Basic Aud Chapter 2: Sound Acoustics

Sound

• A physical phenomenon which a movement or vibration of an elastic medium without permanent displacement of particles

• Three prerequisites:

  - A source of energy

  - A vibrating object which generates an audible pressure wave

  - A medium of transmission

Characteristics of sound

• Mass: quantity of matter present

• Force: a push or pull on an object

• Inertia: tendency to resist any change in motion

• Newtons first law: outside force must be applied to overcome inertia, the greater an objects mass, the greater the inertia

• Elasticity: tendency of an objects resistance to deformity and its return to the rest position

• As air molecules vibrate, waves of pressure fluctuations are created and travel through an elastic medium like air

• Forced displacement of air particles creates areas of condensation (compression) and areas of decreased air pressure and density (rarefaction)

Properties of simple harmonic motion (SHM)

• 4 parameters

  • Frequency- number of complete vibratory cycles per unit time

  • Amplitude- A derived unit of measurement describing an objects distance from rest to maximal displacement

  • Period- amount of time needed to complete one cycle of vibration: inverse of frequency

  • Phase- the relative timing of compressions and rarefactions of waves

Frequency is measured in Hz (hertz) which can be found in audiometric testing

We can hear 20Hz-20,000Hz

There are few speech sounds below 125Hz and past 8000Hz: that is why it is not usually tested

Sound measurement

• Intensity vs loudness: relates to amplitude or power of sound

• What is a decibel (dB): unit of measurement of the intensity of hearing

• Which decibel should I use?:

  • Sound pressure level (dB SPL)

  • Hearing level (dB HL)

  • Sensation level (dB SL)

Measure of relative intensity

• Express a ratio of measured sound intensity to a relative sound intensity

• Logarithmic scale versus linear scale

  • Linear scale: equal increments

  • Logarithmic scale: used with really big/small numbers

Anatomy And Physiology

3 parts of the human ear

Outer ear———> Acoustic

Middle ear———> Mechanical energy

Inner ear———> Fluid energy

(If there is a problem with any of the ear pathways, it will be difficult to get sound to the brain)

Outer ear anat and phys.

• Auricle aka pinna

• External auditory meatus aka ear canal

• Tympanic membrane aka ear drum

Auricle (pinna) anatomy:

• Triangular fossa, concha, tragus, intertragual notch, lobe, antitragus, antihelix, helix

• Collects and resonates sound

• Assists in sound localization

• Provides primary cue for determination of sound source elevation

External auditory meatus (ear canal) anatomy

• Makes wax

• Protective mechanism for middle and inner ear

• Protects from foreign objects

• Boosts/ amplifies high frequency sounds

Tympanic membrane anatomy

• Pars tensa, handle of the malleus, umbo, light reflex, annulus, anterior fold, short process of the malleus, pars flaccida, posterior fold, junction of incus and stapes

Middle ear anat and phys: Begins at the inner layers of the tympanic membrane

• Filled with air

• Ossicles: 3 ear bones (smallest in the human body)

• Malleus: aka hammer; lateral most bone, connected to the ear drum

• Incus: aka anvil; center bone of the chain

• Stapes: aka stirrup; medial most bone, rocks in and of the oval window

Eustachian (auditory) tube:

• Equalizes pressure between middle ear cavity and nasopharyx

• Helps drain fluid which might accumulate in the ME into nasopharynx

• Ligaments suspend ossicular chain loosely in middle space, allowing for movement

• Amplifies signal frequencies between 100 Hz and 2500 Hz

• Stapedius and tensor tympani

Inner ear anat and phys:

• Vestibular and auditory labyrinths

• Pathways in petrous portion of the temporal bone

• Osseous labyrinth: channel in the bone

• Membranous labyrinth: soft tissue fluid filled channels within osseous labyrinth contacting end organs of hearing and balance

• Auditory labyrinth: Cochlea; sensory end organ of hearing

• Vestibular labyrinth: semicircular canals; sensory end organ of balance

• Cochlea: fluid filled space within temporal bone, shaped like a snail with 2 5/8 turns, has three chambers

  • Scala vestibuli; upper portion filled with perilymph fluid

  • Scala media: middle portion filled with endolymph fluid

  • Scala tympani: lower portion filled with perilymph fluid

  • NEVER want these fluids to mix. Can make you very sick!

• Basilar membrane: on floor of scala media, separates it from scala tympani

• Organ of Corti: Sits on the basilar membrane and contains sensory cells of hearing

  • Outer hair cells tuned to sound intensity

  • Three rows of cilia embedded onto tectorial membrane

  • about 13,000 OHC altogether

•Inner hair cells tuned to sound clarity

- One row of cilia in proximity of tectorial membrane

• about 3,500 IHC altogether

membrane
– about 13,000 OHC altogether
• Inner hair cells tuned to sound clarity
– One row of cilia in proximity of tectorial
membrane
– about 3,500 IHC