Pitch problem ........

[DocDrakeRamoray]

I thought it was going to be an easily problem, but I got it wrong. See if you get it right.


A person blowing a whistle is running toward another person who is standing sill. As the runner approaches (at constant velocity), the stationary listerner will hear a sound that:

A. continuosly decreases in pitch and intensity

B. has a fixed pitch but increases in intensity

C. has a fixed pitch but decreases in intensity

D. continuosly increases in pitch and intensity

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[Marjan Islam]

Gosh, this one is tricky! I'll guess with B. constant pitch and increased intensity.

My reasoning goes as follows:

Pitch is just a measure of frequency; increased pitch means increased frequency. From the doppler effect, we know that there is increased frequency when source is moving towards the object (listener), so pitch should be increasing. BUT, since it says constant velocity, the pitch will feel higher than it should be (because he is running towards the listenner), but that's it. It won't be INCREASING, since the source isn't accelerating.

As for intenisy, which is something over area. Mayber energy/area, the area (distance between source and listener) is decreasing as the source is nearing the listener. So, with decreased area, and same number of frequency, intensity should be increasing.

I may be wrong! I'll check back to find out!

 

[LeftyUM]

B, final answer! I think Marjan nailed it. Let us know!

 

[DocDrakeRamoray]

Gosh, this one is tricky! I'll guess with B. constant pitch and increased intensity.

My reasoning goes as follows:

Pitch is just a measure of frequency; increased pitch means increased frequency. From the doppler effect, we know that there is increased frequency when source is moving towards the object (listener), so pitch should be increasing. BUT, since it says constant velocity, the pitch will feel higher than it should be (because he is running towards the listenner), but that's it. It won't be INCREASING, since the source isn't accelerating.

As for intenisy, which is something over area. Mayber energy/area, the area (distance between source and listener) is decreasing as the source is nearing the listener. So, with decreased area, and same number of frequency, intensity should be increasing.

I may be wrong! I'll check back to find out!

Great Job! B is the right answer, Now would you tell me why frequency is not increasing if he is moving towards the listener, why does it matter whether velocity is constant or not the guy is clearly moving towards the listener

 

[Marjan Islam]

Yeah, that part was a little tough to figure out. Intensity would decrease regardless of whether it was constant velocity or not, area between them IS decreasing.

For the frequency/pitch, picture it in your head. The person is hearing the pitch at an increased level compared to what he would normally hear if the source wasn't moving - this is because the source IS moving towards the listener.

BUT, this INCREASED FREQUENCY LEVEL is constant, because there's no acceleration of the source. The increased sound waves are passing by the listener at a constant pace. They aren't aggregating where the listener is, thus increasing in "concentration", if that's what you're thinking? They're passing right by him at a constant pace.

I found that picture online. See the 605 kph car? He's going at a constant velocity, so the frequency/pitch waves are all exactly the same.

 

[DocDrakeRamoray]

Marjan thanks a lot for the explanation. I totally see it now, my brain is fried by the end of the day and can't think straight

salam!

 

[Marjan Islam]

Wa' salaam, and no problem! Totally helped me out too

 

[robrobbberts]

oh wow. even after reading that explanation, it took me a bit to understand.

so the pitch IS higher than normal, but since the velocity is constant, the pitch is also constant, if the velocity was changing, the pitch would be changing..

wow.. what a tricky question.

 

[Will Hunting]

Yeah, that part was a little tough to figure out. Intensity would decrease regardless of whether it was constant velocity or not, area between them IS decreasing.

For the frequency/pitch, picture it in your head. The person is hearing the pitch at an increased level compared to what he would normally hear if the source wasn't moving - this is because the source IS moving towards the listener.

BUT, this INCREASED FREQUENCY LEVEL is constant, because there's no acceleration of the source. The increased sound waves are passing by the listener at a constant pace. They aren't aggregating where the listener is, thus increasing in "concentration", if that's what you're thinking? They're passing right by him at a constant pace.

I found that picture online. See the 605 kph car? He's going at a constant velocity, so the frequency/pitch waves are all exactly the same.

That's not true. I found the source of that image. In the top image the car isn't moving. Consequently, the pitch is the same. In the second image the car is moving at 605 kph, the author says that sound of engine moves at 1210 kph so the car is chasing the sound waves. Look closely, the sound is being compressed IN FRONT of the care and DEPRESSED behind it.

The third picture the car is moving at 1210 kph which is the same as the emitted sound. Consequently, all the sound waves move as one and this is what we call a sonic boom. Instead of the waves being closed together, they all occupy one space.

Moreover, I don't know where you got this question. However, the pitch does change. The constant velocity is irrelevant. Pitch is a function of frequency and despite running at constant velocity, since the person is coming closer, sound waves are being compressed ahead so the PERCEIVED frequency increases.

In addition, intensity is the energy. If I blow a whistle, a person standing next to me feels a more INTENSE sound than someone who is further away. So, in this case the pitch is increasing as is the intensity.

B doesn't not make sense.

 

[Will Hunting]

That's not true. I found the source of that image. In the top image the car isn't moving. Consequently, the pitch is the same. In the second image the car is moving at 605 kph, the author says that sound of engine moves at 1210 kph so the car is chasing the sound waves. Look closely, the sound is being compressed IN FRONT of the care and DEPRESSED behind it.

The third picture the car is moving at 1210 kph which is the same as the emitted sound. Consequently, all the sound waves move as one and this is what we call a sonic boom. Instead of the waves being closed together, they all occupy one space.

Moreover, I don't know where you got this question. However, the pitch does change. The constant velocity is irrelevant. Pitch is a function of frequency and despite running at constant velocity, since the person is coming closer, sound waves are being compressed ahead so the PERCEIVED frequency increases.

In addition, intensity is the energy. If I blow a whistle, a person standing next to me feels a more INTENSE sound than someone who is further away. So, in this case the pitch is increasing as is the intensity.

B doesn't not make sense.

Marjan, you're correct in that the emitted sound doesn't change. Just because someone is running doesn't change anything.

However, to kill the MCAT, you must read carefully. The question is asking what will be the sound that the STANDING person hears. That person will hear a sound with increasing pitch and increasing intensity.

If it asked about the person running, then pitch would be the same.

HTH, based on the wording of the question the answer is D.

 

[DocDrakeRamoray]

The right answer is B.

And now after thinking more about it, it makes perfect sense.

Pitch is increased but it is not increasing.

If one to look at the doppler effect formula, after you plug in the velocity of a sourse, the perceived frequecy is increased but it does not change because plugged in velocity is not changing. The f prime value is greater than f but it is fixed.

 

[LeftyUM]

The way I saw it is that since the velocity is constant, the sound wave "lines" are not bunching together, they are at a constant distance apart as they travel through the air. Now, if there was an acceleration, the "lines" would bunch together, which means that there is an increasing frequency (pitch). Intensity continues to increase just because the distance between them is decreasing. Thats my take on it.

 

[Will Hunting]

The right answer is B.

And now after thinking more about it, it makes perfect sense.

Pitch is increased but it is not increasing.

If one to look at the doppler effect formula, after you plug in the velocity of a sourse, the perceived frequecy is increased but it does not change because plugged in velocity is not changing. The f prime value is greater than f but it is fixed.

I should note that I originally picked B. When one takes the MCAT pick the best answer. After some further analysis. It didn't make sense. However, I'm used to answers of frequency heard by listener compared to frequency emitted. The listener does hear a higher pitch, but since speed is constant, that higher pitch doesn't change. You got it, that's what they were emphasizing.

 

[Will Hunting]

The right answer is B.

And now after thinking more about it, it makes perfect sense.

Pitch is increased but it is not increasing.

If one to look at the doppler effect formula, after you plug in the velocity of a sourse, the perceived frequecy is increased but it does not change because plugged in velocity is not changing. The f prime value is greater than f but it is fixed.

You answered your own question. F prime is greater, which is the pitch that the listener hears. The question is asking if F prime greater than F, which it is. The sound that the person hears is of a higher pitch, however, the actual pitch doesn't change.

The listener does hear a higher pitch, so by higher I thought they wanted increasing. It's constant but higher. I guess the key is to look for the best answer which you guys did.

I just wanted you guys to understand the 3 images in Marjan's figure.

The first is of an unmoving source.

The second is of a moving one so in front the frequency is compressed but at a constant rate. So, the listener hears a sound of higher pitch that has a constant value. I guess this is what confused me. Just because a source is moving at constant velocity does NOT mean it doesn't compress in front of it. If it didn't compress, then one would NOT hear a higher pitch. The compression is the reason for the INCREASED pitch heard by the receiver. However, in this case, since the source is moving at CONSTANT velocity, the person hears a HIGHER BUT CONSTANT pitch.

The last one would never happen to a car because of speed, it's more relevant with Jets. They move at or faster than the speed of sound, so the emitted sounds don't separate and pool together in space and time. So, when it gets to you, you hear one large SONIC BOOM. Well done guys.

I'm happy this was asked because I got a better conceptual understanding. where did you get this ramoray?

 

[DocDrakeRamoray]

I should note that I originally picked B. When one takes the MCAT pick the best answer. After some further analysis. It didn't make sense. However, I'm used to answers of frequency heard by listener compared to frequency emitted. The listener does hear a higher pitch, but since speed is constant, that higher pitch doesn't change. You got it, that's what they were emphasizing.

Exactly. I picked D, but it was wrong and after reading Marjan's answer I gained a better insight.

The important thing in this question is not to confuse INCREASED (but constant) VS INCREASING. But it can be tricky when one is tired and under time pressure like during the actual test.

 

[DocDrakeRamoray]

where did you get this ramoray?

I have an old (1991) princeton science workbook with a zillion of practice questions.

 

[MD2BEEE]

Thanks to the OP and everyone who contributed with input... learned a new perspective on doppler with this question.

 

[BerkReviewTeach]

The question is asking if F prime greater than F, which it is.

I think that is the mistake everyone is likely to make on this question. We are so used to being asked whether the frequency is higher or lower in Doppler questions, that we subconsciously give that answer on all Doppler questions. The question writer did a really good job of presenting a unique spin on a typical question.

This question asks what the listener hears, while never asking whether it is higher or lower in frequency than what the sender emits. It's an atypical question in that sense.

There is no ambiguity here, because there are two facts addressed correctly in the best answer.

1) The intensity increases because the sender is getting closer.
2) The perceived frequency stays constant, because the relative velocities of the sender and receiver remain constant.

(1) eliminates choices A and C while (2) eliminates choices A and D.

 

[Will Hunting]

I think that is the mistake everyone is likely to make on this question. We are so used to being asked whether the frequency is higher or lower in Doppler questions, that we subconsciously give that answer on all Doppler questions. The question writer did a really good job of presenting a unique spin on a typical question.

This question asks what the listener hears, while never asking whether it is higher or lower in frequency than what the sender emits. It's an atypical question in that sense.

There is no ambiguity here, because there are two facts addressed correctly in the best answer.

1) The intensity increases because the sender is getting closer.
2) The perceived frequency stays constant, because the relative velocities of the sender and receiver remain constant.

(1) eliminates choices A and C while (2) eliminates choices A and D.

Yea, great write up. I caught that after awhile. I see the importance of thinking about every question so you're not tricked.