Module 8 - Doppler Effect, Standing Waves, Beats, The Ear and Hearing

Question 1

you are standing on a platform and a train approaches, moving at a constant velocity, directly. it whistle is blowing continuously. the train does not stop but continues to move past the platform at constant velocity. what is true about the frequency of the train’s whistle?

Answer 1

the whistle’s frequency that you hear remains constant as the train approaches the platform and then decreases after the train passes the platform, after which it remains constant at a lower pitch than originally.

reason: doppler affect

Question 2

a stationary source S generates circular outgoing waves on a lake. the wave speed is 5.0 m/s and the crest-to-crest distance is 2.0m. a person in a motor boat head directly toward S at 3.0m/s. What frequency does the person in the motor boat detect?

Answer 2

4.0Hz.

reason: third doppler equation (observer moves towards stationary source)
3m/s + 5m/s / 2m = 4Hz

Question 3

you’re on a hot air balloon ride, carrying a buzzer that emits a sound of frequency f. if you accidentally drop the buzzer over the side while the balloon is rising at a constant speed, what can you conclude about the sound you hear as the buzzer falls to the ground?

Answer 3

the frequency decreases and the intensity decreases

Question 4

two loudspeakers are placed next to each other and driven by the same source at 500 Hz. a listener is positioned in front of the two speaker and on the line separating them, thus creating a constructive interference at the listener’s ear. what minimum distance (change in y) would one of the speakers be moved back away from the listener to produce destructive interference at the listener’s ear?
(speed of sound = 343 m/s)

Answer 4

0.343m

reason: λ = v/f
= 343/500
λ = 0.686m

destructive path length: Δ = (m + ½)λ
m=0
Δ=0.68/62
Δ=0.343m

Question 5

when a standing wave occurs on a string.

Answer 5

many points along the string move up and down, some move more than other. some are stationary.

Question 6

you have a guitar string tied to both ends. you pluck it and you get a fundamental frequency an “A” note of 440Hz. If you wanted to adjust the string to get the next “A” note one octave higher (880Hz) as a fundamental frequency what could you do?

Answer 6

quadruple (4x) the string tension.

reason: T=f^2 tension is equal to the frequency squared.

Question 7

a pipe of length L will support the lowest fundamental frequency in which of theses cases?

Answer 7

if it were open at one end and closed at the other.

reason: f=v/4L
bigger denominator (longer pipe), smaller frequency

Question 8

a large pipe organ has valves that allow either one end of the pipe to be open and the other end closed, or else both ends open. if one end is open and the other end is closed the pipe can produce fundamental frequency of 40Hz, what will be the fundamental frequency when both ends are open?

Answer 8

80Hz

doubles

Question 9

a pipe open at both ends resonates at a fundamental frequency fopen. when one end is covered and the pipe is again made to resonate, the fundamental frequency is fclosed. which of the following expressions describes how these two resonant frequencies compare?

Answer 9

fclosed = 1/2 fopen

Question 10

which of the following wavelengths is common to a 10com tube open at both ends and a 15cm tube open at one end and closed at the other?

Answer 10

λ=20cm

Question 11

you are tuning a guitar by comparing the sound of the string with that of a standard tuning fork. you notice a beat frequency of 5Hz when both sounds are present. you tighten the guitar string and the beat frequency rises steadily to 8Hz. to tune the string exactly to the the tuning fork you should;

Answer 11

loosen the string. this causes beat frequency to decrease

Question 12

you have two tuning forks that have a beat frequency of 2Hz with respect to each other. one of them has a frequency of 440Hz, but you do not know the frequency of the other. you place a small piece of clay on the other, an amount just capable of decreasing its resonance frequency by 1Hz, and you repeat the experiment. this time the beat frequency is 3Hz. what is the frequency of the other tuning fork when it has no clay on it?

Answer 12

438Hz

440-2

Question 13

the term “timbre” refers to?

Answer 13

the quality of sound from instruments due to the mixture of harmonics