Acoustics Flashcards
Acoustics
- 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
Oscillations
Periodic oscillations
- 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
Waves
- The propagation of the oscillation in space
- the oscillating particles interacts with its neighbours, make them oscillate
Types of waves according to their nature
- 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
Types of waves according to the oscillation direction
-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
Main physical characteristics of waves
> Period of the wave T
- unit; second, s
> Frequency of the wave v
- unit; hertz, Hz
> Propagation velocity v
- unit; m/sBasic physical characteristics of waves
> 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
The nature of sound
> 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.
Velocity of sound
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
Sound pressure
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
Acoustic impedance
Za= pv
> reflection and refraction of the sound wave at the boundary between two different media depend on the acoustic impedances of the media.
Acoustic spectrum
> the information about the frequencies and intensities of all harmonic oscillations, comprising the sound
wave
Types of sound
look at the diagrams on the book
> 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
subjective characteristics of the human auditory perceptions
> Subjective because
- specific characteristics of the human ear
- individual variation of these characteristics
- Pitch, Timbre, Loudness
Pitch
> depends on it’s fundamental frequency
- higher frequency sounds have higher pitch
- sensation of pitch increases= arithmetic progression
- frequency of the sound increases = geometric
Timbre
> 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
Loudness
> level of the sound sensation above its threshold]
-Loudness L depends on sound intensity I,and fundamental frequency
Hearing threshold
the sound intensity for a given frequency, resulting in minimal sound sensation
- I0=10^-12w/m^2
Spectral dependence of the hearing threshold
> The dependence of the hearing threshold on sound frequency
Pain threshold
> The sound intensity for a given frequency, which results in a sensation of pain
- I1 = 10 W/m^2
Hearing area
> Range limited by the hearing and pain thresholds for all audible frequencies
Measurements of sound
> Based on the logarithmic dependence of loudness on sound intensity
Sound intensity level measurement scale
> measured on a logarithmic scale
-zero sound intensity is at I0= 10^-12 w/m^2
Loudness measurement scale
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
Loudness measurement scale
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
Sound loudness and the area of speech
whisper = 20-40 phon
normal speech = 45-75 phon
loudness range = 45-75 phon
frequency range = 200-8000 Hz
Audiometry
> 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
Ultrasound
> a mechanical wave with frequency above 20,000 hz
- main properties
- exhibits reflection, refraction, absorption
- at high intensities can destroy viruses, bacteria
Detectors of ultrasound
> Piezoelectric material- production of electric voltage when the stress is applies to a plate, made of that material
Generators of ultrasound
> 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.
Applications of ultrasound
> Diagnostics, therapy, surgery
Principle of ultrasonic imaging
> 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
Ultrasound Doppler Techniques
> 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.
Ultrasonic physical therapy
> common is 800 kHz, intensity is 1 W/m^2
- changes the permeability of cell membranes
- speeds up the metabolic processes
- Ultrasonic osteosynthesis
Extracorporeal lithotripsy
> 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
Infrasound
> a mechanical wave with frequency bellow 20 Hz
- sources, explosions, typhoons, earthquake
- absorption of infra sound by matter is small
Effect of infrasound on the human body
> Not sensed by human ear,
- harmful effects- deteriorates vision
- causes neural disorders
- causes memory loss
Vibrations
> mechanical oscillations of structures and machines -harmful- vibration illness -useful- vibrotherapy > characteristics of vibrations - fundamental frequency and spectrum -energy and power -velocity and acceleration
