Auditory System

21
Speech Processing mohitgoel4u.net(Mr. Feb)  Human Auditory System

description

working of human hearing system

Transcript of Auditory System

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 1/26

Speech Processing 

mohitgoel4u.net(Mr. Feb)

  Human Auditory System

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 2/26

Why Do We Have Two E

• Sound localization – spatially locate sou

sources in 3-dimensional sound fields, batwo-ear processing, loudness differencestwo ears, delay to each ear 

• Sound cancellation – focus attention on‘selected’ sound source in an array of sousources – ‘cocktail party effect’, BinauralMasking Level Differences (BMLDs)

• Effect of listening over headphones =>

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 3/26

Overview of Auditory Mecha

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 4/26

The Human Ear 

Outer ear : pinna and external canal

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 5/26

Ear and Hearing

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 6/26

Human Ear • Outer ear: funnels sound into ear canal

• Middle ear: sound impinges on tympanicmembrane; this causes motion – middle ear is a mechanical transducer, consisti

hammer, anvil and stirrup; it converts acousticawave to mechanical vibrations along the inner e

• Inner ear: the cochlea is a fluid-filled champartitioned by the basilar membrane – the auditory nerve is connected to the basilar m

via inner hair cells

 – mechanical vibrations at the entrance to the coc

create standing waves (of fluid inside the cochlecausing basilar membrane to vibrate at frequen

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 7/26

The Outer Ear 

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 8/26

The Middle Ear The Hammer (M

 Anvil (Incus) an

(Stapes) are thebones in the bod

they form the cou

between the vibr

eardrum and the

exerted on the ovthe inner ear.

These bones can

of as a compoun

achieves a multipforce—by a facto

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 9/26

   O  u   t  e  r  e  a  r  g  a   i  n   (   d   B   )

0.2 0.3 0.5 1.00.7 2 3 5 107

Frequency (KHz)

20

10

0

-10

Transfer Functions at the Perip

M   i   d   d   l  e

  e  a  r  g  a   i  n   (   d   B   )

20

10

0-5

20

0

-20

-40   R  e  s  p  o  n  s  e   G  a   i  n   (   d   B   )

Combined respo(outer+middle ear)

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 10/26

The CochleaMalleus

Incus

Stapes

Ossicles

(Middle Ear Bo

Oval Windo

Vestibule

 Auditory nerv

Tympanic

Membrane

Round Window

Cochlea

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 11/26

The Inner Ear The inner ear ca

of as two organs

semicircular can

serve as the bod

organ and the c

serves as the bo

microphone, con

sound pressure

the outer ear int

impulses which

on to the brain v

auditory nerve.

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 12/26

The Auditory Nerve

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 13/26

 Auditory Nerves

IHC IHCInner 

Hair Cells

Basilar Membran

Cochlear

(Implic

Vestibular System

Oval Window

Perilymph

Malleus

Incus Stapes

RoundWindow

Eustachian Tube

Middle Ear Cavity

Tympanic Membrance

Middle and Inner Ear 

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 14/26

Schematic Representation o

Ear 

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 15/26

Stretched Cochlea & Basilar Memb1600 Hz

800 Hz

400 Hz

200 Hz

100

50 Hz

0 Distanc

Basilar 

Membrane

Scala

Vestibuli

C

Cochlear Base

(high frequency)

Unrolled

Cochlea

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 16/26

Basilar Membrane Mecha

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 17/26

Basilar Membrane Mecha• characterized by a set of frequency responses at differe

along the membrane

• mechanical realization of a bank of f ilters• filters are roughly constant Q (center frequency/bandwidt

logarithmically decreasing bandwidth

• distributed along the Basilar Membrane is a set of about 3sensors, called Inner Hair Cells (IHC), which act as mech

motion-to-neural activity converters• mechanical motion along the BM is sensed by local IHC c

firing activity at nerve fibers that innervate bottom of eac

• each IHC connected to about 10 nerve fibers, each of difdiameter => thin fibers fire at high motion levels, thick fibe

lower motion levels• 30,000 nerve fibers link IHC to auditory nerve

mohit

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 18/26

Human Auditory System

mohitgoel4u.net(Mr. Feb)

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 19/26

Human Auditory System

• The pinna (or informally the ear) is the surface

surrounding the canal in which sound is funneled.

• Sound waves are guided by the canal toward the

eardrum—a membrane that acts as an acoustic-to-mechanic transducer. The sound waves are then

translated into mechanical vibrations that are passed

to the cochlea through a series of bones known as

the ossicles.

mohitgoel4u.net(Mr. Feb)

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 20/26

Human Auditory System

• Presence of the ossicles improves sound propagation by

reducing the amount of reflection and is accomplished by

the principle of impedance matching.

• The cochlea is a rigid snail-shaped organ filled with

fluid. Mechanical oscillations impinging on the ossiclescause an internal membrane, known as the basilar

membrane, to vibrate at various frequencies.

• The basilar membrane is characterized by a set of

frequency responses at different points along the

membrane.

mohitgoel4u.net(Mr. Feb)

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 21/26

Human Auditory System

•Motion along the basilar membrane is sensed by theinner hair cells and causes neural activities that are

transmitted to the brain through the auditory nerve.

• Due to this arrangement, the human auditory system

behaves very much like a frequency analyzer.

mohitgoel4u.net(Mr. Feb)

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 22/26

Absolute Threshold

•The absolute threshold of a sound is the minimumdetectable level of that sound in the absence of any other

external sounds.

The horizontal axis is frequency measured in hertz (Hz);

while the vertical axis is the absolute threshold in decibels

(dB), related to a reference intensity of watts per

square meter—a standard quantity for sound intensity

measurement.

1210

mohitgoel4u.net(Mr. Feb)

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 23/26

Absolute Threshold

• As we can see, human beings tend to be more

sensitive toward frequencies in the range of 1 to 4

kHz, while thresholds increase rapidly at very high

and very low frequencies. It is commonly acceptedthat below 20 Hz and above 20 kHz, the auditory

system is essentially dysfunctional.

•These characteristics are due to the structures of thehuman auditory system.

mohitgoel4u.net(Mr. Feb)

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 24/26

Absolute Threshold

We can take advantage of the absolute threshold curvein speech coder design. Some approaches are the

following:

• Any signal with an intensity below the absolutethreshold need not be considered, since it does not

have any impact on the final quality of the coder.

•  More resources should be allocated for the

representation of signals within the most sensitive

frequency range, roughly from 1 to 4 kHz, since

distortions in this range are more noticeable.

mohitgoel4u.net(Mr. Feb)

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 25/26

Masking

• Masking refers to the phenomenon where one sound is

rendered inaudible because of the presence of other

sounds.

mohitgoel4u.net(Mr. Feb)

7/17/2019 Auditory System

http://slidepdf.com/reader/full/auditory-system-568edfbc2997a 26/26

Masking

• The presence of a single tone, for instance, can mask

the neighboring signals—with the masking capability

inversely proportional to the absolute difference in

frequency.

• Masking capability increases with the intensity of the

reference signal, or the single tone in this case.

mohitgoel4u.net(Mr. Feb)