section 3 - human hearing

7/23/2019 Section 3 - Human Hearing

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Human hearing

The structure and function of the ear

The structure of the ear can be

divided into three main parts: theouter ear, the middle ear and the

inner ear. The outer structure of the

ear is responsible for helping us

place the original location of a

sound source. It also helps to funnel

and focus sound waves on their way

to the middle ear and auditory

canal. The middle ear contains the

auditory canal, which terminates in

the eardrum, or tympanic membrane. Attached to the other side of the eardrum,

in a small space of air, are three tiny bones which then attach to a uid-lled

structure called the cochlea of the inner ear at a point called the oval window. It

is in the cochlea that the vibrations transmitted from the eardrum through the

tiny bones are converted into electrical impulses sent along the auditory nerve to

the brain. The inner ear, which is surrounded by bone, also contains semicircular

canals, which function more for purposes for euilibrium than hearing. The chain

of ossicles can be sti!ened or muted by a contraction of the stapedius muscle.

This provides a form of protection against loud sustained sounds, but not sharp

sudden ones, such as a gunshot. This ree" is far less e#cient in older people,

which along with di!ering tastes may e"plain their lower tolerance to louder

music as well as an increased ris$ level for hearing loss.

The most fascinating aspect of perception ta$es place in an area of the cochlea

called the basilar membrane. The cochlea is a tapered tube, which circles around

itself li$e the scroll on a violin. The basilar membrane divides the tube lengthwise

into two uid-lled canals, which are %oined at the tapered end. The ossicles

transmit the vibration to the cochlea where they attach at the oval window. The

resultant waves travel down the basilar membrane where they are sensed by

appro"imately &' ( )*,*** hair cells attached to it which po$e up from a third

canal called the organ of +orti. It is the organ of +orti that transforms the

stimulated hair cells into nerve impulses. ecause of the tapered design ofcochlea, waveforms travelling down the basilar membrane pea$ in amplitude at

di!ering spots along the way according to their freuency. igher freuencies

pea$ out at a shorter distance down the tube than lower freuencies. The hair

cells at that pea$ point give us a sense of that particular freuency ( it is thought

that a single musical pitch is perceived by &* ( &) hair cells. ue the tapered

shape of the cochlea, the distance between pitches follows the same logarithmic

distance as our perception of pitch. This arrangement is responsible for the fact

that a higher freuency can mas$ or hide a lower one, but a lower one can mas$

a higher one.

The limits of hearing



The threshold of hearing is the /ound pressure level 0/123. The absolute

threshold of hearing is the minimum amplitude of a pure tone that the average

ear with normal hearing can hear in a noiseless environment. 4n the other hand,

the threshold of pain is the /12 beyond which sound becomes unbearable for a

human listener. This threshold varies only slightly with freuency. The table

below shows the di!erent values for the threshold of pain.

Threshold of pain

SPL: Sound pressure:

120 dB 20 Pa

130 dB 63 Pa

134 dB 100 Pa

137.5 dB 150 Pa

140 dB 200 Pa

The average person can hear sounds down to about *d, the level of rustling

leaves. /ome people with very good hearing can hear sounds down to -&5d. If a

sounds reaches 65d or stronger, it can cause permanent damage to your

hearing.

1sychoacoustics

1sychoacoustics is the branch of psychology, with the perception of sound and

its physiological e!ects.

The aas e!ect or the

1recedence 7!ect is a

psychoacoustic e!ect

described by elmut aas as

the ability of our ears to localisesounds coming from anywhere

around us. This can be

demonstrated using a modulation delay e!ect in 1ro Tools.

The coc$tail party e!ect is the phenomenon of being able to focus one8s auditory

attention on a particular stimulus while ltering out a range of other stimuli9 the

same way that a partygoer can focus on a single conversation in a noisy room.

The auditory mas$ing e!ect occurs when the perception of one sound is a!ected

by the presence of another. The unmas$ed threshold is the uietest level of the

signal which can be perceived without a mas$ing signal present. The mas$ed

threshold is the uietest level of the signal perceived when combined with a



specic mas$ing noise. The amount of mas$ing is the di!erence between the

mas$ed and unmas$ed thresholds.

ith tuning instruments that can produce sustained tones, beats can readily be

recognised. Tuning two tones to a unison will present a peculiar e!ect: when the

two tones are close in pitch but not identical, the di!erence in freuencygenerates the beating. The volume varies li$e a tremolo as the sounds

alternately interfere constructively and destructively. As the two tones gradually

approach unison, the beating slows down and may become so slow as to be

unnoticeable.

The oppler e!ect on the other hand, is an increase 0or decrease3 in the

freuency of sound,

light, or other waves

as the source and

observer move towards 0or

away from3 each other. The e!ect

causes the sudden change in

pitch noticeable in a passing

siren, as well as the

red shift seen by

astronomers.

ealth and safety

7ar monitors and headphone monitors do not generally meet the necessary

criteria to be categorised as personal hearing protection devices, although some

wor$ers in the music and entertainment sector may wrongly consider

them to be. owever, I7;/ and headphone monitors play a valuable role

in reducing the ris$ of hearing damage as they allow a reduced

level of reproduced sound on stages and in other

wor$ areas. 1ersonal hearing protection should be used

where e"tra protection is needed above what can be

achieved using noise control and as a short-term

measure while other more permanent solutions are

being sought. To be of value, hearing protection needs to: control

the ris$s, not over- protect, reduce the noise level to no less than

about <*d, be more comfortable and suitable for the wor$ing environment, be

properly used, be worn at the right time, and be properly maintained.

y A$ai. ;

section 3 - human hearing

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