L1: Basic aspects of sound

What is sound

• small fluctuations in air pressure

As any vibration can be hear as sound, what must the ear do?

• resolved complex vibrations into the sum of individual sinusoidal vibrations

Frequency

Pitch

• number of cycles per second

• 1/period

Ampltitude

• pressure variations around the mean

• LOUDNESS

Phase

Relative position in the cycles

• cannot really detect

Wavelength

• important for the localisation of sound

• lambda = c/f

What is the speed of sound

340 m/s in air

Period

time duration for one cycle

Different sounds usually involve what

• sets of multiple frequencies

• at different ampltitudes

Why is sound intensityexpressed on logarithmic scale

• we can hear immense range of intensities

Unit of power of sound

Decibel

What are decibels

Ratios

• strength is indicated relative to reference value

What is the reference value for huamsn

20 uPa

• this is close to the threshold of human hearing at 2kHz

How much is 1Pascal in N/m²

1 N/m²

All signal levels expressed relative to this 20uPa standared are known as what

Decibels sound pressure level

• dB SPL

Equation for dB SPL

dB SPL = 20 Log10(Pressure/20uPa)

• 0 dB: sound has the same pressure as reference signal

• Cannot have level of 0 sound (log 0 infinite)

• Negative values of dB are LESS than the reference

Pa and SPL of normal conversation

1. Pa: 2 ×10^-2

2. SPL: 60

Threshold of damage to hair cells

• 120SPL

• 20Pa

What is the human frequency range

20 - 20k Hz

• depends on the animal

Filtering

• Low pass: cut off higher freq

• Band pass: specific freq range

• High pass: cut off low freq

• Band stop: only passes freq outside restricted bandwidth

What is the freq range for speech

300 - 3,400 Hz

What is human speech loudness

• 55-65 dB

What is sound spectrum

• graph representing a sound based on its freq compoistion

• acts as a recipe that dhows amount of vibration at each indivudal freq

What is the wavelength of 200Hz tone

wavelength = 340/ f

340/200 = 1.72 metres

Three parts of the ear

1. Outer ear

   1. Pinna

   2. External auditory meatus (ear canal)

2. Middle ear

   1. Tympanic membrane

   2. ossicles

   3. middle ear cavity

3. Inner ear

   1. cochlea (with vestibular apparatus)

1. External meatus (EAM) ear canal (what is it)

1. open ended tube

2. has resonant peaks that are predictable from knowing its length

EVIDENCE: measured gain matches calculated gain

• peaked at around 3-5 kHz

• coincides with human speech frequency range

• HOW MEASURED: sound pressure measured at the entrance (input) and end (tympanic membrane: output)

Length of ear canal

2-3 cm

What does the ear canal do

1. Conducts sound waves from outer eat to eardrum

   • passage for sound to each tympanic membrane (ear drum)

2. amplifies speech freq sounds

3. protects inner ear from debris, foriegn objects and bacteria

4. akes ear wax

1. Pinna what is it

• visible cartilaginous outer part of ear

What does Pinna do

• assists sound localisation

• modifying the spectra in space-dependent manner

  • can detect the azimuthal location

HOW:

1. sound to ear

2. sound input enter the ear canal directly AND reflecting at different parts of pinna

3. depends on their elevation

4. the elevation modified the spectral pattern

When the sound is at 90 degrees what does the sound spectrum look like?

• directly in line with the left pinna

What happens to the spectral pattern at different locations?

• the input wave has a wavelength that is equal or shorter than that of the head

• RESULT: different spectra seen at the end of the ear canal

• this is the Head related transfer function:

  • how sound is altered by the head and pinnae BEFORE entering the ear canal on to the tympanic membrane

How does the Head related transfer function change with azimuth

The functions are not flat

• just characterised the FIRST spectral dip (NOTCH)

  • this varies with the rotation

  • and particularly in the vertical plane

• Notches are over 5 Hz

Evidence that there is plasticity in sound localisation

• EXP: molds fitted to external ear

  • disrupt the pinna shape

• RESULT

  • at first: lost localisation

  • With time: regained it back

  • If removed the mold: still had localisation preserved from before (did not deteriorate)

• THERFORE: plasticity:

  • neural growth

  • novel synaptic connections

What does the thirst result suggest?

• multiple representations of auditory space can co-exist

How does the ear canal change the sound inputs?

• natural resonator

• ampltifying sound waves between 2-5 kHz

• funneling the sound waves in

How does pinna change the sound inputs?

• reflect sounds

• creates HRTF

  • different notches to signify localisation

How does the outer ear contribute to hearing?

1. collect sound wave

2. channel them

3. amplify specific frequencies before reach eardrum

4. Protection

5. sound localisation

What are the functions of the Middle ear

1. Impedance matching

   • so the sound from the air does not get absorbed by the high resistance in the cochlear fluids

2. Protection from loud sounds

   • including own vocalisations

3. Anti-masking of high-frequency sounds by low-frequneecy sounds

   • particulalry at high sound levels

what are the oscicles

Three ear bones

1. Malleus: hammer

2. Incus: anvil

3. Stapes: stirrup

How do the ossicles work to transmit the sound from the canala to the fluid filled cochlea

1. Sound in air

2. vibrates the tympanic membrane (ear drum) (60mm²)

3. Causes Malleus and Incus lever action

4. Connects to the stapes which is connected to the oval window (3.2mm²)

5. Pascles principle P1A1= P2A2 means that pressure to a smaller area will INCREASE and thus AMPLIFY the vibration/ sound when it gets to the fluid

6. Therefore the sound is not just absorbed once it reaches the high resistance fluid in the chochlea

What do the middle ear muscles do? (MEM)

PROTECTION

• dampen the vibrations of the ossicles

• reduce the acoustic signal that reaches the ears

• Via the MEM reflex

they are the smallest skeletal muscle in the human body

When do middle ear muscles contract

1. 100ms after exposure to loud sound

2. before a person vocalises

Why are they absent in frogs?

• they do not vocalise

What frequencies do they attenuate more?

Low

What are the two middle ear muscles and where are they connected

1. Tensor tympani

   • connects the neck of the malleus

2. Stapedius

   • connected to the neck of the stapes

Which muscle is active when exposed to loud sounds in humans

• Stapedius

What level of sound activates the MEM reflex

• sounds 80-90 dB above a person’s hearing threshold

What happens in the MEM acoustic reflex circuit (i.e how does sound cause the stapedius to contract)

Ipsilateral side: Where sound is heard

1. Sound heard at cochlea

2. Cochlea sends signal to the central cochlear nucleus

3. goes to the superior olivary complex ON BOTH SIDES

4. projects to the facial nerve nuclei on both sides

5. Effect output from the facial nuclei

6. enables contract of stapedius muscles IN BOTH EARS

7. THEREFORE: DAMPENS the vibrations of the osccislces to PROTECT from loud sounds

OVERALL: contracts the stapieus (and sometimes the tensor tympani) at high intensity sounds

• stiffens the ossicular chain

• reduces sound transmission

• THEREFORE: proteective

Evidence 1: Bird stapeisus put under tension

RESULT: decrease attenuation to sound with increased stapieus contraction

• SUGGESTS: birds reduce intensity of sound produced when the bird cries/calls

Evidence 2: Effect when stapidus is absent

• Intact stimulus→ hearing lat lower dB levels

  • Keeps the amplitute of soundheard at relatively constant level

• Inactive stapedius due to hearing loss→ hearing at higher levels

What happens in conductive hearing loss

1. middle ear cavity fills with fluid

2. low frequnecy hearing loss of 30dB or greater develops

What is sensorineural hearing loss

1. cochlea or autitory pathway is damaged

How to distinguish between the conductive and sensorinueral hearing loss

Rinne test

1. place a vibrating tuning fork on the mastoid process

   • Patient indicates when they can no longer hear sound

     • Indicates Bone conduction of sound

       • (transmission of sound to the cochlea via the skull)

2. Fork at the entrance of the external auditory meatus (EAM)

   • Tests for air conduction

RESULT:

• Conductive:

  1. bone conduction is unimpaired

  2. response to sound conducted by the EAM is reduced

     • Bone > Air

• Sensorineural (or normal):

  1. Bone conduction impaired

  2. Greater sensitivity to air conduction at the EAM

     • Air > Bone

Inner ear: What is the cochlea + structure

• Snail-shaped fluid-filled cavity

  • 3 Chambers + 2 Partitions

    1. Scala tympani

    2. Scala vestibuli

       • (not always partitioned from one another, can meet at the end at the helicotrema)

    3. Scala media

       • partition that runs longitudinally

       • between the two scalae

       • contains the organ of corti

       • NEXT TO THIS: Stria vascularis

         • makes the K+ high endolymph fluid

• converts sound vibrations into neural impulses: Transduction

• When unravelled:

  • straight tube

  • compartmentalised longitudinally

• Contains spiral ganglion (auditory nerve cell bodies)

• Connects to VIII cranial nerve fibres (Auditory nerve)

• Modiolus

  • porous bony central axis

  • around which canal makes 2.75 turns

Why need the helicotrema

• allow fluid to pass between the chambers

• pressure equalisation

• detecting low-frequncing sounds

What is the principle function of the cochlea

• Decompose the acoustic signal into component frequencies

• sight of sound transduction

What is the organ of corti

• receptor structure for hearing

Structure of the organ of corti

1. Basilar membrane at the bottom (33mm)

2. Inner hair cells

3. Outer hair cells

4. Tectorial membrane

5. Sulcus

6. Connecting auditory nerve

How does the width of the basilar membrane change from base to apex

1. Base: where sound comes in thinnest AND stiffer

2. Apex: widest

How is sound made into its different components

1. Stapes vibrates oval window

2. incompressible fluids in cochlea vibrate

3. cause round window to bulge outwards

4. At the base: stiffer and narrow→ only high freq sounds vibrate here

5. At the apex: less stiff and wider→ only LOW freq sounds vibrate here

6. Therefore components of the same sound are separated out be frequency

7. Mapped tonotopically along its length

8. note: the distance corresponds to equal increments in logarithmic frequency: NON-LINEAR MAPPING