7. Sound

Question 1

Know transverse wave vs longitudinal wave and their examples

Answer 1

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)

Question 2

Know amplitude, period, frequency and one cycle

Answer 2

yep

Question 3

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

Answer 3

v = flambda; lambda = 2L/n (n = # of antinodes) vs lambda = 4L/n (n = odd integers); f = (nv)/(2L) vs f = (n*v)/(4L)

Question 4

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

Answer 4

source moving: f[v/(v +/- vs)]

you moving: f[(v +/- vo)/v]

Question 5

Know Decibel scale formula. How much does decibel and intensity increase?

Answer 5

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)

Question 6

What’s I in decibel scale?

Answer 6

Intensity = power/area = Watt/m^2

Question 7

Does speed of sound change when you inc/dec the freq and/or lambda?

Answer 7

Nope, you only change speed of sound by changing medium

Question 8

Is speed of sound faster or slower in denser mediums? Why?

Answer 8

slower; b/c low bulk modulus: v = sqrt(bulk modulus/density)

Question 9

What’s so special about 10^-12 W/m^2?

Answer 9

It’s the softest intensity humans can hear

Question 10

What’s the formula if the intensity is changed by some factor?

Answer 10

Bf = Bi + 10log(If/Ii)
If = final intensity, Ii = initial intensity; Bf = final Decibel, Bi = initial Decibel

Question 11

What happens to sound if amplitude is bigger? How does amp relate to intensity? How does distance relate to intensity?

Answer 11

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

Question 12

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?

Answer 12

343 m/s. 20-20000 Hz. Below 20 = infrasonic waves, above 20000 = ultrasonic waves

Question 13

Characteristics of open vs closed pipe

Answer 13

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

Question 14

Closed vs open boundaries

Answer 14

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)

Question 15

Angular freq

Answer 15

Omega = 2pif (units: radians/s)

Question 16

In phase vs out of phase vs partial constructive and destructive

Answer 16

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

Question 17

Forced oscillation and force freq

Answer 17

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

Question 18

Resonating

Answer 18

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)

Question 19

Equil position

Answer 19

A central point around which the wave oscillates

Question 20

Traveling wave vs standing wave

Answer 20

A moving wave vs a wave with amplitudes fluctuating at a fixed point

Question 21

Damping/attenuation

Answer 21

Dec in amp caused by an applied or non conservative force

Question 22

Pitch

Answer 22

Freq of sound

Question 23

Fundamental freq aka first harmonic

Answer 23

Lowest freq aka longest wavelength of a standing wave that can be supported in a given length of a string; basically n=1

Question 24

How do you tell which harmonic the wave is in?

Answer 24

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

Question 25

What does first overtone correspond to in open vs closed pipes?

Answer 25

Second harmonic (n=2) vs third harmonic (n=3)

Question 26

Shock waves have the greatest impact when source is traveling at:

Answer 26

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

Question 27

How to find beat freq

Answer 27

abs value(first freq - second freq)