EAR - 'ERE

HUMAN EAR - 'ERE
The final link in the sound reproduction chain is the human ear. How can you ignore the basic knowledge of it if you like or make music? It is a vital element to all those involved in audio engineering too. A read of this will cast doubt on Darwinian ideas of "natural selection".

DIRECTIVITY
Sounds from a source situated to one side of the listener arrive at the furthest ear fractionally later than the nearest. Thus there is a delay in phase which the brain interprets in terms of direction. Long wavelengths, (lower frequencies/notes), have less phase shift than the higher frequencies which fact is why it is much more difficult to locate the source of a low frequency note. The rattle of a box of matches is a bone-fide test used to demonstrate this.

Refer now to the drawing of the human ear below. The convolutions of the Pinner or outer ear produce reflections and phase delays which differ according to the angle of incidence with which the sound wave arrives. {A wave in this context is a variation in air pressure of a cyclic nature}. This also aids in direction location, especially, it is believed, in the vertical registration of the source. Again only the higher frequencies are effectively so identified. At mid frequencies the masking effect of the head plays a part by introducing small difference in amplitude.

IMPEDANCE MATCHING
The ear drum or tympanum vibrates in sympathy with the received sound and transmits the vibrations through three small bones called the ossicles in the middle ear. These are the hammer, anvil and stirrup {or stapes as some say}. The first two form a pair of pivoted levers that produces a nominal leverage ratio of 3:1, and the third communicates vibrations to the window of the inner ear. The ratio matches the mechanical impedance of the ear drum to that of the window, so obtaining optimum power transfer.
The tiny bones are held in place by small muscles which permit the pivotal positions, and hence the sensitivity, to alter. Thus the sensitivity is not linear, following a logarithmic law, being at a maximum for quiet sounds reducing to a minimum for loud ones.
This automatic volume control allows the ear to process an enormous range of sound levels being in the order of 10 to the power of 12 which is 1 million X 1 million = 1 trillion levels.
The Eustachion tube equalises the atmospheric pressure on both sides of the eardrum by venting the middle ear to the throat.

FREQUENCY DIVISION
The inner ear consists of a long liquid filled tube that is rolled up like a shell called the cochlea. A horizontal basilar membrane divides the tube along its length into upper & lower compartments except at the sealed far end where there is a connecting gap called the helicotrema. The compartments are termed the scala vestibule and scala tympani respectively.
Sound vibrations are applied to the Oval window at the entrance to the upper chamber by the Stirrup bone. From there they travel through the fluid to the gap at the end, then down into and along the lower compartment and back to its round window where they are absorbed. En route they pass through thousands of sensitive hair cells located on the upper surface of the membrane, which are linked to nerve fibres. These cells respond to different frequencies and are divided into 24 bands with one third octave spacings, starting with the highest band near to the entrance and the lowest at the far end. Individual bands occupy about 1.3mm of space, each being termed a bark.
The centre frequencies of the bands start at 50Hz for the No.1 and go up to 13.4KHz for band 24. Cut off outside of each band is sharp at the lower side but more gradual as the bands rise in frequency. The lowest band is 100Hz while the highest is 3.5KHz. The overall response in a healthy person under 30yrs is 16Hz to 16KHz with the girls generally having better HF hearing than the boys.

FREQUENCY RESPONSE
The frequency response of the ear is not flat, being at a maximum from 2 to 4KHz. The rest of the curve varies according to the sound level. At lower volumes the response to both treble & bass is less which is why some audio units boost these at low listening levels. Speech frequencies come well within the overall range but music encompasses greater range in both frequency & loudness. These contours show the sound pressures required to produce sensations of equal volume at various frequencies. They are known as "Equal Loudness Contours" and are the inverse of frequency response curves.
The contours are an averaged sample taken from an age group of 18 to 25 yrs.

HEARING DAMAGE
Temporary damage to hearing sensitivity results from exposure to loud sounds. It can become permanent if the exposure is prolonged. Damage is greater if the sound contains percussive energy bursts. Impairment is in the 4 kHz range (bark 18 in the cochlea) irrespective of the nature of the sound that caused the damage. As exposure extends the damage tends to reach down to 1 kHz.
Industry regs. give the following maximum exposures shown in the chart. It should be noted that disco music and headphone listening levels in excess of 100dBA can easily be realised. The dangers are obvious.

PRESBYCUSIS
This is an almost inevitable condition where hearing deteriorates with age. It starts slowly from 20 - 30 yrs and worsens over time. Exposure to loud noise plays its part. This chart shows the expected deterioration.