.  When you hear a sound, that means that the little column of air is vibrating in a pattern specific to that sound, and that air is vibrating in your ear.  Those vibrations make your eardrum, then auditory bones (remember the anvil, stirrup, and hammer?), vibrate.   Once these tiny bones vibrate they create identical patterns of vibration in a spiral of fluid they connect to.  The spiral is called the cochlea.
Now, lining the spiral of the cochlea are the nerves of hearing.  Each of these nerve cells have a border featuring little hairs.  Each cell has a set of these hairs that go from very short to longer, creating a brush like structure of varying hair lengths.
These hairs have one end inside the nerve cell, and the other end is dipped in the fluid that fills the spiral of the cochlea.  When the fluid in the cochlea vibrates, hairs that are the right length to vibrate at those frequencies begin to vibrate.  Whenever a hair on the nerve cell begins to vibrate, it creates an electrical signal.   Now air vibrating has been translated to an electrical nerve signal.

3.  The nerve signal is delivered to the brain.  Once the nerve fires, that electrical signal is shot down the auditory nerve from the inner ear to the auditory part of the brain.

4.  You hear.  This happens when patterns of these nerve signals excite the auditory part of the brain in such a way that you perceive hearing that sound.

Here is the sad news.  If you make the hairs on the nerve cells of hearing vibrate too strongly, they break off or separate, and will never again translate a vibration into a nerve signal.  If you lose all your hairs on all the nerves of hearing, you would be forever unable to hear again on your own.

Seeing involves no such moving parts, and neither does touch or smell, so they are not as prone to damage as this mechanism of hearing.

The ear does have a muscle in it called the stapedius muscle that can protect.  When we hear very loud sound, the muscle contracts and thereby keeps one of the bones of hearing, the stirrup (stapes) from vibrating, making the sound much softer.  But it takes a moment for that to happen, and if the sound is loud enough, even that protection will not prevent permanent loss of some hairs on the nerves of hearing.

What level is harmful?

Now that we know that loud sounds can cause permanent loss of hearing, what levels of sound are safe, and in what settings is sound not safe but harmful?

Loudness can cause harm in two ways, as noted above.   The first is a burst of loud sound.  Even a few milliseconds of sound loud enough can damage hair cells of hearing.  
The other way is by sustained sound.  By sustained we mean either for a few minutes, or few hours, or all day.

Loudness is a real physical force.   It relates to how powerful the vibrations are in the air.  A more powerful vibration is experienced as louder.   Sound vibrations are not usually experienced as physical forces, but the vibration of a loud bass can be felt in your body and on tables and floors, and certainly cars.

The unit of loudness usually used is the decibel.  The decibel is a complex number for two reasons: it is created by the ratio of two measures- the actual loudness of the sound, divided by a reference value for loudness.  The other aspect of decibels that make it complex is that they are the logarithm of that ratio.   But good news, all we need to know is that the higher the decibel level of a sound, the louder it is.

The rate at which increasing decibel levels gets louder goes like this.
Start with a sound that is generating 0 decibels of loudness, and call that 1 Unit of Loudness- this is barely loud enough to hear.

If you go up to 10 decibels, you go up to 3.162 Units of Loudness- the loudness of a rural night
If you go up to 20 decibels, you go up to 10 Units of Loudness- the loudness of a whisper
If you go up to 40 decibels, you go up to 100 Units of Loudness- the loudness of a library
If you go up to 50 decibels, you go up to 316.2 Units of Loudness- the loudness of a dishwasher one room over
If you go up to 60 decibels, you go up to 1,000 Units of Loudness- the loudness of normal conversation at a distance of 3 feet away
If you go up to 80 decibels, you go up to 10,000 Units of Loudness- the loudness of a garbage disposal 3 feet away.
If you go up to 100 decibels, you go up to 100,000 Units of Loudness- the loudness of a lawn mower 3 feet away.]
If you go up to 110 decibels, you go up to 316,200 Units of Loudness- the loudness of a rock band 15 feet away.


You can see doubling the decibel level more than doubles the loudness of the sound.

Sounds that are softer than 80dB do not hurt the hair cells of the nerves of hearing over any length of time.  Note- that means sounds should be softer than the loudness of a typical garbage disposal 3 feet away.

If you go over 80dB, permanent hearing loss begins to occur.  The louder the sound, the faster that happens.  At 85dB, one needs to be exposed to this level of loudness several hours a day for many years to lose hearing.

But, at 100dB you can start to lose hairs on the nerves of hearing in two hours, and at 115dB it only takes 15 minutes.  Much over 115dB, one loud burst of sound at that level can cause permanent loss so at least some hearing.

How does that connect to our audio devices?  The main problem is with earbuds and headphones.
One study found students listen to music at these levels:
55% of kids listen to music at 85dB or higher

Another reference is that the earbuds that come with Apple products, when plugged into an iPod at maximum volume, will deliver 102 decibels of loudness, loud enough to cause hearing loss in 2 hours.

For most devices, 80% of maximum volume will deliver about 90 decibels of loudness.  This is safe for listening for under 90 minutes, but over many hours for many years, hearing loss will happen even at this level.

Bottom Lines
1.  Sound is just vibrating air, but it has power in those vibrations.  Too much power and the delicate hairs on the nerves of hearing can be lost, never to return.  As these hairs are lost, hearing begins to weaken, and can lead to real hearing loss.
2.  The damage to these delicate hairs and loss of hearing are related to how powerful the air is vibrating and how long the nerve cells are exposed.   More force takes less time to cause damage, but some lighter forces can cause damage if the exposure lasts long enough.
3.  The force of sound is measured in decibels.  The very, very faint sound in a hearing test can be as low as 0 decibels.   The safe level of loudness for our ears is 80 decibels, or 10,000 times louder than that barely audible signal at 0 decibels.   
4.  The loudness of a garbage disposal at 3 feet, or ear buds on a device set at about 75% of maximum is what 80 decibels sound like.
5.  Keep sounds in your home, around your infants, children, adolescents, and adults to 80 decibels or less and sound will not harm your hearing.
6.  If you do listen to TV or music above 80 decibels, hearing loss begins to happen.  At 85 decibels you need to have sound of that loudness blaring in your ear for many hours a day for years to see actual hearing loss.  But at the max on many devices, you hit 100 decibels and hearing loss can appear within 2hours.   115 decibel blasts can hurt those hairs on the nerve cells in 15 minutes.

So, keep sound levels at 80 decibels or below and spare your hearing for your long life!

To your health,
Dr. Arthur Lavin

PS- Many thanks to a family with a young infant in the practice for asking me the threshold of loudness that is safe!




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