How Ears Tune In and Out Certain Sounds

Ever wonder how humans with normal hearing are able to "hear" what one person is saying in a crowded room where many people are talking? How is it that the ear is able to tune in and out of conversations going around, especially when everybody in a room is talking equally loud?

To understand this ear "tuning" ability, one needs to first understand that the ear incorporates two "aspects" that allow for good hearing: a volume knob and an antenna. Should the volume knob not work, hearing aids boost the frequencies where hearing loss is present. However, if the antenna does not work, simply increasing the volume does not necessarily make sounds/speech more understandable. This situation is analogous to an old radio where turning the volume up on a station with a lot of static does not make sounds more understandable (fiddling with the antenna to get a clear signal is better).

We have a very good understanding the volume knob of the human ear. What we do NOT have a good understanding (nor how to fix) is the antenna.

However, researchers have begun to clarify how the human ear "fiddles" the antenna to make certain sounds more understandable relative to others providing humans the ability to tune into what one person is saying in a loud restaurant.

Apparently, part of the answer is the tectorial membrane found within the cochlea (membrana tectoria in the Gray's illustration below).

Note that this membrane found inside the cochlea rests on the inner and outer hair cells which allow for hearing (the volume knob). When the ear hears a sound, "waves" are created within the cochlea stimulating the hair cells which allow us to hear. Depending on which hair cells are stimulated, a specific sound frequency is heard.

Using mutant mice, MIT researchers have determined that the viscosity of tectorial membrane depends on the size and distribution of tiny pores. This viscosity provides mechanical filtering that helps to sort out specific sounds by influencing the speed and decay of tectorial membrane waves when a sound is heard.

Although the precise way how the tectorial membrane allows for such exquisite frequency selectivity is still not fully understood, this may be the first step towards that comprehension.

Because once we firmly understand how the ear's antenna works, we hopefully can make better hearing aids that not only act as a volume knob, but can also fiddle the antenna as well.

Reference:
Porosity Controls Spread of Excitation in Tectorial Membrane Traveling Waves. Biophysical Journal, Volume 106, Issue 6, 1406-1413, 18 March 2014 doi:10.1016/j.bpj.2014.02.012

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Permanent Hearing Loss From Noise May Depend on Time of Day!

Research out of Sweden have discovered that noise-induced hearing loss sustained at night is more likely to become permanent than if it occurred during the day... at least in mice.

Apparently, there are certain genes found to cycle in the cochlea of mice in a pattern that followed the hours of the day. By measuring the auditory nerve activity, they found that mice exposed to moderate noise levels during the night suffered from permanent hearing loss while mice exposed to similar noise levels during the day did not. The ability to heal after noise-induced hearing loss (temporary threshold shift) was linked to the circadian rhythm due to levels of BDNF (brain derived neurotrophic factor).

Specifically, concentration of BDNF increases in the cochlea during the day which protected hearing from permanent damage whereas levels were absent at night. Furthermore, researchers were able to artificially increase BDNF levels at night when normally absent and confer hearing protection with noise exposure.

What does this mean for humans?

IF human ears also have a biologic clock similar to mice, than researchers MIGHT be able to develop a specific way to reverse (or prevent) noise-induced hearing loss. Currently, the only way to treat acute noise-induced hearing loss is high doses of steroids whether orally or by injection directly into the ear.

Also, humans should probably avoid listening to loud music at night (i.e. concerts, night-clubs, restaurants, etc) which elevates risk of permanent hearing loss (compared to during the day).

Reference:
TrkB-Mediated Protection against Circadian Sensitivity to Noise Trauma in the Murine Cochlea. Current Biology, 27 February 2014. 10.1016/j.cub.2014.01.047

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Hearing Protection and NRR (Noise Reduction Rating)

For all hearing protection devices from ear plugs to ear muffs, there is a NRR (Noise Reduction Rating) score that informs consumers how much of a decrease in noise exposure to expect with use. For example, if an ear plug has a NRR score of 30, use of the device would be expected to reduce a 90 decibel sound by 30 decibels resulting in a noise exposure of 60 decibels.

If two hearing protection devices are used, an ear plug inside the ear canal followed by ear muffs over the ear, the ultimate noise exposure would be an additional 5 to 10 decibels greater than the higher rated NRR of the two devices.

How is NRR determined anyways? This single number rating is determined by the Environmental Protection Agency as specified under 40 CFR (Code of Federal Regulations) Part 211, Product Noise Labeling, Subpart B - Hearing Protective Devices. The raw data compiled from test subjects is than plugged into a formula which than produces the NRR:

Given that, there are MANY different types of hearing protection devices on the market. Regardless of style, most devices offer a NRR between 20-30. However, as a general rule, ear plugs typically offer a higher NRR than ear muffs. Among high NRR earplugs, disposables offer higher NRR than reusables. BUT... ear muffs typically block sound more consistently than ear plugs which require some degree of molding and fit for optimal effectiveness. Using both obviously is better than using just one.

Also, keep in mind that unless hearing protection devices are correctly fit and used properly, the attenuation suggested by NRR may not over-estimated. Indeed, lay-use of such devices without training typically result in sound attenuation far below what NRR may suggest.

The highest rated NRR disposable ear plugs are around 33:


The highest rated NRR ear muffs are around 30:

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Teens (People) Who Listen to Music Loudly... A Possible Solution

Now parents may not like this, but I do have a possible solution to getting teenagers to turn the music volume down so not to damage their hearing... Get them noise-cancelling headphones.

As an occasional audiophile myself, I understand that when listening to music with headphones, you invariably HAVE to turn the music volume up in order to hear everything clearly, especially when listening to music in noisy environments. Such environments include city sounds while walking, airplane buzz when flying, and car engine/road drone while driving.

Test this out yourself next time you drive a car. First, listen to the car radio with the engine turned off. Note the volume.

Now, see how well you hear the music from the car radio when driving on the highway.

It's the main reason why one may constantly fiddle with the volume knob while driving. It's also why people have to raise their voice in order to be heard in a loud restaurant.

For teens and other people who are audiophiles, the only solution is to turn the music volume up in order to hear the music clearly... which can damage their hearing over time.

So what to do? One can listen to music only when the environment is silent, but that's not going to happen for music-loving individuals who want to hear it all the time.

The other solution is to get a nice pair of noise-cancelling headphones.

A good pair of NC headphones significantly reduces if not eliminates the buzz, drone, white noise of the environment which is a very common cause of requiring a volume increase.

Now, not all NC headphones are created equal. Bose NC headphones are perhaps the best but are pricey at around $300.

However, cheaper brands do not noise-cancel at all or do so poorly even if they are labelled as noise-cancelling.

Generally speaking, Bose is by far the best followed by Panasonic, Sony, and AKG. You can read a Pogue's New York Times review of different noise-cancelling headphones here.

So... perhaps you can make a deal with your teen or adult loved one to turn the music volume down to protect their ears by getting them a Bose NC headphones which come in headphones and earbud models. Available for purchase on Amazon.

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Dr. Chang Quoted in Washington Post Regarding Hearing Damage from Noise

The Washington Post on April 8, 2013, published a story about how loud noises can lead to hearing loss, even in young teenagers.

Dr. Chang was liberally quoted throughout this informative article.

"...That’s according to Christopher Chang, an ear, nose and throat doctor at Fauquier ENT Consultants in Warrenton, who sees patients every day with hearing-related issues. “What he’s hearing is way too loud, because it’s concentrated directly into the ear itself,” he says of my son, adding that the anatomy of the ear magnifies sound as it travels through the ear canal." 

"And when someone turns the music up louder? Well, in short spurts, that’s okay. Loudness is only one part of the equation. How long you’re exposed to loud noises also affects your risk. “The biggest concern is that teens and 20-somethings are using music players so regularly,” Chang says. 

"Cheap earplugs that you can buy at the drugstore offer some protection, Chang says. In addition, there are specialized ear plugs, such as ones for hunters that amplify low-level sounds and block out gunshots. Musicians’ earplugs reduce noise with higher fidelity than the muffling effect that regular plugs have: You want to hear the music that you and your orchestra mates are playing, after all."

Read the entire article here.

Source:
Hearing loss may be caused by cities’ noises and music that’s played too loud. Washington Post 4/8/2013.

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People DO Suffer Hearing Loss After a Loud Concert

Almost everybody at some point in their lives have attended a loud concert or loud nightclub.

Almost everybody will also recall a temporary reduction in their hearing with or without ringing afterwards for a short period of time.

This phenomenon is called temporary threshold shift (TTS) whereby the affected person has a temporary hearing loss that may last hours to days due to temporary damage of the outer hair cells of the cochlea.

If loud nose exposure happens enough times, permanent hearing loss can occur.

For some, permanent hearing loss may occur after attending numerous loud concerts whereas in other more "susceptible" individuals, it can happen after a single loud concert.

It doesn't even have to be a loud music concert. ANY loud noise exposure can do including shooting guns, listening to a portable music player like the iPod, fireworks, playing in an orchestra/band, etc.

However, loud concerts are of particular interest given how many individuals are exposed all at once and provided a unique research opportunity which was presented on May 21, 2012 at the American Otologic Society Meeting in Los Angeles.

The researchers (led by Dr. Jennifer Derebery of the House Ear Institute) essentially took hearing test before and after the concert of 29 teenagers.

72% suffered objective hearing loss after the concert though only 53% reported subjective hearing loss. 25% reported new onset tinnitus after the concert.

Of note, hearing ear plugs were offered to the study subjects, but only 3 accepted them.

Adult researchers who sat with the teenagers under investigation measured sound levels throughout the concert. During the 26 songs played during the three hour concert, sound levels ranged from 82 to 110 decibels with an average of 98.5 decibels. The mean level was greater than 100 decibels for 10 of the 26 songs.

As a reference, ear pain occurs at 125dB. Jet engine noise is at 140dB. Sustained loud noise exposure of 90-95dB can result in permanent hearing loss.

OSHA (Office of Safety and Health Administration) mandates workers to never be exposed to sound levels of 100dB for more than 2 hours per day (no more than 4 hours per day at 95dB).

Clearly something more needs to be done to protect teenagers' hearing, especially given how few accepted hearing ear plugs. A fundamental cultural shift would most likely have to occur with encouragement from not only the musicians but also parents and teachers...

OR, the music volume at concerts should be turned down... No boos please.


Reference:
Teenagers Hear Worse After Attending Concert. LiveScience 5/21/12

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