3) sound and ultrasound Flashcards
Acoustics
Section of physics that studies
-mechanical oscillations and waves
Physiological acoustics studies the work of organs for sound detection and production
Oscillations
-repeated movement or change of state
Periodic oscillations are repeated at regular time intervals
V=1/T
V= frequency of oscillations per unit of time (Hz) T = the time after which the oscillation is repeated
Example
Example of periodic oscillations
The swinging of pendulum
The rhythmic contraction of the heart
Harmonic oscillations
Periodic oscillations are shown by a sine function
Y= Asin(Wt+@)
Y-displacement at time t
A- amplitude
W-angular frequency
@-initial phase
W=2piV = 2pi/T
Waves
If oscillating particles interact with its neighbors then it can make them oscillate
Wave-propagation of oscillation in space
Eg: waves on surface of water
Types of waves according to nature
Mechanical waves
-oscillations of material particles
Propagated in material media only
Eg:sound and water waves
Types of waves according to nature 2
Electromagnetic waves
Periodic oscillations of an electromagnetic field
Propagated in material media and in free space
Eg: light, X rays, radio waves
Types of waves according to oscillation direction
Longitudinal waves
The oscillations are parallels to the propagation of the wave
Eg:sound
DRAWING
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Types of waves according to oscillation direction 2
Transverse
The oscillations are perpendicular to the propagated direction of the wave
Eg:electromagnetic waves and water
Main physical characteristics of waves
Period of wave T
-measured in seconds
Frequency of wave
-measured in hertz Hz
Propagation velocity
-measured in m/s
Basic physical characteristic of waves
Wavelength (lander)
Distance travelled by the wave for one period - lander=vt
Drawings
Basic physical characteristics of waves 2
Intensity I
-the energy fluency rate
The energy transferred by the wave through unit area normal to the propagation direction per unit of time
Units- w/m^2
I=dE/dsdt
DRAWING
The nature of sound
Sound-mechanical wave with frequency from 20-20,000Hz.
Other mechanical waves
Ultrasound- frequency above 20khz
Infrasound -frequency below 20hz
In liquids sound propagates as longitudinal waves and in solids it can be longitudinal or transverse
Velocity of sound
Gases : v alpha root T
T- absolute gas temp
In air V = 330m/s
Liquid: v= root(k/p)
K-bulk modulus is liquid
p- liquid density
In water V=1500m/s
Velocity of sound 2
In solids
-metals v=4000m/s
Longitudinal waves:
V=root(E/p)
E is Young’s modulus of elasticity
Transverse :
V=root(G/p)
G is the sheer modulus of the body
Acoustic impedance
Z_A=PV
The reflection and refraction of the sound wave at the boundary between two different media depends on the acoustic impedances of the media
Acoustic spectrum
Acoustic spectrum-the information about the frequencies and intensities of all harmonic oscillations.
The acoustic spectrum can be represented graphically or as a table
The various sounds have different acoustic spectra
Types of sound
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
Types of sound
Noise-has a continuous complex randomly varying acoustic spectrum
Sound strokes-a loud sound of short duration with a continuous acoustic spectrum
Types of sound 2
DRAWING
Acoustic spectra of the various types of sound
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The subjective characteristics of the human auditory perception
Sound is evaluated subjectively to different people due to
- characteristics of the human ear
- individual variation of these characteristics
The main subjective characteristics are
- pitch
- timbre
- loudness
Pitch
The pitch of the sound depends on its frequency
Higher frequency means higher pitch
If a sound has higher pitch than another sound then it is said to have s higher frequency aswell
If our sensation of pitch increases in arithmetic progression then frequency increases in a geometric progression
Timbre
The changes of the overtones of a compound tone is sensed as variations in sound timbre
Eg:sounds with identical intensities and frequencies when played on different instrument are said to have different timbre
Human ears sense timbre difference between 2 tones if they have a 10% intensity difference
Loudness
Sound loudness characterises the level of sound sensation above the threshold
Loudness L depends on sound intensity I as well as fundamental frequency
Loudness is complex and is determined by the sensitivity of the ear to sounds with different frequencies
Hearing threshold
The sounds intensity for a given frequency that results in the minimum sound sensation
Normal herring threshold is 1khz
I_0=10^-12 w/m^2
Spectral dependence of the hearing threshold
DRAWING
Pain threshold
The sound intensity for a given frequency that results in a pain sensation
Normal pain threshold is 1KHz and is assumed to be
I_1= 10w/m^2
Hearing area
The range limited by hearing and pain threshold for all audible frequencies
DRAWING
The psychophysical law of Weber-fechner
The changes of the stimulus 🔺I resulting in a minimal change of the sensation 🔺L. Depends of initial value of the stimulus I.
🔺L=K 🔺I/I = L= klg I/I_0
For the sensation of hearing:
Loudness is proportional to the logarithm of the ratio of intensity I
Measurements of sound
Sound measurements are based on the logarithmic dependence of loudness on sound intensity
This feature reflects the ability of the ear to adapt
-the sensitivity of the sensory organ changes depending on the strength of the stimulus
Sound intensity level measurement scale
It is measured on a logarithmic scale
Zero sound intensity: I_0= 10^-12
Loudness measurement scale
The loudness measurement scale divides the interval between hearing and the pain threshold for each frequency into 130 equal parts = loudness measurement unit is the phon
1kHz change in loudness = change of sound intensity by one decibel
Sound loudness and area of speech
Whisper = 20-40 phons
Normal speech = 45-75 phons
Street traffic = 70-90
Normal speech area:
Loudness range 47-75
Frequency = 200-8000 Hz
Audiometry
The investigation of the sharpness of hearing
Audiogram- graphical representation of hearing threshold versus frequency
audiometer- sound generator that that adjustable sound intensity and frequency
Ultrasound
A mechanical wave with frequency above 20,000Hz
Main properties:
- reflection and refraction
- propagates as narrow sound beams
- high intensity can destroy viruses, bacteria
Detectors of ultrasound
Piezoelectric effect
-production of electric voltage when stress is applied by sound pressure
Generators of ultrasound
Natural sources: dolphins, bags waterfalls
Artificial:
Converse piezoelectric effect
Production of electric field when stress is applied
Ultrasound in medicine
Ultrasound diagnostics
Ultrasound therapy
Ultrasound surgery
Production of aerosols
Principles of ultrasonic imaging
Diagnostic imaging of soft tissue
- delay of echo is used to determine here depth of boundary
- amplitude corresponds to the density of medium
DRAWING
Ultrasonic Doppler technique
Measurements about the velocity of blood is blood vessels
- emitted ultrasound js reflected using moving target
- difference between frequency V1 and V2 is used to determine velocity
Ultrasonic physical therapy
The commonly used ultrasound frequency is 800Hz and intensity is 1W/m^2 intensity
- changes the permeability of the cell membranes
- speeds up metabolic process inside the cells
- speeds up healing process of bones
Extracorporeal lithotripsy
The destruction the kidney and gall bladder stones
- powerful ultrasonic wave is focused on the stone and high sound pressure shatters it only small pieces
- soft tissue is not harmed as they have low density and low sound pressure
Infrasound
Mechanical wave with frequency below 20Hz
Eg:
Explosions
Road traffic
Earthquakes
The absorption of infrasound by matter is small and it travels long distances.
Effect of infrasound on the human body
Not sensed by human ear
- harmful at high intensities
- deteriorates vision
- causes memory loss
Vibrations
Mechanical oscillations and structures of machines
Effect on human body:
Harmful - vibration illness
Useful- vibrotherapy
