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The “Doppler effect” refers to the universally perceived cha

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Carcass
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The “Doppler effect” refers to the universally perceived cha [#permalink] New post   16 Jul 2018, 01:25
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The “Doppler effect” refers to the universally perceived change in the apparent pitch of a sound when that sound is approaching or receding from the listener. Specifically, whenever the distance between the listener and the source of the sound is decreasing, the sound will be perceived by the listener as higher than its true pitch; whenever that distance is increasing, the sound will be perceived by the listener as lower than its true pitch. If the distance between the listener and the source is not changing, the listener will perceive the true pitch of the sound.


If the above principle holds, which of the following should be observed as an eastward-traveling train is approached by a westward-traveling train that is blowing its horn?

(A) Passengers in the eastbound train should hear the true pitch of the horn; passengers in the westbound train will hear a sound lower than the true pitch.
(B) Passengers in the westbound train should hear the true pitch of the horn; passengers in the eastbound train will hear a sound lower than the true pitch.
(C) Passengers in the eastbound train should hear the true pitch of the horn; passengers in the westbound train will hear a sound higher than the true pitch.
(D) Passengers in the westbound train should hear the true pitch of the horn; passengers in the eastbound train will hear a sound higher than the true pitch.
(E) Passengers in both trains should hear a sound that is higher than the true pitch.



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Re: The “Doppler effect” refers to the universally perceived cha [#permalink] New post   16 Sep 2020, 05:46
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Dopler Effect says sound perception changes with distance. If you stay at your distance, you hear true pitch but if move you move toward source, you hear louder pitch and if you move back then you hear softer pitch.

So people on the west train, since they aren't moving from the sound, will hear true pitch whereas people on eastbound train are reducing distance between them and the source of the sound, so will hear an increasing sound.
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Re: The “Doppler effect” refers to the universally perceived cha [#permalink] New post   16 Sep 2020, 05:49
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ArneyViel wrote:
Hi....I'm not sure why this popped into my head earlier, but it did. If you have a performer running around the stage with an FM transmitter, why doesn't the Doppler effect cause problems?
From my basic understanding of FM radio, this should be causing DC offset by moving the "base line" frequency above or below the carrier frequency the receiver is tuned to. Is it the case that this DOES happen but it's filtered out in the receiver?


My two cents would be avoid bringing 'worldly' knowledge to the question. Read the question for what it's worth and don't expand or reduce the scope of the argument.

By this we should just understand Doppler effect as pitch increasing when moving to the source, pitch reducing when moving away from the source and hearing true pitch when not moving.

The trick with this question is to realise that train passengers on Westbound train are not moving 'from the source'.
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Re: The “Doppler effect” refers to the universally perceived cha [#permalink] New post   16 Sep 2020, 17:23
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While we may know about the "Doppler effect" from our high school, the key to answering such questions is to focus on the verbal issues and NOT bring in your own knowledge when answering the questions.
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Re: The “Doppler effect” refers to the universally perceived cha [#permalink] New post   29 Sep 2020, 21:33
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The “Doppler effect” refers to the universally perceived change in the apparent pitch of a sound when that sound is approaching or receding from the listener. Specifically, whenever the distance between the listener and the source of the sound is decreasing, the sound will be perceived by the listener as higher than its true pitch; whenever that distance is increasing, the sound will be perceived by the listener as lower than its true pitch. If the distance between the listener and the source is not changing, the listener will perceive the true pitch of the sound.

Inferences:
If two objects are coming close to each other, then the sound will increase.
If they are moving away, their sound will decrease.
If they stay stationary, the sound pitch will be stationary
Westward travelling train is stationary and eastward-travelling is approaching westward-train

If the above principle holds, which of the following should be observed as an eastward-traveling train is approached by a westward-traveling train that is blowing its horn?

(A) Passengers in the eastbound train should hear the true pitch of the horn; passengers in the westbound train will hear a sound lower than the true pitch.
Wrong. It's the opposite. Westward travelling train should hear the original sound as it is stationary.

(B) Passengers in the westbound train should hear the true pitch of the horn; passengers in the eastbound train will hear a sound lower than the true pitch.
Wrong. First part is right, but the eastbound train will hear increased pitch as it is approaching the westbound train.

(C) Passengers in the eastbound train should hear the true pitch of the horn; passengers in the westbound train will hear a sound higher than the true pitch.
Wrong. The voice pitch for eastbound trains will increase or decrease as it is not stationary.

(D) Passengers in the westbound train should hear the true pitch of the horn; passengers in the eastbound train will hear a sound higher than the true pitch.
Right. Westbound is stationary and hence will hear the original pitch. Eastbound is approaching westbound and hence the voice pitch will increase.

(E) Passengers in both trains should hear a sound that is higher than the true pitch.
Wrong. One is stationary and one is moving.
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Re: The “Doppler effect” refers to the universally perceived cha [#permalink] New post   04 Sep 2018, 03:58
Hi Carcass,

Can you please explain how (D) is the answer for this question?

As per the question stem "an eastward-traveling train is approached by a westward-traveling train that is blowing its horn" is the case. However, as per the passage given, "whenever the distance between the listener and the source of the sound is decreasing, the sound will be perceived by the listener as higher than its true pitch".

In the case that is discussed, the distance between the trains is reducing. So, would'nt (E) Passengers in both trains should hear a sound that is higher than the true pitch. be the right answer?
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Re: The “Doppler effect” refers to the universally perceived cha [#permalink] New post   04 Sep 2018, 09:13
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OFFICIAL EXPLANATION


Since the westbound train is blowing its horn, the westbound train is “the source of the sound.” Therefore, the passengers on the westbound train are traveling along with the source of the sound, and thus are neither approaching the source nor receding from it. Hence the westbound passengers should hear the true pitch of the sound.

According to the passage, the passengers in the eastbound train are approaching the source of the sound, the westbound train; the distance between the eastbound passengers and the sound is decreasing. Therefore, according to the given description of the Doppler effect, the eastbound passengers should perceive a sound that is higher than the true pitch.

(A) Passengers in the eastbound train do not hear the true pitch of the horn; passengers in the westbound train do.

(B) It is true that the westbound passengers hear the true pitch; the eastbound passengers, however, hear a sound that is higher than the true pitch.

(C) Passengers in the eastbound train do not hear the true pitch of the horn; passengers in the westbound train do.

(D) CORRECT. Westbound passengers do hear the true pitch of the sound. Eastbound passengers do hear a sound that is higher in pitch than the true sound.

(E) It is true that eastbound passengers hear a sound that is higher than the true pitch; the westbound passengers, however, hear the true pitch.


Actually, the passengers on the west traveling train are on it so they hear the true sound. The passengers on the east train hear a higher sound because the west train is approaching the east.

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Re: The “Doppler effect” refers to the universally perceived cha [#permalink] New post   13 Apr 2020, 09:32
Hi....I'm not sure why this popped into my head earlier, but it did. If you have a performer running around the stage with an FM transmitter, why doesn't the Doppler effect cause problems?
From my basic understanding of FM radio, this should be causing DC offset by moving the "base line" frequency above or below the carrier frequency the receiver is tuned to. Is it the case that this DOES happen but it's filtered out in the receiver?
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Re: The “Doppler effect” refers to the universally perceived cha [#permalink]
13 Apr 2020, 09:32
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