7. Sound Flashcards
Know transverse wave vs longitudinal wave and their examples
perpendicular to propagation (ex: microwaves, radio waves, visible light, x-rays, basically EM waves cuz their oscillating electric and magnetic field vectors = perpendicular to propagation); ONLY TRANSVERSE WAVES HAVE POLARIZATION - a property of a wave when it can oscillate w/ more than one orientation vs parallel to propagation (ex: sound)
Know amplitude, period, frequency and one cycle
yep
Formula for propagation speed (v); formula for wavelength (lambda) of a standing wave nmlly vs for closed pipe; formula for possible freq of standing wave and open pipe vs closed pipe
v = flambda; lambda = 2L/n (n = # of antinodes) vs lambda = 4L/n (n = odd integers); f = (nv)/(2L) vs f = (n*v)/(4L)
Know the DOPPLER EFFECT formula of freq for source moving towards/away from you and know formula of freq when you’re moving towards/away from source
source moving: f[v/(v +/- vs)]
you moving: f[(v +/- vo)/v]
Know Decibel scale formula. How much does decibel and intensity increase?
B = 10log(I/10^-12)
Going up 10 dB makes intensity go up by factor of 10 (going up 20 dB –> intensity go up 100)
What’s I in decibel scale?
Intensity = power/area = Watt/m^2
Does speed of sound change when you inc/dec the freq and/or lambda?
Nope, you only change speed of sound by changing medium
Is speed of sound faster or slower in denser mediums? Why?
slower; b/c low bulk modulus: v = sqrt(bulk modulus/density)
What’s so special about 10^-12 W/m^2?
It’s the softest intensity humans can hear
What’s the formula if the intensity is changed by some factor?
Bf = Bi + 10log(If/Ii) If = final intensity, Ii = initial intensity; Bf = final Decibel, Bi = initial Decibel
What happens to sound if amplitude is bigger? How does amp relate to intensity? How does distance relate to intensity?
louder. Intensity (and energy of a wave) = amp^2 (ie. If amp doubles, intensity inc 4x). Intensity = inversely proportional to square of distance from source
What’s the speed of sound in m/s? What’s the range human ears can hear in Hz? What’s the range called outside of our hearing?
343 m/s. 20-20000 Hz. Below 20 = infrasonic waves, above 20000 = ultrasonic waves
Characteristics of open vs closed pipe
Open both ends, ends in antinodes, f=2L/n with n = number of nodes, has 1st/2nd/3rd harmonic and 1st/2nd overtone etc. vs closed one end and open at another, closed end has node & open end has anti node, f=4L/n with n = odd integers, has 1st/3rd/5th harmonic and 1st/2nd overtone etc
Closed vs open boundaries
Don’t allow oscillation and correspond to nodes (ex: closed end of pipe, secured ends of string) vs allow max oscillation and correspond to antinodes (ex: open end of pipe, free end of flag)
Angular freq
Omega = 2pif (units: radians/s)
In phase vs out of phase vs partial constructive and destructive
waves waving in sync, phase diff = 0, take sum of resultant displacement vs waves waving completely out of sync, phase diff = lambda/2, take difference of resultant displacement vs waves neither waving perfectly in phase or out of phase but still add up together, they’re basically in between in phase and out of phase
Forced oscillation and force freq
When a system gets contacted by a force, the system will tune to the freq that matches the freq of the force (ex: a parent pushes a child on a swing at the same rate the child is moving back towards the parent). This freq is called the force freq
Resonating
When the freq of a force matches the natural freq of the system (this causes glass to shatter when singing cuz the song’s freq exactly matches the freq of glass; if the freq = above or below natural freq —> glass won’t shatter)
Equil position
A central point around which the wave oscillates
Traveling wave vs standing wave
A moving wave vs a wave with amplitudes fluctuating at a fixed point
Damping/attenuation
Dec in amp caused by an applied or non conservative force
Pitch
Freq of sound
Fundamental freq aka first harmonic
Lowest freq aka longest wavelength of a standing wave that can be supported in a given length of a string; basically n=1
How do you tell which harmonic the wave is in?
if the strings are closed at both ends, # of antinodes can tell you which harmonic the standing wave is. In an open pipe, # of nodes can tell you which harmonic the standing wave is
Still use lambda = 2L/n
What does first overtone correspond to in open vs closed pipes?
Second harmonic (n=2) vs third harmonic (n=3)
Shock waves have the greatest impact when source is traveling at:
The speed of sound
Shock waves are just buildup of wave fronts as distance b/w hose wave fronts dec. when they move faster than speed of sound —> shock wave = migrated because all wave fronts will trail behind object —> destructive interference
How to find beat freq
abs value(first freq - second freq)
