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How Hearing Works

There is a common misconception that our ears hear things.

It is, however, our brain that does the hearing and our ears are simply a collection mechanism for the sounds that our brain interprets into words and meaning.

Natural hearing

A healthy hearing system can recognise both low pitch sounds like traffic  or vowel sounds and high pitch like bird song or the clarity of consonants. In technical terms, that means frequencies from as low as 20 Hertz (Hz) and up to 20,000 Hz are audible to a young ear. It can process very quiet sounds such as the rustling of leaves and extremely loud sounds like a road drill. Sound is measured in decibels (dB). The human ear can hear sounds at zero dB or below, and tolerate sounds up to 120 dB.

Understanding speech

When we process sound our brain takes in all types of sound in varying volume levels, and at different frequencies, in varying situations, and copes with it all in real time. Whether we are sitting in a restaurant or in a car, shopping  or walking on the beach, our brain filters out a mass of irrelevant or background sounds to concentrate on those that we need or want to hear. It is thanks to this facility that we are able to focus on a single voice, or participate in intimate conversations in a noisy environment. This is an acquired skill and we lose this skill as our hearing deteriorates.

It is important to know that when you first hear through hearing aids, you may need to give your brain time to relearn this filtering process.

Surround sound

Our brain hears everything irrespective of the direction it has come from. Because of the shape of our ears, we know whether the sound has come from above or below us, or from in front or behind us. Using sound not only do we communicate,  but we discern a great deal about the environment in which we find ourselves.  The best and latest modern hearing aids can mimic this process.

Three main components make up the human ear

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Outer ear

The outer ear includes the pinna, the visible part of the ear, as well as the ear canal which terminates at the eardrum, also called the tympanic membrane. The pinna serves to focus sound waves through the ear canal toward the eardrum. It is because of the peculiar shape of the outer ear, that sound arrives at the brain at different times and volumes, allowing the brain to determine the direction it is coming from. This gives us the ability to localise sound accurately.  Sounds from above arrive at the ear drum sooner than sounds from below, acting as a natural defense mechanism from danger. Similarly we will be alerted to sounds from behind us before sounds in front of us.

The eardrum is an airtight membrane only 3 cell layers thick. When sound waves arrive at the eardrum, they cause it to vibrate following the waveform of the sound. This vibration is transferred to the first of the 3 bones in the middle ear, the malleus or hammer as it is more commonly known.

Middle ear

The middle ear consists of a small air-filled chamber that is located beyond the eardrum. Within this chamber are the three smallest bones in the body, known collectively as the ossicular chain which is made up with the malleus, incus and stapes (sometimes referred to as the hammer, anvil and stirrup respectively). They aid in the transmission of the vibrations from the eardrum to the inner ear. The middle ear serves to amplify the sound waves prior to them being transmitted through to the cochlea. The stirrup is actually the smallest bone in your body.

The inner ear

The inner ear consists of the cochlea, which is a snail shaped hollowing out in the skull and the semi circular canals which control our balance. Processing begins in the inner ear as the sound waves are transformed into electrical impulses. It is filled with fluid, sound waves cause this fluid to move and the movement is picked up by thousands of microscopic hair cells . The hairs sit on  a membrane which rises and falls as the sound waves enter the cochlea having been transmitted from the middle ear. The membrane is divided into sections each responding to a different pitch of sound. High pitch sound is located at the entrance to the cochlea which is why we tend to lose high frequency sounds first. The hairs trigger an electrical nerve impulse which is sent to your brain via the auditory nerve.

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The brain and rehabilitation

cochlea-curl-frequenciesMany people find it challenging to follow conversations in places like a noisy restaurant. This is because the low pitched vowel sounds interfere with the consonant sounds.

The high-pitched consonant sounds like f, s and t are easily drowned out by louder, low pitched vowel sounds like a, o and u. This results in a person with hearing loss complaining that they can hear that others are talking, but not what they are saying.

Speech is made up of a large number of different sounds, put together in very rapid flow. Our brain constantly prioritises and organises all these sounds for us.

When it comes to hearing, it may come as a surprise to learn that the brain works harder than the ears. This is why  it can be very frustrating just trying to follow conversation in noise. Even people with no hearing loss can find this challenging. Ordinarily your brain will be able to sort through all information and filter out the unwanted or irrelevant sounds.

Once impulses are sent to the brain, it processes the data so that we can select what is relevant to the situation and follow it. When acclimatising to hearing aids is is important to understand and accept that the brain needs to relearn the sounds it has forgotten and to re-prioritise what it needs to hear. In the early stages of this rehabilitation clients will hear many new sounds and gradually learn to process out the ones they do not want to hear in the same way that we learn to ignore the trains running at the bottom of the garden after a few weeks of moving to a new house.

For people with hearing loss, however, the brain has to work much harder to make sense of sound because the input it receives from the ears is softer, less detailed, or unclear, with elements missing.