Department of Electrical & Computer Engineering Meena Ramani 04/07/04

Department of Electrical & Computer Engineering

Meena Ramani

04/07/04


Topics

• Anatomy of the Ear and Hearing

• Auditory perception

• Hearing aids and Cochlear implants.


The Incredible sense of HearingThe Incredible sense of Hearing

“

Behind these unprepossessing flaps ... lie structures of such delicacy that they shame the most skillful craftsman"

Stevens, S.S. [Professor of Psychophysics, Harvard University]


Why study hearing?

• Best example of Speech Recognition

• Mimic Human Speech Processing

• Hearing Aids/ Cochlear implants

• Speech Coding


• The stapes or stirrup is the smallest bone in our body. It is roughly the size of a grain of rice ~2.5mm

• The movement of the eardrum in response to the minimum audible ## sound is less than the diameter of a hydrogen atom

• The inner ear has reached its full adult size and shape when the fetus is 20-22 weeks old.

• Even during sleep the ear continues to function with incredible efficiency

• The ears are responsible for keeping the body in balance

• Hearing loss is the number one disability in the world.

• Percentage of people who loose their hearing at age 19 and over: 76.3%


Dynamic Range of Hearing• The practical dynamic range could be said to be from the threshold of hearing to the threshold of pain

• Sound level measurements in decibels are generally referenced to a standard threshold of hearing at 1000 Hz for the human ear which can be stated in terms of sound intensity:

Dynamic range is enhanced by an effective amplification structure which extends its low end and by a protective mechanism which extends the high end.


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Outer Ear

• Focuses sound waves (variations in pressure) into the ear canal

• Sound spreads out according to Inverse Square Law

• A larger pinna captures more of the wave and hence more sound energy.

• Elephants: Hear Low frequency sound from up to 5 miles away

• Human Pinna structure: Pointed forward & has a number of curves. • More sensitive to sounds in front

• Dogs/ Cats- Movable Pinna => focus on sounds from a particular direction

Pinna /Auricle

Auditory Canal


Outer EarPinna /Auricle

Auditory Canal

Is sound on your right or left side?

Interaural Time Difference (ITD)

Interaural Intensity Difference (IID)

Interaural Differences

Horizontal localization

Vertical localization

Sound Localization


Interaural differences

The direct path from the acoustic source to the two ears will generally be different

-The signal needs to travel further to more distant ear

-More distant ear partially occluded by the head

Two types of interaural difference will emerge

- Interaural time difference (ITD)- Interaural intensity difference (IID)


Schematic illustration of interaural differences

Leftear

Rightear

soundonset

left right

time



Leftear

Rightear

soundonset

time

arrival timedifference



Leftear

Rightear

soundonset

time

ongoing timedifference



Leftear

Rightear

soundonset

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nsity

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Interaural time differences (ITDs)

• Threshold ITD 10-20 s (~ 0.7 cm)

Interaural intensity differences (IIDs)

• Threshold IID 1 dB

Thresholds


Interaural time differences (ITDs) Low frequencies

Ongoing disparities can only be detected for frequencies up to around 1500 Hz

sensitivity declines rapidly above 1000 Hz

• Auditory system assumes that the smallest phase difference corresponds to the true ITD

• For frequencies below 700 Hz, this strategy will always give the correct answer

Interaural intensity differences (IIDs) High Frequencies

The amount of attenuation varies across frequency

• below 500 Hz, IIDs are negligible (due to diffraction)

• from 2 – 4 kHz, IIDs of 10 dB occur for sources located at 90º

• IIDs can reach up to 20 dB at high frequencies

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Auditory Canal

Is sound above or below?

Pinna Directional Filtering

Horizontal localization

Vertical localization

Sound Localization

•Pinna amplifies sound above and below differently

•Curves in structure selective amplifies certain parts of the sound spectrum


Sound localization of Barn Owls and cats

In a Barn Owl, the left ear left opening is higher than the right - so a sound coming from below the Owl's line of site will reach the right ear first.

Hearing sensitivity comparison of Barn Owls, Cats & Humans

•Both the cat and the Barn Owl have much more sensitive hearing than the human in the range of about 0.5 to 10 kHz. •The cat and Barn Owl have a similar sensitivity up to approximately 7 kHz.• Beyond this point the Barn Owl's sensitivity declines sharply.


Project1:Using Head-Related Transfer Functions to deliver speech for

virtual reality applications

•The simplest spatial audio systems are limited to localizing in azimuth only.

•To go beyond the limited capabilities of these approaches, we need to use Head-Related Transfer Functions (HRTF's).

•The impulse response from the source to the ear drum is called the Head-Related Impulse Response (HRIR), and its Fourier transform H(f) is called the Head Related Transfer Function (HRTF)

•It accounts for diffraction around the head, reflections from the shoulders and most significantly, reflections from the pinnae.


Project 2 Beamforming and Direction of Arrival

Most DOA algorithms apply Eigen Decomposition for the Spatial correlation matrix and noise subspace eg. MUSIC, ESPRIT

More biologically inspired DOA algorithm should do better

Frequency Dependent Frequency Independent



Auditory Canal

•Auditory canal length 2.7cm

•Can model the canal as a ¼ wave resonator

•Resonance frequency ~3Khz

•Boosts energy between 2-5Khz upto 15dB

•Correspondingly, the hearing curves show a significant dip in the range 2000-5000 Hz with a peak sensitivity around 3500 -4000 Hz.

•High sensitivity region at 2-5kHz is very important for the understanding of speech.


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Middle Ear

Functions of Inner Ear

Impedance matching– Between vibrations in air and the liquid medium in the inner ear.

– Acoustic impedance of the fluid is 4000 x that of air. => All but 0.1% would be reflected back.

Stapedius reflex (explained later)

Eardrum

Ossicles

Oval window

The tympanic membrane or "eardrum" receives vibrations traveling up the auditory canal and transfers them through the tiny ossicles to the oval window.


Middle Ear

Eardrum MalleusIncusStapesOval Window

Ossicles: 3 bones Malleus (Hammer), Incus (Anvil), Stapes (Stirrup)

Eardrum

Ossicles

Oval window

An amplification by lever action < 3x

Area amplification 15x• Large area of ear drum ( 55mm2), small area of

stapes (3.2 mm2)• Increases effective Force/Unit area.

Stapedius Reflex:

Protection against low frequency sounds

Tenses muscles stiffens vibration of Ossicles reduces sound transmitted (20dB)

Reflex is triggered by loud sounds


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Inner EarSemicircular Canals

Cochlea

•The semicircular canals are the body's balance organs.

•Hair cells, in the canals, detect movements of the fluid in the canals caused by angular acceleration

•The canals are connected to the auditory nerve.



Cochlea

The inner ear structure called the cochlea is a snail-shell like structure divided into three fluid-filled parts.

Two are canals (Scala tympani and Scala Vestibuli) for the transmission of pressure and in the third is the sensitive organ of Corti, which detects pressure impulses and responds with electrical impulses which travel along the auditory nerve to the brain.

This mid-modiolar section shows the coiling of the cochlear duct (1) the scala vestibuli (2) and scala tympani (3).The red arrow is from the oval window, the blue arrow points to the round window. Within the modiolus, the spiral ganglion (4) and auditory nerve fibres (5) are seen.



Cochlea

•The organ of Corti can be thought of as the body's microphone.•Perception of pitch and perception of loudness is connected with this organ.

• It is situated on the basilar membrane in the cochlea duct• It contains inner hair cells and outer hair cells.•There are some 16,000 -20,000 of the hair cells distributed along the basilar membrane.

•Vibrations of the oval window causes the cochlear fluid to vibrate.•This causes the Basilar membrane to vibrate thus producing a traveling wave.•This causes the bending of the hair cells which produces generator potentials•If large enough will stimulate the fibers of the auditory nerve to produce action potentials•The outer hair cells amplify vibrations of the basilar membrane


The cochlea works as a frequency analyzer It operates on the incoming sound’s frequencies

Place TheoryFrequency Theory


Frequency Theory

BM vibrates in synchrony with the sound entering the ear, producing action potentials-- in auditory nerve cells -- at the same frequency

(e.g., 50 Hz sound -> 50 APs/sec).

Limitations: max APs/sec = 200 Hz.

Use this theory for Frequencies <100Hz


Place Theory

• High frequency sounds selectively vibrate the BM of the inner ear near the oval window.

• =>Each position along the BM has a characteristic frequency at which it has maximum vibration.

• Lower frequencies travel further along the membrane before causing excitation of the membrane.

• The place along the basilar membrane where maximum excitation of the hair cells occurs determines the perception of pitch

32-35 mm long

4mm21mm2

At the base, the basilar membrane is stiff and thin (more responsive to high Hz)At the end or “apex”, the basilar membrane is wide and floppy (more responsive to low Hz)


Tuning curves of auditory nerve fibers

Response curve is a BPF with almost constant Q(=f0/BW)

Method to verify

•Apply 50ms tone bursts every 100ms

•Increase sound level until discharge rate increases by 1 spike

•Repeat for all frequencies

Tonotopic map on Cochlea: Cells in different spots on the cochlea respond to different frequencies, with high frequencies near the base, and low frequencies near the apex.


Auditory NeuronThe auditory nerve takes electrical impulses from the cochlea and the semicircular canals

Makes connections with both auditory areas of the brain.

Information from both ears goes to both sides of the brain - binaural information is present in all of the major relay stations.

----- Left ear information

___ Right ear information

Auditory Area of Brain


Auditory Neurons Adaptation

•When a stimulus is suddenly applied spike rate of an auditory neuron fiber increases rapidly

•If the stimulus remains (a steady tone for eg.) the rate decreases exponentially

•Spontaneous rate: neuron firings in the absence of stimulus.

Neuron is more responsive to changes than to steady inputs


Perception of Sound

Threshold of hearing– How it is measured– Age effects

Equal Loudness curves

Bass loss problem

Critical bands

Frequency Masking

Temporal Masking


Threshold of Hearing

Hearing area is the area between the Threshold in quiet and the threshold of pain.

Note:

Shift in threshold of quiet for those who listen to loud music

The sound intensity required to be heard is quite different for different frequencies.

Threshold of hearing at 1000 Hz is nominally taken to be 0 dB.

Marked discrimination against low frequencies so that about 60 dB is required to be heard at 30 Hz.

The maximum sensitivity at about 3500 to 4000 Hz is related to the resonance of the auditory canal.


Bekesy Tracking

Used to measure Threshold in quiet or JNL of a test tone

STEPS:

•Play a tone

•Vary its amplitude till its audible

•Then tone’s amplitude is reduced to definitely inaudible and the frequency is slowly changed

•Then increase the SPL till you can hear and so on.

Whole recording will last atleast 15minutesChange in level at fine steps <2dB else clicks become audible and act as a cue to listener


Threshold in Quiet variation with age

•Hearing sensitivity decreases with age especially at High frequencies

•Note we also loose the sensitivity at 3.5-4Khz

•Presbycusis: hearing loss because of age

•Hair cells which process HF are closest to the oval window and are often the first to be damaged.


Equal Loudness Curves Loudness is not simply sound intensity!

Subjective term describing the strength of the ear's perception of a sound.

Have to include the ear's sensitivity to the particular frequencies contained in the sound as in the equal loudness curves.

Sound must be increased in intensity by a factor of ten for the sound to be perceived as twice as loud.


The Bass Loss Problem

Eg. Rock music

Too lowno bass

Too hightoo much bass

For very soft sounds, near the threshold of hearing, the ear strongly discriminates against low frequencies.

For mid-range sounds around 60 phons, the discrimination is not so pronounced

For very loud sounds in the neighborhood of 120 phons, the hearing response is more nearly flat.


Ohms law of hearing

The sound quality of a complex tone depends ONLY on the amplitudes

and NOT relative phases of its harmonics.


Elephants

Sound Production

A a typical male elephant’s rumble is around an average minimum of 12 Hz, a female's rumble around 13 Hz and a calf's around 22 Hz.

Produce sounds ranging over more than 10 octaves, from 5 Hz to over 9,000 Hz Produce very gentle, soft sounds as well as extremely powerful sounds. (112dB

recorded a meter away)

Hearing

Wider tympanic membranes Longer ear canals (20 cm) Spacious middle ears.

Low frequency detection

Documents

Department of Electrical & Computer Engineering Meena Ramani 04/07/04