Weeks 1&2 intro, sound waves, attenuation and refl Flashcards
Two Eqns to calculate level dB
Level dB = 10log(I/Io)
Level dB = 20log (A/Ao)
The 3 dB rule
if you increase level by 3 dB, intensity doubles
if you decrease level by 3 dB.,
intensity halves
Eqn with period and freq
T = 1/f
Formula to calculate intensity
I = P^2/(2ρc)
P is pressure amplitude
ρ is density
c is speed of sound
Formula for speed of sound
c = fλ
speed of sound in soft tissue
1540 m/s
acoustic impedance defn, eqn, units
acoustic impedance is resistance to transmit sound
z =ρc
units for z are Rayles or kg/m^2*s
Eqn for reflection coeff
R = Ir/Ii = [(Z2-Z1)/(Z2+Z1)]^2
x 100 to get percentage
get reflection when perpendicular incidence
Eqn for refraction/ snells law
critical angle?
Sinθ1/sinθ2 = c2/c1
if c2 > c1, θ2 > θ1
get refraction and reflection when not perpendicular incidence
critical angle when θ2 is 90; there is no reflection or refraction
Types of reflectors: specular, scatterers, rayleigh scatterers
specular: large smooth surface,»_space;1λ, eg diaphram
scatterers/diffuse reflectors: rough irregular surface, <=1λ, eg most tissue
get weak signal but same from all angles
Rayleigh scatterers: lots of scattering, «1λ, eg RBC
Scattering = f^4
Three things that increase absorption (lose E to heat)
what happens to absorption and intensity when you double the freq?
- incr viscosity
- incr relaxation time
- incr freq
Doubling the freq, doubles the absorption and halves the intensity
Eqn to calculate attenuation
Attenuation (dB) = α (dB/cm) * d (cm)
α = attenuation coeff; values are given at 1 MHz, so MULTIPLY BY FREQ to adjust
d = distance; to structure OR there and back question?
What is the freq used in DMS?
2-20 MHz
What is intensity? definition and units
rate energy is transmitted by wave over area
units are W/m^2
sound waves are
mechanical longitudinal waves
