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Medical Physics > Acoustics > Flashcards

Acoustics Flashcards

1
Q

Acoustics

A
  • branch of physics which studies mechanical waves, including vibrations, sound infrasound and ultrasound
  • Acoustics observes generation, propagation of mechanical waves, their interaction with environment and the process of hearing.
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2
Q

What is a mechanical wave?

A
  • spreading disturbance (forced displacement of particles) in elastic medium
  • MW transfers energy but not mass.
  • The MW may take a form of an elastic deformation in solids or a variation of pressure in gases
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3
Q

What is a sound?

A
  • Sound is called any MW/mechanical vibrations producing hearing perception
  • transmitted only through elastic media
  • sound transfers energy but not mass
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4
Q

What is the acoustic range?

A

freq. from 20 Hz to 20kHz

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5
Q

Where can sound propagation occur

A

in elastic media only

  • vibrations in elastic matter are transmitted consecutively from particle to particle causing recurrent alteration of density

P(t) = ΔP sin φ

ΔP = maximal pressure increase/decrease

φ - phase of the given mechanical wave showing the rate of alterations

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6
Q

How do mechanical waves propagate?

A

Longitudinal and Transversal

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7
Q

Longitudinal

A

typical to gasses and liquids

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8
Q

Transverse waves

A

typical for solids where particles displacement occurs perpendicularly to wave propagation

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9
Q

Physical characteristics of sound: Sound Intensity, I

A

I = E/t.S

  • the sound energy transmitted per sec. through unit area, placed perpendicularly to the sound propagation
  • Intensity is measured in watt per square meter. [W/m2]
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10
Q

Physical characteristics of sound: Sound pressure, P

A

P = ΔP sin φ added to Patm

  • the relation between intensity and pressure:

I = p2/2Za

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11
Q

Physical characteristics of sound: Sound frequency, f

A
  • Frequency is the number of vibrations (full alterations of the sound pressure or repetitions of motion of the particles) per sec
  • It is measured in Hz
  • 1 Hz equals to 1 vibration per sec
  • A quantity which is directly related with f is the period: the time duration of 1 vibration
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12
Q

Physical characteristics of sound: sound velocity

A
  • quantity which depends on the properties of the medium through which sound propagates
  • For example – At 20ºC SV in the air is about 340 m/s, but in water SV is about 1500 m/s
  • The exact relationship between sound speed and matter properties threats elasticity as major factor.
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13
Q

Physical characteristics of sound: sound wavelength

A

The distance between two consecutive sound fronts\

λ = v.T=v/f

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14
Q

Physical characteristics of sound: Acoustic impedance

A

Z = ρ.v

ρ -density of the given medium
v – sound velocity

  • Sound propagation through certain medium is determined from acoustic impedance Z of the latter
  • When the sound wave encounters a border between two different media – a part of the wave reflects, and the other part passes due to differences in acoustic impedances Z1 and Z2
  • The degree of reflection/transmission depends on difference sbetween Z1 and Z2
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15
Q

Simple tone and complex tone sounds

A

Simple - if vibration has sine shape and possesses certain frequency

Complex - when it consist of several simple tones as superposition of mechanical waves.

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16
Q

Acoustic Spectrum

A
  • Combination of certain amplitudes and frequencies proportional to participant simple waves frequencies
  • Each complex sound can be illustrated by graph representing its acoustic spectrum
  • f0 - basic freq. - always has maximal amplitude

2f0, 3f0 etc. - amplitudes smaller

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17
Q

Psychophysical characteristics of sound

A
  • Each physical (objective) characteristic of sound corresponds to respective psychophysical (subjective) analogue
  • Human perception of sound is based on fundamental physiological Weber - Fechner law:

Perception ~ log(stimulus)

(Perceptions increases logarithmically with the stimulus)

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18
Q

Psychophysical characteristics of sound: Sound intensity

A
  • Sound Intensity level E (E is defined at 1000 Hz sound frequency only)

E=k. lg( I/I0 ),
I = intensity of sound

I0 = threshold (at 1000Hz, 10-12 W/m2 - lowest in power vibration audible for human ears)

Intensity level is measured in bel B/ decibel dB (at k=1 and k=10 respectively)

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19
Q

Psychophysical characteristics of sound: Loudness of sound, L*

A
  • quantity representing subjective human perception of the sound magnitude

L=k.lg(I/I0)

k = coefficient depending on frequency

I = intensity of the given sound

I0 = threshold of audibility of this sound

L is measured in Phon.

figure bellow - lines represent equal-loudness contours

  • Each point of these curves corresponds to certain frequency and sound level in dB making the relation loudness- intensity level accessible.
20
Q

Why is it that doubling the sound intensity to the ear does not produce a dramatic increase in loudness?

A
  • there are saturation effects
  • Nerve cells have maximum rates at which they can fire, and it appears that doubling the sound energy to the sensitive inner ear does not double the strength of the nerve signal to the brain

This is just a model, but it seems to correlate with the general observations which suggest that something like ten times the intensity is required to double the signal from the innner ear.

  • One difficulty with this “rule of thumb” for loudness is that it is applicable only to adding loudness for identical sounds
  • If a second sound is widely enough separated in frequency to be outside the critical band of the first, then this rule does not apply at all.
  • While not a precise rule even for the increase of the same sound, the rule has considerable utility along with the just noticeable difference in sound intensity when judging the significance of changes in sound level.
21
Q

Psychophysical characteristics of sound: audibility area

A
  • On diagram bellow the closed area between threshold of audibility and threshold of pain/feeling represents audibility area
  • All the sounds we can perceive are inside this area
  • Our ears are most sensitive to sounds from 1000 – 4000 Hz range. Human speech is spread over 400 – 1000 Hz frequency range.
22
Q

Psychophysical characteristics of sound: pitch

A
  • subjective characteristic, corresponding to sound frequency
  • The higher frequency – the higher pitch.
  • The pitch of human voice is physiological feature. It is determined by the vibrating characteristics of vocal cords (their basic resonant frequency)
  • For example: male basic frequency is in range 80-100 Hz and men’s voice sounds with lower pitch, whereas females basic frequency of vocal cords is about 400-500 Hz and this is the reason for the higher pitch of female voice.
23
Q

Psychophysical characteristics of sound: timbre

A
  • characterizes complex sounds
  • Timbre allows different sound sources to be distinguished by their acoustic spectra.
24
Q

Sound as a diagnostic instrument: Auscultation

A
  • The basic tool for auscultation is stethoscope.
  • Mechanical stethoscopes amplify sound due to Standing wave phenomenon.
  • There are optimal recording sites (sites to place chest-piece of stethoscope) for the various heart sounds
  • Sound conductivity of a stethoscope is characterized by some features:
  • firm application of the chest piece makes the diaphragm taut with pressure thereby causing an attenuation of low frequencies

loose-fitting earpiece cause leakage which reduces the coupling between the chest wall and the ear.

25
Q

Sound as a diagnostic instrument: Phonocardiography

A
  • Heart sounds and murmurs have extremely small amplitudes with frequencies from 0.1 to 2000 Hz
  • Thus the recording device must be carefully selected for wide band frequency response characteristics
  • Specially designed acoustically quiet environment is needed for noise free recording of heart sounds
  • The device used for recording of these tones and their transformation into audible sounds is called phonocardiograph
26
Q

What does phonocardiography allow us to do?

A
  • The method allows to eliminate subjective interpretation of the heart sounds
  • It enables evaluation of the heart sounds and murmurs regarding electric and mechanical events in the cardiac cycle
  • Evaluation of the result is based on the basis of changes in the wave shape and various timing parameters
27
Q

Sound as a diagnostic instrument: Audiometry

A
  • method for experimental examination/determination of the personal threshold of audibility
  • It is carried out by means of generator of simple tones with a certain frequencies (125, 250, 500, 1000, 2000, 4000, 8000 Hz )
  • Audiometry is used to determine the relation between objective sound Intensity and subjective Loudness.
28
Q

Sound as a diagnostic instrument: Extracorporeal Lithiotripsy

A
  • EL is based on the concept of remote destruction of kidney and bladder stones by sound wave
  • When a mechanical wave (of type shock wave) passes trough the stone, it starts to vibrate
  • A resonance occurs and the amplitude of vibration reaches critical value at which the structure stability is impaired
  • The stone has been destroyed to small pieces

The latter is due to rapid increase of pressure inside the stone and impairing of its structure stability.

29
Q

Infrasound

A
  • vibrations in elastic media with frequency from 0 to 20 Hz
  • Humans do not hear these vibrations
  • Natural sources of IS are: earthquakes, volcanoes, sea waves (tsunami), typhoons, waterfalls
  • Some animals use IS to communicate – elephants, hippopotamus.
  • Artificial sources: some machines, transport systems, factories, large air conditioners
  • IS is absorbed weakly due to its long wavelength e.g. if two waves with frequencies of 1000 Hz and 10 Hz respectively propagate through the same medium – the latter will be absorbed ten times less! Therefore IS penetrates deeper than audible sound.
30
Q

Effect of IS

A
  • Because of its low frequency IS causes vibrations of large objects (machines, buildings, ships ) and also can produce resonance vibrations in human body – internal organs and body cavities have characteristic resonant frequencies:

Heart – 1-1.5 Hz

Circulation of the blood flow across the body – 1.2 Hz

Vestibular apparatus – 0.3-13 Hz

Most of Internal organs – 2-8 Hz

Head – up to 20 Hz

31
Q

Exposure to IS effect

A
  • Exposure to infrasound has been demonstrated to effect recipients with symptoms including fear, sorrow, depression, anxiety, nausea, chest pressure and hallucination
  • It can cause objects to move through vibration and some believe the body’s internal organs can be effected
  • It is suggested that levels above 80 decibels at frequencies between 0.5 to 10Hz may start to effect the vestibular of the inner ear thus causing disorientation
  • Any high volume sound can trigger the body to react by increasing respiration, heart rate and blood pressure, but when they cannot actually hear the sound recipients are left with no explanation for the sudden onset of these symptoms
  • This may then lead to further effects caused by the minds possible reaction to the unknown, as outlined below.
32
Q

Ocean waves IS

A

Ocean waves are known to sometimes generate infrasound and it has been suggested to have been a possible “trigger” causing ships crews to abandon their craft in fear, only to have the ship later found mysteriously drifting about unmanned.

33
Q

IS Hallucinations

A

Tests by NASA have revealed that the human eyeball resonates at around 18Hz, to which infrasound exposure may cause a reaction and lead to hallucinations.

34
Q

Subsonic sound (IS)

A

Subsonic sound can travel long distances, pass through walls and may be amplified in tunnel like structures. Standard hearing protection is of little use for subsonic sound as it often can pass straight through and may even be amplified.

35
Q

Diffusion

A
  • Diffusion is the net movement of molecules or atoms from a region of high concentration (or high chemical potential) to a region of low concentration (or low chemical potential)
  • Quantitatively the mass of transferred substance can be evaluated using Fick’s laws of diffusion:
J = - D dc/dx
J = mass flux – the mass flowing per second through unit area (M/ S.t)

D = diffusion coefficient

dc/dx = concentration gradient

or: M= -D.dc/dx .S.t

36
Q

Gradient

A
  • Gradient is vector quantity representing the rate of change of given scalar quantity over the distance
  • Its value equals to the difference in quantity’s values in two points/regions and direction – from low toward high (denoted in the formula by “-“).
  • Particularly in diffusion, the concentration gradient dc/dx represents difference in concentration over the distance.
  • The substance flows against the gradient and down the gradient/osmotic forces
  • Diffusion is a spontaneous process (no input of energy is required)
37
Q

Diffusion applications in medicine

A
  • Diffusion across membrane (transfusion) is physical basis of hemodialysis
  • Hemodialysis is called the process of (out of body) purification of blood from toxins by controlled transfusion
  • For this purpose a high concentration gradient is used
  • This therapy is applied in cases of kidney deficiency
  • Toxin molecules diffuse through the membrane separating two counter-fluxes - blood from a special solution named dialysis fluid.
  • The permanent flow of dialysis fluid provides a high concentration gradient/difference
38
Q

Internal friction

A
  • Impulse – physical quantity equal to the product of velocity and mass of a moving object:

p = m.v

When neighboring parts of a fluid move with different velocities v1 and v2, transfer of impulse is occurred between two layers

  • The faster layer “drags” the slower one transferring impulse to the latter
  • As in the same time the faster one experiences decelerating action of the slower layer
39
Q

Heat Conduction

A
  • Another type of physical transfer appears when thermal non- homogeneities exist in given matter medium
  • Heat is transferred when there is difference in the temperature of a two contacting objects or two parts of one medium
  • gradient is presented as difference Ta > Tb .
40
Q

Medical applications of heat transfer and thermal conduction

A
  • The body temperature can be controlled by heat transfer decrease or increase of the temperature depending on the physiological status
41
Q

Thermal therapy

A
  • warming pads and compresses, baths with warm liquids, paraffin.
  • A heating pad is a pad used for warming of parts of the body in order to manage pain
  • Localized application of heat causes the blood vessels in that area to dilate, enhancing perfusion to the targeted tissue.
  • Many episodes of pain come from muscle exertion or strain, which creates tension in the muscles and soft tissues.
  • This tension can constrict circulation, sending pain signals to the brain.
42
Q

How does heat application ease pain?

A
  • dilating the blood vessels surrounding the painful area
  • Increased blood flow provides additional oxygen and nutrients to help heal the damaged muscle tissue.
  • stimulating sensation in the skin and therefore decreasing the pain signals being transmitted to the brain
  • increasing the flexibility (and decreasing painful stiffness) of soft tissues surrounding the injured area, including muscles and connective tissue.
43
Q

Hypothermia

A
  • Reducing cerebral metabolism (approximately 6-8% per 1oC)
  • Reducing excitatory amino acids (glutamate release)
  • Attenuation and/or reversibility of ischemic depolarization of the CNS, leading to membrane stabilization, electrolyte redistribution, and normalization of intracellular water concentration and intracellular pH (stabilization of the blood-brain barrier)
  • Attenuation of oxygen free radical production and lipid peroxidation
  • Restoration of normal intracellular signaling mechanisms (including calcium modulation) and inhibition of deleterious signaling mechanisms, such as apoptotic signaling
  • Restoration of protein synthesis and gene expression
  • Inhibition of deleterious inflammatory products (ie, cytokines, interleukins, arachidonic acid cascade end products)
  • Attenuation of CSF platelet-activating factor (PAF)
  • Inhibition of cytoskeletal breakdown
  • In the heart, hypothermia may decrease the area of injury, promote epicardial reflow, decrease myocardial metabolic demand, and preserve intracellular high-energy phosphate stores.
44
Q

Cyrosurgery

A
  • Application of extreme cold to destroy or impair diseased tissues
  • Cryosurgery is a minimally invasive procedure
  • Tissue cooling is done by liquid nitrogen and more rarely – carbon dioxide
  • Modern cryosurgery uses argon gas sprayed from ultra-thin needles
  • Cryosurgery works by taking advantage of the destructive force of freezing temperatures on cells
  • Once the temperature falls below -40°C, ice crystals may form within the cells - Once it occurs, cell death is almost certain.
45
Q

Progressive failure of microcirculation in cyrosurgery

A
  • due to a cascade of events:
  • endothelial layer destruction causing vessel walls to become porous, interstitial edema, platelet aggregation, microthrombi, and ultimately vascular congestion and obliteration
  • It was theorized that during cryosurgery, the immune system of the host became sensitized to the tumor being destroyed by the cryosurgery
  • Any primary tumor tissue undamaged by the cryosurgery and the metastases were destroyed by the immune system after cryosurgery. This response was termed the “Cryo- Immunological response”.
46
Q

Transplantation (cyrosurgery)

A

Transplantation of organs requires low temperature for conservation. Cold suppresses enzyme activity of tissues and provides their intactness.

Medical Physics (10 decks)

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