The Ear and Hearing

The Outer Ear

The external ear (pinna) and the ear canal act as a guide, directing sound waves that vibrate the ear drum. The acoustic transfer functions formed by the shape, size and position of the head and pinna also provide information on the relative direction of the sound source. The roughly horn shape of the pinna and ear canal create an acoustic transfer function which is directionally sensitive and also amplifies sound at certain frequencies e.g. 10 dB to 20 dB gain around 2.5 kHz.

The Middle Ear
The vibration of the eardrum is transferred through the chain of small bones (ossicles) in the middle ear that in turn vibrate the oval window of the cochlea.  This impedance matching chain of transmission acts to reduce power loss as the air-borne vibrations are transferred to the fluid medium in the cochlea. Excessive movement of the ossicles is constrained by a neuro-muscular feedback mechanism that acts to prevent damage due to loud sounds.

The Inner Ear

Vibration of the oval window produces pressure waves in the cochlear fluid that stimulate cochlear structures that perform a spectral analysis. Sensor "hair" cells within the cochlea cause neurons connected to the auditory nerve to "fire”. They transmit timing, amplitude and frequency information to the auditory brain stem where a hierarchy of neural processing commences.

Human Hearing and Speech
Human hearing covers a range of frequencies from about 20 Hz to 20 kHz and can respond to an enormous range of sound levels.
Within this range are those frequencies and levels generated by conversational speech. A rough guide to the separation in frequency and level between vowel sounds and consonants is shown.
The lowest sound pressure level (SPL) that humans can hear varies with frequency and is called the Hearing Threshold.
Greatest sensitivity is normally in the frequency range 1 kHz to 4 kHz.

MPEG/MP3 Audio Coding

The use in MP3 of a lossy compression algorithm is designed to greatly reduce the amount of data required to represent the audio recording and still sound like a faithful reproduction of the original uncompressed audio for most listeners. An MP3 file that is created using the setting of 128 kbit/s will result in a file that is about 11 times smaller than the CD file created from the original audio source. An MP3 file can also be constructed at higher or lower bit rates, with higher or lower resulting quality.

The compression works by reducing accuracy of certain parts of sound that are considered to be beyond the auditory resolution ability of most people. This method is commonly referred to as perceptual coding. It uses psychoacoustic models to discard or reduce precision of components less audible to human hearing, and then records the remaining information in an efficient manner.