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Medical Physics > Acoustics > Flashcards

Acoustics Flashcards

1
Q

Acoustics

A
  • studies the mechanical oscillations and waves
  • their propogation in various media
  • physiological; studies the work of the organs for sound detection and production in humans
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2
Q

Oscillations

Periodic oscillations

A
  • repeated movement or change of state
  • the movement/state is repeated at regular time intervals
  • period T- the time after which the oscillation is repeated
  • Frequency v- the no. of oscillations per unit of time
    unit: hertz, Hz
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3
Q

Waves

A
  • The propagation of the oscillation in space

- the oscillating particles interacts with its neighbours, make them oscillate

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4
Q

Types of waves according to their nature

A
  • Mechanical waves- oscillations of material particles
    • propagate in material media only
    • eg. sound waves, water waves
  • Electromagnetic waves- periodic oscillations of an electromagnetic field
    • propagate in material media and free space
    • e.g. light, x rays, radio waves
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5
Q

Types of waves according to the oscillation direction

A

-Longitudinal waves- the oscillations are parallel to the propagation direction of the wave
> e.g. sound waves

-Transverse waves- The oscillations are perpendicular to the propagation direction of the wave
> electromagnetic waves

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6
Q

Main physical characteristics of waves

A 
> Period of the wave T  
     - unit; second, s
> Frequency of the wave v
     - unit; hertz, Hz
> Propagation velocity v
     - unit;  m/s
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7
Q

Basic physical characteristics of waves

A

> Wavelength λ- distance travelled by the wave for one period λ=vT
- measurement unit: metre, m

> Intensity I- energy fluence rate. i.e. the energy transferred by the wave through unit are.
-unit: W/m^2

I= dE/ dSdt

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8
Q

The nature of sound

A

> Sound- a mechanical wave with frequency from 20Hz to 20000 Hz.
Mechanical waves- ultrasound frequency above 20kHz
-infrasound- frequency below 20 Hz
Longitudinal wave propagation for liquids and gases
logintudinal/transverse wave propagation for solids.

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9
Q

Velocity of sound

A

In gases v (wierd alpha sign) √ T

  • T is absolute gas temperature
  • in air v= 330 m/s

In liquids v= √ k/p

  • K is the bulk modulus of the liquid
  • p is the liquid density
  • in water: v=1,500 m/s

In metals - v=4000 m/s
-longitudinal wave: E is the modulus of elasticity of the solid body V=√ E/p

Transverse wave

  • G is the shear modulus of the body
  • v= √ G/p
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10
Q

Sound pressure

A

Longitudinal sound wave induces local compression and rarefaction of the particles of the particles of the medium
the amplitude p0 of sound pressure and intensity are interrelated I=p0^2 / 2pv

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11
Q

Acoustic impedance

A

Za= pv

> reflection and refraction of the sound wave at the boundary between two different media depend on the acoustic impedances of the media.

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12
Q

Acoustic spectrum

A

> the information about the frequencies and intensities of all harmonic oscillations, comprising the sound
wave

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13
Q

Types of sound

look at the diagrams on the book

A

> Tone- the acoustic spectrum consists of a small number of frequencies:

  • Pure tone- one frequency
  • compound tone- several frequencies
  • Fundamental tone- with the lowest frequency
  • overtones- all other components of the acoustic spectrum

> Noise- has a continuous, complex, randomly varying acoustic spectrum
Sound stroke= loud sound of short duration with a continuous acoustic spectrum

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14
Q

subjective characteristics of the human auditory perceptions

A

> Subjective because

  • specific characteristics of the human ear
  • individual variation of these characteristics
  • Pitch, Timbre, Loudness
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15
Q

Pitch

A

> depends on it’s fundamental frequency

  • higher frequency sounds have higher pitch
  • sensation of pitch increases= arithmetic progression
  • frequency of the sound increases = geometric
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16
Q

Timbre

A

> Changes of the overtones of a compound tone are sensed as variations in sound timbre
-human ear, timbre difference between two tones of 10% intensity in their overtones

17
Q

Loudness

A

> level of the sound sensation above its threshold]

-Loudness L depends on sound intensity I,and fundamental frequency

18
Q

Hearing threshold

A

the sound intensity for a given frequency, resulting in minimal sound sensation
- I0=10^-12w/m^2

19
Q

Spectral dependence of the hearing threshold

A

> The dependence of the hearing threshold on sound frequency

20
Q

Pain threshold

A

> The sound intensity for a given frequency, which results in a sensation of pain
- I1 = 10 W/m^2

21
Q

Hearing area

A

> Range limited by the hearing and pain thresholds for all audible frequencies

22
Q

Measurements of sound

A

> Based on the logarithmic dependence of loudness on sound intensity

23
Q

Sound intensity level measurement scale

A

> measured on a logarithmic scale

-zero sound intensity is at I0= 10^-12 w/m^2

24
Q

Loudness measurement scale

A

divides the interval between the hearing and the pain thresholds for each frequency into 130 equal pares to define the Phon.
1phon= 1 dB
1 dB = 0.1 B

25
Q

Loudness measurement scale

A

divides the interval between the hearing and the pain thresholds for each frequency into 130 equal pares to define the Phon.
1phon= 1 dB
1 dB = 0.1 B

26
Q

Sound loudness and the area of speech

A

whisper = 20-40 phon
normal speech = 45-75 phon
loudness range = 45-75 phon
frequency range = 200-8000 Hz

27
Q

Audiometry

A

> investigation of the sharpness of hearing
-Audio gram- a graphical representation showing the difference between the hearing and the normal threshold value versus frequency

-Audiometer- a specialized sound generator with adjustable sound frequency and intensity

28
Q

Ultrasound

A

> a mechanical wave with frequency above 20,000 hz

  • main properties
    • exhibits reflection, refraction, absorption
  • at high intensities can destroy viruses, bacteria
29
Q

Detectors of ultrasound

A

> Piezoelectric material- production of electric voltage when the stress is applies to a plate, made of that material

30
Q

Generators of ultrasound

A

> Natural sources, dolphins, bats
Artificial
- Converse piezoelectric effect- the production of stress and or strain when an electric field is applied

-Magnetostriction effect- change of shape of ferromagnetic materials when subjected to a magnetic field.

31
Q

Applications of ultrasound

A

> Diagnostics, therapy, surgery

32
Q

Principle of ultrasonic imaging

A

> Diagnostic imaging of the soft tissue

  • ultrasound pulse is reflected by boundaries between different tissues
  • delay of the echo = depth of the boundary
  • amplitude of the echo= density of the medium
33
Q

Ultrasound Doppler Techniques

A

> Measurement of the velocity of blood in blood vessels
- the difference between the frequency v1 of the emitted wave and the frequency v2 of the reflected wave is used to calculate velocity v.

34
Q

Ultrasonic physical therapy

A

> common is 800 kHz, intensity is 1 W/m^2

  • changes the permeability of cell membranes
  • speeds up the metabolic processes
  • Ultrasonic osteosynthesis
35
Q

Extracorporeal lithotripsy

A

> Powerful ultrasonic pulsed wave is focused onto the kidney stone and high sound pressure shatters it into smaller pieces
- soft tissues are not harmed as a they have a low density

36
Q

Infrasound

A

> a mechanical wave with frequency bellow 20 Hz

  • sources, explosions, typhoons, earthquake
  • absorption of infra sound by matter is small
37
Q

Effect of infrasound on the human body

A

> Not sensed by human ear,

  • harmful effects- deteriorates vision
  • causes neural disorders
  • causes memory loss
38
Q

Vibrations

A 
> mechanical oscillations of structures and machines 
-harmful- vibration illness
-useful- vibrotherapy
> characteristics of vibrations
- fundamental frequency and spectrum
-energy and power
-velocity and acceleration

Medical Physics (2 decks)

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