Audiology - Week 1 - Hearing & Measurement

PHYSICS
1. In order for sound to be heard, has to travel through a medium - the only thing that cannot be a medium is a vacuum
2. Air mollecules move together an apart to increase and decrease air pressure
3. Pressure wave is longitudinal - direction of vibration the same as wave
4. 1. VIbrating object (in a medium)
  1. 2. Surrounding air mollecules vibrate
    1. 3. Movement of air mollecules causes oscillations of increased and decreased pressure - ‘compression’ and ‘rarefaction’
5. SOUND MEASUREMENT
  1. AMPLITUDE - measured in decibels dB
    1. Represents the amount of change in pressure relative to normal atmospheric pressure (100 000Pa)
  2. FREQUENCY / wavelength - measured in Hertz (Hz)
    1. Number of cycles / second
    2. Distance between 2 corresponding points on 2 consecutive waveforms measured in metres
EARS - Peripheral Auditory System
1. OUTER: Pinna & Ear canal
  1. Pinna - cartilage, channels sound into external auditory meatus, aids sound localisation via bumps and cavities,
    1. The Concha (bit in middle of pinna) serves to amplify particular freq 5kHz, this is known as 'transfer function of the outer ear'
      1. External Auditory Meatus - 2.5cm long and diameter of 0.8cm, 1/3 cartilaginous 2/3 bony and narrows towards the ear drum, lined with hairs and glands which produce cerumen (wax),
        Canal is like an organ pipe). Sound which enters bounces off the walls. The natural resonance characteristics of the ear canal and the TM will boost the frequencies around 2.5kHz.
  2. ACTS as RESINATOR
2. MIDDLE: Eardrum (Tympanic Membrane) Middle ear bones / Ossicles
  1. Malleus, Incus and Stapes transfer sound energy from the TM to the oval window of the cochlea
    1. Protection mech: If there is a sudden loud sound, the stapedius muscle contracts and reduces compliance of the ossicles
  2. TRANSMIT SOUND PRESSURE ENERGY from external ear - 'impedance transformer'
  3. If the middle ear were not present then only 1% of the energy would be transmitted into the inner ear
3. INNER: Cochlea, Vestibular Organs, VII Cranial Nerve
  1. COCHLEA
    1. Size of a small pea and embedded deep in temporal bone
    2. Divided into three scalae (stairs): Scale vestibuli, Scale media, Scale tympani
    3. Consists of membranous and bony labyrinths.
    4. Hearing section called the cochlear duct.
      1. Scala Tympani is connected to the round window and is separated from the scala media via the Basilar membrane.
        Scala Media- self contained within the other two scala. Contains the organ of corti. This is an important structure in the transduction of neural impulses in the auditory nerve. This is basically where sound vibrations are turned into sound impulses to the brain.
        Scala Vestibuli communicates with stapes via oval window and is in the top portion of the cochlear duct. SV is separated form the scala media by the Ressiner’s membrane.
        ORGAN of CORTI: Contains inner and outer hair cells and their nerve endings, hair cells have stereocilia (hairs) protruding from top into tectorial membrane
        Inner hair cells are in charge of sending the signals to the brain.
        Outer hair cells make the ear more sensitive to quiet sounds by helping to amplify the sound vibrations.
        The stereocilia = bathed in endolymph - high in K+ ions (positive charge). Bases of hair cells = bathed in perilymph which is high in Na+ ions. The stereocilia themselves contain ion channels and within the hair cell membrane there are negative ions.
        The movement (shearing action) of hairs causes the ion channels open which causes K+ ions to enter and the HC to depolarise. The depolarization of the cell causes the release of a neurotransmitter (glutamate) into the synaptic cleft. The neurotransmitter in turn causes the depolarization of the neuron. Hence an action potential is made as this impulse is carried up to the brain.
        lamina provides the boundry between endolymph and perilymph and is at the bottom of the stereocilia.
      2. Movement of stapes/oval window  causes displacement of the perilymph within SV and ST Creating a wave which travels along the Basilar Membrane (BM) Each frequency of sound relates to a particular place along the BM at which it causes maximum level of vibration - tonotopic organisation/place principle
        High frequency sounds create max. vibration at the basal end Low frequency sounds create max. vibration at the apical end
4. NERVES
  1. Cochlea goes to the cochlear nucleus
    1. Splits into two streams going to:
      1. 1. Ventral cochlear nucleus
        2.Dorsal cochlear nucleus
        This stream analyzes the quality of sound, picking apart the tiny frequency differences which make "bet" sound different from "bat" and "debt".
        The ventral cochlear nucleus cells project to a collection of nuclei in the medulla called the olivary nucleus. There, minute differences in the timing and loudness of the sound in each ear are compared to localize sound.
MEASUREMENT
1. Hearing measured by lowest threshold - quietest sound that can be heard
  1. Standard definition threshold when 50% of sounds can be heard
2. Defined by comparison to hearing threshold level of young, healthy ears - measurements compared to national standards
3. Can be done by MAP-headphones
  1. MAF-anechoic chamber with a free field - sit person infront of speakers
  2. Determine thresholds for both of these and then take an average
4. PURE TONE AUDIOLMETRY
  1. Uses headphones
  2. Patient presses button when hears sound
  3. Tone presented 30 dB above presumed threshold, if client responds you drop by 10 dB
    1. No response, increase by 5 dB
  4. Threshold = when patient responds 2/3 times
  5. Frequencies defined = 250 Hz up to 8 kHz for both ears
  6. Produces AUDIOGRAM
5. TYMPANOMETRY
  1. Gives physical properties of the middle ear
  2. DOESN'T GIVE THRESHOLDS
  3. Tests how well TM and Ossicles are working - known as 'compliance' / how well they accept sound
  4. Only passive cooperation req from the patient
  5. PROBE w soft tip in ear of patient, measures over 2 seconds
    1. Contains: Source to produce probe tone - 220Hz
      1. PUMP to change pressure
        MIC to record sound
        If compliance is LOW (bad ear) lots of sound will be reflected back towards the microphone
        Pump alters air pressure - high and low pressure compliance is low as ear drum bends towards / away from middle ear in contrasting pressure to canal, when ear drum is flexible at equal (atmospheric) pressures compliance is high - HIGHEST AT THIS POINT if ear normal
        At low and high ear canal volume, line is flat:
  6. Produces TYMPANOGRAM
6. ACOUSTIC REFLEXES
  1. Normal ear will impede loud sounds by stiffening the stapedius muscle, if ear damaged this may not happen
  2. Causes for abnormal acousitic reflex / response can be: damaged cochlea, damaged acoustic nerve VIII, damaged facial nerve VII THIS ACTIVATES STAPEDIUS, conductive hearing loss
  3. As sound gets louder, muscle contracts, shown by a drop on the verticle axis
  4. Both ears measured as if you play loud sound in one ear, muscles in the other ear should also contract

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Audiology - Week 1 - Hearing & Measurement

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