MCAT Study Guide Physics Ch. 10 – Sound Waves 2017-08-15T06:45:05+00:00

## I.          10.1:  SOUND WAVES

### A.     Longitudinal waves

Sound waves are different than mechanical waves; the particles oscillate in a direction parallel to the direction the waves travel (not 丄 to)

1.     Compressions – the regions of high pressure in sound waves

2.     Rarefactions – the regions of low pressure in sound wave

## II.          10.2:  STANDING SOUND WAVES IN TUBES

### B.     Tube open at both ends

1.     Both ends are antinodes

a)     λn = 2L/n

b)     fn = nv/2L

c)     n is any positive integer ### C.    Tubes closed at one end

1.     Both ends are nodes

a)     λn = 4L/n

b)     fn = nv/4L

c)     n is an odd number!

## III.          10.3:  BEATS

### A.     Beats

Sound waves of nearly equal frequency that interfere with each other

1.     Because frequencies don’t match, sometimes they are in phase and sometimes out of phase

2.     In phase = amplitudes add

3.     Out of phase = amplitudes subtract

### B.     Beat frequency

Frequency at which the beats are heard equals the difference between the frequencies of the two original sound waves

1.     fbeat = |f1 = f2|

## IV.          10.4:  INTENSITY AND INTENSITY LEVEL

### A.     Intensity

Energy (power) transmitted per second per unit area (W/m2)

1.     I = power/area

2.     I is inversely ∝ to r2 (r = distance from source); area = πr2

### B.     Threshold of hearing

Lowest intensity human ear can perceive, 10-12 W/m2, = I0

### C.    Intensity level (sound level) – base 10 log of ratio I/I0, in β

1.     β = 10log10(I/I0)

a)     Every time we multiply I by 10, we add 10 to β

b)     Every time we divide I by 10, we subtract 10 from β

## V.          10.5:  THE DOPPLER EFFECT

### A.     Doppler effect

Perceived frequency of sound is different than emitted sound due to the movement of a source (either towards or away from detector)

1.     fD = fS

[(v ± vD)/(v ∓ vS)]

a)     Remember, top sign is toward

(1)   If vD is moving toward source, use the + sign

(2)   If vS is moving toward the detector, use the – sign

b)     fD = frequency heard by the detector

c)     fS = frequency emitted by the source

d)     vD = speed at which the detector is moving

e)     vS = speed at which the source is moving

f)       v = speed of the wave (usually 340 m/s) # 10.

#### Ch. 11 Reflection + Refraction

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