Lecture - Hearing

Audio,Image and Video processing

Overview

1. The Human Ear
2. How The Ears Work
3. Impedance Matching Mechanisms
4. Human Hearing

The Human Ear

There are three levels of hearing that we will experience as a human.

The first is called Primitive Level and an example of this would be if you are sitting in a room with a window that faces passing traffic and the ear can find it mellow and comforting or harsh and disruptive. Regardless of your reaction your ear will still process the sound and it is the level that is Primitive Level.

The second is called Symbolic Level and an example of this would be that if you are making a decision about going out or leaving the house and you hear a report that a road is busy but that as you come nearer to this road the noises of cars become louder. It is this background noise of the cars and the vocal noise of the report that will be processed in your brain that the road will be congested with traffic. This is Symbolic Level.

Finally the last level is call Warning Level and a prime example of this is if you are driving and you hear the siren of an Ambulance or a Police car you instinctively know to move aside or move out of the way has the siren has warned you that they will need to move quickly to their destination unhindered. Your brain will process this sound in the same manner as an instruction. This level of noise and sound is called the Warning Level.

How The Ears Work

The human ear is composed of three main parts. The outer ear is made up of the Pinna, Ear Canal and the Eardrum. The middle ear is made up of the Ossicles and the Eardrum. The inner ear is made of up the Cochlea, the Auditory Nerve and the Brain.

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Sound is taken in when the ear flap funnels sound waves into your Outer ear canal. The waves then travel along the passage until the reach the Eardrum and vibrate. Once these vibrations are picked up the Ossicles start to move and the vibrations are sent on to a thin layer of tissue of the inner ear called the Oval Window. The movement that is passed through the Oval Window creates a wave motion in the fluid in the Cochlea.

Inside the Cochlea there is a spiral shaped organ called the Corti. The Corti is made up of thousands of tiny sensory hair cells attached to a single membrane. These hair cells also have another set of sensory hair cells that are attached to their own membrane. When the fluids of the Cochlea have the wave like motion, created by the Oval Window, they press the hairs against the second membrane and the movement of these hairs are changed into nerve impulses which travel along the Cochlear nerve and are sent to the brain.

The fact that humans have two ears allows us to have the ability to locate where a sound originates from. If a sound is louder on the left ear and quieter on the right our brain will tell us that the sound is originating to our left.

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Impedance Matching Mechanisms

Impedance matching is an important function that is used in the middle ear. The middle ear's primary function is to transfer all incoming vibrations and sort them from the large, low impedance membrane to the smaller, high impedance oval window. The middle ear acts as a transformer of sorts as it converts low pressure, high displacement vibrations into high pressure, low displacement vibrations that will be at a suitable level for the cochlear fluids.

Auditory processing is the two channel set of contiguous frequency bands. The separation of the ear's left and right signals, the low and high frequency information and then timing and intensity information. 

Ohm's law stated that the perception of the tone of a sound is a function of the amplitudes of the harmonics rather than the phase relationships between them. 

Human Hearing 

Human hearing can cover a variety of frequencies that can start at 20 Hz and go all the way up to 20,000 Hz. Although unless you are a bat then the much higher frequencies of 18,000 Hz - 20,000 Hz will be useless as they are outwith most of our day to day life. 

source - the lecture slides

Humans have a Decibel range as well as a Hertz range. An example of normal dB range would be having a conversation with a friend. This will be at roughly 60 dB whereas a 110 dB noise would be the sound of a Jack hammer on concrete or a rock concert. At 120 dB the ear will start to experience discomfort and we will not want to listen to the noise any longer but at 140 dB we will start to feel physical pain from the sound.