coursework Flashcards
what is sound?
sound is small fluctuations in pressure level around an ambient state created by vibration of elastic medium and propagate through the elastic medium.
is sound wave longitudinal or transverse?
sound waves are longitudinal (compression and rarefraction) in fluid medium and transverse in solids.
what does speed of sound depend on?
physical properties of the elastic medium of propagation, speed of sound in air is sqrt(gamma * R * T)
define wavelength and range of wavelength in human audibility range?
wavelength is the distance the pure tone travels to during one complete period.
where pure tone is sinusiodal wave which contains single frequency.
wavelength ranges from 17.2m to 17.2mm
what are sound pressure range?
2 * 10-5 pa to 20 pascal
why sound pressure level is measured in RMS value?
RMS is the the root of average of square of each element which gives an equivalent amount to a varying signal (for eg sinusoidal wave for which avg value is 0).
human ear cannot react to instantaneous sound, instead it integrates the sound pressure over a period of 30ms to 300ms and gives an average value.
what is a db scale in sound? what is the range of hearing?
db level is a scale in which the lowest level of sound that can be detected by humans is made as scale zero by dividing the avl pressure level by min pressure level and taking log scale to it.
SPL = 0 db (hearing threshold) to 120 db (threshold of pain)
what is the difference in db level required for significant difference in sound level?
3 db.
how addition of extra sources of sound affects the sound perception?
if the two sources are coherent and in same phase the sound is perceived to be 6 db more if the two sources are non coherent then the sound is perceived to be 3db more.
what is relation between intensity and pressure of sound?
pressure is a scalar quantity where as intensity is a vector qty which is a product of pressure and velocity.
sound pressure level and distance relation?
when sound is from point source intensity is inversely proportional to square of the distance (doubling the distance will give 6db attenuation) eg: any source from sufficient distance.
in line source intensity is inversely proportional to the distance (doubling the distance will give 3db attenuation). eg: flow in duct, traffic noise
what is acoustic impedance and characteristic impedance?
acoustic impedance is the resistance offered by the material to the sound propagation. Z= P/v this is a complex number since pressure and velocity need not to be in same phase
characteristic impedance Z = rho*c
formula for intensity and pressure?
for plane wave P = A/sqrt(2) , I = p^2/ (rho*c). for spherical wave P = A/(sqrt(2)*r) , I = p^2/(rho*c). for standing wave P = 2A * cos(kx)/sqrt(2) and I = 0. where k is the wave number = 2 * pi / lamda
what is octave?
octave can be considered as a band of frequency with fmax = 2 fmin
and in one third octave fmax=cuberoot(2) * fmin
what is pitch?
pitch is human perception to the increase in frequency of the sound.
define the bode plot of human ear to sound.
at lower frequency our sound perception is low and keeps increasing till around 4 khz and starts to increase till 9 khz and then again reduces till 14khz and then reduces again.
ie if sound amplitude is kept constant and frequency is increased it is percieved as increase in amplitude till
what is phone level?
it is unit of sound level. this is used to compensate for the effect of frequency on the perception of sound.
the number on the phone represents the db SPL (sound pressure level) that 1khz frequency sound should be to be perceived the same.
what is sone level?
sone is linear scale for compensation for effect of frequency on the perception of sound.
what is reverberation?
it is the persistence of sound even after the source has stopped due to reflection from the surrounding.
Reverberation time?
The time it takes for the sound intensity to reduce 60db. It is given by sabine`s formula t = 0.161*V/(s alpha) where V volume of room, alpha absorption coeff at that freq, s surface area of walls.
reverberation radius?
critical radius/reverberation radius is the locus of the point at which the intensity of sound due to direct source is equal to the intensity from reflection.
= 0.057sqrt(gammaV /T) where gamma is directivity factor , T reverberation time, V volume of the room
what are the types of sound sources?
monopole: due to change in volume / mass flow.
Dipole: 2 opposite monopole sources which are opposite in phase
Quadruple: two opposing dipole
different types of genteration of noise?
air borne noise
structure borne noise
types of airborne noise?
mechanically induced displacement: p § v^4 monopole
flow induced : Local change in velocity: p§ v^6 dipole
Detaching eddies : dipole at low reynolds no 40-200 f=sv/d where s is stroughal no v-velocity d-diameter
turbulence : p§v^8 at 0.72 quadruple small eddies near jet high freq, large eddies at end low freq.
thermal excitation: monopule.
