Psychoacoustics The Human Ear

For this report we will explore the many wonders of the human ear. We will start off by looking at the mechanisms of the outer, middle and inner ear, and then we will look at the weird world of auditory illusions, the perception of sound. And finally we will explore the types of hearing losses you can get, and as you will find out some hearing losses can be there from birth but others can come later in life.

Outer Ear

The first section of the human ear we will indulge ourselves into is the outer ear. The outer ear consists or the Pinna (Auricle), Auditory canal and the Tympanic membrane, which is also known as the ear drum. The pinna is the visible part of your ear, which is made up of a thin plate of elastic cartilage. Your skin covers the cartilage and it’s connected to other parts of the ear by ligaments and muscles. Different mammals can actually move the pinna so that they can focus on the sound coming from a certain direction, for example you’ve probably noticed that dogs have there ears back, this is so they can listen to what’s behind them because they are unable to see behind themselves.

http://www.affordablehearing.com.au/wp-content/uploads/2012/07/2.jpg

http://upload.wikimedia.org/wikipedia/commons/c/ca/Gray904.png

Next is the Auditory canal, this is what connects your pinna to the tympanic membrane. This is where you ear wax is located. Your earwax is very useful because it catches the dust that enters your ear, then over it get pushed our or until you use a Q-Tip. The auditory canal is about 2.5 cm long and 0.7 cm in diameter.

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http://hyperphysics.phy-astr.gsu.edu/hbase/sound/imgsou/msens2.gif

At the end of the outer ear you have the tympanic membrane, which separates the outer ear from the middle ear. The tympanic membrane or eardrum is shaped like a cone and is thin; its job is to transmit the sound you hear to the ossicles, which are three tiny bones, located in the middle ear, which we will look at later on. The eardrum is made of two main parts, the pars tensa and the pars flaccda. The pars tense is the main portion of the eardrum and the flaccda is the flaccid portion and is relatively small compared to the tensa. You should try your best to look after your eardrums as they can be damaged, this could lead to conductive hearing loss, which we will also talk about later on. Hear are some images to help you visualise what I have been going on about.

http://flylib.com/books/1/588/1/html/2/44.%20anatomy%20and%20physiology%20of%20the%20ear_files/loadbinary_017.gif

http://upload.wikimedia.org/wikipedia/commons/b/bc/View-normal-tympanic-membrane.png

The Middle Ear

Ok now lets take a look at the middle ear. The main function of the middle ear is to transfer the energy of longitudinal waves to fluid – membrane waves in the cochlea. Lets talk about the ossicles. The ossicles are three bones in the middle ear, which are all some of the smallest bones in the body. Here’s a quick interesting fact, the word ossicle literally means tiny bone. Ok before we talk more about these bones you should know that each one has its own name, they’re called the malleus, incus and stapes. The malleus is the first of the first of the three bones and it is attached to the eardrum. This bone pacifically transmits sound vibrations from the tympanic membrane to the incus, you should also note that he malleus is unique to mammals. The incus is the anvil shaped bone. Once the sound vibrations have been transmitted, it’s then transmitted to the stapes where the sound is then transmitted again to the oval window. Think of the oval window as the crossroads between the inner and middle ear. By the time the vibrations from the stapes reach the oval window, the vibrations have been amplified 20x what they originally were.

The Eustachian tube or otherwise known as the auditory tube and the pharyngealtympanic tube (good luck pronouncing that one) is a small tube that connects the middle ear to the back of the nose. It has two main functions; the first function is to let the air from the back of the nose up into the middle ear. The second function happens when you have an infection or fluids in the ear, what happens is when the infection is about to clear it will drain down the Eustachian tube into the back of the nose. The auditory tube is usually closed, but opens up every time we swallow, yawn or chew gum. When we swallow we only get air come up the auditory tube and fluid only drains drown auditory tube when we swallow. If he air in the middle ear isn’t replaced and absorbed it can cause negative pressure, which will then pull on the eardrum and cause it to decrease in motion along with the ossicles. This will cause temporary conductive hearing loss. If your auditory tube is working just fine you will always equalize the pressure between the air in the middle ear and the air in the back of your nose and there’ll be no pressure. But if its not you will feel a pressure just like when you’re driving on a mountain or on a plane just like when your ear pops. Some people can actually create this popping of the ear themselves and open the Eustachian tube, they’re even able to decrease and increase the air pressure inside of the middle ear. If you can do this then you should of noticed that in your head your voice actually sounds louder than what it would be if it were closed. Lastly your ear will also catch dust that enters and over time will be pushed out.

http://cdn.ent-surgery.com.au/wp-content/uploads/2012/02/the-eustachian-tube.jpg

The Inner Ear

The inner ear is what’s responsible for you being able to balance and detect sound. I bet your wondering how on earth could our ears be responsible for balance, well stick with it and you’ll find out. Our inner ear contains the bony labyrinth, which contains the cochlea, vestibule and the semi-circular canals. The two main operative parts of the labyrinth are the cochlea and the vestibular system.

The cochlea is a cavity inside of the labyrinth, which looks a bit like a snail. Its job is to transduce (convert) the sound pressure into electricity, which then gets passed through the brain by the auditory nerve. The cochlea is full of fluid called perilymph, which moves when vibrations come from the middle ear. Perilymph is rich in sodium but lacks calcium and potassium. There are two different types of perilymph; the first is the perilymph scala vestibula, which comes from your blood plasma. The second is the scala tympani, which comes from the spinal fluid. When the fluids move all of the tiny hairs that are inside of the cochlea sense the movement by their stereocila. Steroclia are what change sound into electricity for the hair cells.

One of the main parts of the cochlea is the organ of corti. This is the receptor organ of hearing; this is where all the transduction of sound happens so as you can probably guess its pull of hair cells. If you damaged your inner ear you could potentially go completely deaf. The hair cells in the cochlea can’t be fixed once they have broken, that’s why it’s important that you look after them or you could loss your hearing and possibly get tinnitus.

One last thing you should know about the cochlea is that it’s attached to the oval window. If you recall I mentioned how the stapes transfers the vibrations on to the oval window. This how the sound reaches the cochlea by the stapes going in and bout of the cochlea and passing on the vibrations. These pictures should help you visualise how it all fits together.

Although I said that if you damage the hair cells in the cochlea you would eventually go deaf, you can actually get these devices called cochlea implants that

help give hearing to those who have damaged their hair cells. The implants make it easier for them to understand speech better, but the sound that they hear isn’t the same as natural hearing. It’s different because your brain actually will receive less sound information. These implant are great because patients are even able to use their implants for activities like swimming. Even though they may be able to hear it doesn’t cure deafness, it’s simply just a substitute for sound. Some people with the implants find them really good but some actually find them to be irritating and would rather not have one in because they find it worse. This all depends on thinks like if the patient grew up being deaf or if they became deaf later in life.

Ok now lets talk about how we balance. The vestibular system inside the labyrinth is what is what gives us the ability to balance. The vestibular system has to main parts the semi-circular system and the otoliths. The semi-circular system is made of three canals that all are filled with a motion sensing fluid. The first canal of the three, the horizontal semi-circular canal senses movements like looking both ways before crossing the road. The second the superior semi-circular canal which senses movements like cartwheels and touching your shoulders with your head. The last one is the posteror semi-circular, which is responsible for sensing when you nod your head. Now on to the ototlith, which consists of the saccule and the utricle, which sense acceleration and gravity however it sense linear acceleration. For example when you’re going up in an elevator. You’re probable wondering if this has anything to do with being dizzy, well you’re in luck it does. Being dizzy happens because the fluids in the canals are whooshing around.

Perception, Subjectivity and Auditory Illusions

In this section we will discuss how what we hear maybe false even when were sure of what we just heard. First let me explain what and auditory illusion is. You can probably guess what they are just by the name but if you are unsure this is what they are. Auditory illusions are the hearing equivalent to optical illusion, so you’re essentially hearing sounds that aren’t really there.

Let take a looks at an auditory illusion called the McGurk effect. This illusion will show you how our eyes can effect what we hear. Lets says your watching a video that’s been dubbed and the person in the video is saying the word bar, but then it shows another clip of the same person but this time you hear far. On the second clip the first thing you would have noticed is that the way his mouth moved was different this time. The way it moved looked like he was pronouncing an F. What if I told you that because of the way it looked to you, it actually affected what you heard? What actually happened was because vision is our primary sense we perceive sound through our eyes first, so our brains try to make a guess with the visual information on what we are hearing. Hear is a video of the McGurk effect.

https://www.youtube.com/watch?v=G-lN8vWm3m0

One of the most famous psychologists Diana Deutsch has crated many well known auditory illusions. We will take a look at an illusion she made called the tritone paradox. Before we get into it lets first talk about what a paradox is. A paradox is something which cant be possible happened or a statement that contradicts itself which could be true. For example if I asked you to draw me a square circle, it’s impossible to do. Here is a paradox called Russell’s Paradox. If there was a country where all men were shaved daily and there is only one barber, and he shaves all the men who don’t shave themselves then who shaves the barber. However who shaves the barber, if he doesn’t shave himself then that means he has to shave himself but if he does that then according to the rule he will not shave himself.

Now we know what a paradox is lets look at the tritone paradox. In this paradox the listeners are played tones and they are asked to figure out if it goes from high to low or low to high. When you compare what you heard with enough people you’ll see that all of your answers are different. But how could all the answers be different if your were all listening to the same tone, well yes its true that you listened to the same tone, however what you didn’t know was the tone is created with a both a high and low frequency. Our brains actually has a fondness of which one it wants to listen to, this is why what you heard is different to what your friend would of heard.

As a species we tend to do things that are considered as cognitive bias. This means that we have a tendency to do and think things in a specific way, for example people tend to think that they are above average drivers, however if everyone thinks that then surly that’s the average. Another type of cognitive bias is the bandwagon effect, which I’m pretty sure we’ve all experienced in our lives. The bandwagon effect is wen you believe something or do something because others do so. We humans also have a strong tendency to infer. To infer

something to basically read between the lines, for example when someone implies something to you and you understand what the mean without them actually having to say it. That right there is what inference is. We infer things all the time about other, even music, and ourselves. When people next to you are whispering to each other you will infer that they’re saying things about you when in reality there most likely not, we will infer when is comes to music to. When we hear a piece of music we will infer weather it’s a happy or sad bit of music. This ability to infer is amazing because it allows us to communicate without having to completely know what the other person is saying, however it cause mistakes in our perception.

Hearing Loss/Disorders

In this section I will talk about the different types of hearing losses that you can get, some you get from birth, some overtime and some are inevitable. I will talk about a total of seven types, and then you will be able to see if you have any of them.

Conductive Hearing Loss

Conductive hearing loss happens anywhere within the outer ear and middle ear due to problems with conducting sound, usually because there is some kind of blockage in the ear. I could be due to earwax or snot in the Eustachian tube; this is easily fixable. However in some rare cases a tumour in the ear canal can cause it, which isn’t as easily fixable. People with conductive hearing loss usually given hearing aids so that it can amplify the sound waves they receive.

Sensorineural Hearing Loss

Sensorineural hearing loss can be a really big deal if you have it, because it some cases it might not be curable. This type of hearing loss is congenital which means that its and abnormality that you have had since birth. This type of hearing loss usually accrues because of abnormalities of the hair cells in the cochlea, you probably remember me telling you about cochlea implants, well this is what they used for. This is one of the most challenging problems in medicine to solve. Sometimes you can get sensorineural hearing loss later in life instead of being born with it, this can be due to exposure to loud music for a long periods of time, like being in a loud space or having your headphones to loud.

Central Hearing Loss

Central hearing loss is a bit like auditory dyslexia because people with it have trouble separating sounds apart. It usually happens when the patient is in a loud environment with a lot of noise, what happens is their brains have trouble with processing and knowing what there’s sound mean. It’s believed that it’s related to the central nervous system because people with this type of loss have damage to their auditory nerve. If you have this you should avoid listening to loud music because it will just make your condition worse. Sadly they’re no usual treatments

because it’s not a problem with the ear but the central nervous system, however saying that there is a pill that could help. This pill is called the hearing loss pill and it improves the nerves in the brain and ear.

Functional Hearing Loss (PTSD)

Functional hearing loss is from posttraumatic stress disorder, which I’m sure you know of. If not it’s a disorder that you get when you have experienced either one or multiple traumatic events; it’s usually a disorder that soldiers in the army get. You can also get PTSD from drug misuse, foster care and there’s even some evidence to suggest that it’s also genetic, which means that some people are more likely to get it than others after experiencing a traumatic event. Many woman in the army are nine times as likely to get its if they have been sexually assaulted while in the army, even some soldiers that have been racially discriminated against have shown to have PTSD to an extent. PTSD is really strange because you can loose your sight or hearing, your brain could literally just decide to stop letting you see or hear. I guess in a way it’s a type of defence mechanism because its like your brain is saying you’ve seen something that people shouldn’t have to so its going to shut your sight down. Its been shown that a lot of people that are prepared and know that they could see a traumatic even, are less likely to get it because they have in a way mentally prepared themselves for it. Here’s a fact that will make you understand just how bad PTSD is, more American soldiers and killed themselves than killed who they are fighting.

Presbycusis

Presbycusis is inevitable because its ageing hearing loss. What happens is that the high frequency hairs in the cochlea will start to go first over time, its has been found to happen as early as the age of 18, and its seems to affect men more often then women. There are available treatments for this such as hearing aids and cochlea implants to enhance the hearing of the elderly.

Mixed Hearing Loss

Mixed hearing loss simply means that you have more than one type of hearing loss. You could possibly have any of the types of hearing losses I have discussed so far however.

Tinnitus

Tinnitus is a whistling, humming or ringing sound that you hear. About 10 – 15% of people on earth have it. Tinnitus is what we a phantom sound because its subjective and the sounds not real there, this is why there isn’t a cure because how can you sure something that isn’t there. It’s not sure what exactly causes it but it’s known that it’s a problem with neural hearing. This believed that tinnitus happens because the patents that have this condition have lost the high frequency hairs at the start of the cochlea, which is were tinnitus happens. The weird thing about it is that the patients are hearing the high frequencies they’ve

lost, its like their brain are over compensating for the lost hairs, this is why it’s believed to be connected to the high frequency hairs in the cochlea.

It’s a bit like phantom limb, were people that have lost there limbs still believe they have them. About 60 – 80% of amputees have this feeling as if their limbs are still there. Like tinnitus there isn’t a cure because is a mental thing. People that have phantom limbs are still able to warmth, cold and even pain in the limb they have lost its as if there brain is recreating these feelings. Patents also feel the need to itch the lost limb, could you imagine trying to itch something that wasn’t there, it would drive you insane. This is one of the reason prosthetic limbs were made, so that they can do things like itch there arm. Just like phantom limbs tinnitus will drive you crazy to if it gets to bad, some people cant even sleep at night because the raining is so loud. A lot of people with this actually commit suicide sadly because it becomes so unbearable. For people with it you can get music called white noise to play in your ears at night so you can sleep better, but other than that there isn’t much else that can be done.

Preferences

When it comes to humans we know that there is a universal preference to consonance over dissonance. Consonance are sounds that are considered to be pleasing and nice, where as dissonant sounds are considered bad and unpleasing sounds. This preference we have for pleasant sounds is unique to humans, for some reason other animals don’t seem to like or hate either what we consider nice or bad sounds. You’re probably wondering what we consider to be a really bad sound; well I shall know tell you. The sound we consider the most dissonance is the sound of sick, because we know that if we are near someone that was sick we could perpetually become ill to. This is how we have evolved to avoid things that could make us ill or kill us, its survival of the fittest. This is why we have evolved to dislike the sound of being sick.

Equal Loudness Contours

These two images show my hearing compared to the human hearing.

https://www.google.co.uk/search?q=cochlea&es_sm=91&source=lnms&tbm=isch&sa=X&ei=UleMVOzAEozyUIXlgJAE&ved=0CAgQ_AUoAQ&biw=1280&bih=701#facrc=_&imgdii=_&imgrc=0IyhtwL12xmp1M%253A%3BygodZCNucjiHOM%3Bhttp%253A%252F%252Fwww.brainhq.com%252Fsites%252Fdefault%252Ffiles%252Fstyles%252Fmedia_gallery_large%252Fpublic%252Fcochlea-xsection.jpg%253Fitok%253DZdmxX5Fd%3Bhttp%253A%252F%252Fwww.brainhq.com%252Fmedia-gallery%252Fdetail%252F159%252F47%3B582%3B600

http://upload.wikimedia.org/wikipedia/commons/5/5e/Cochlea.png

http://www.open.edu/openlearnworks/pluginfile.php/66526/mod_page/content/1/SD329_1_018i.jpg

http://www.thesoundlearningcentre.co.uk/images/head_move.jpg

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http://newt.phys.unsw.edu.au/jw/hearing.html