Question

I have some concept questions. :)

1. Regarding sound intensity and the equation I=P/(4pir^2)

Does the power (P) ever change like for a speaker emitting a certain sound? I was doing a bunch of problems where I and r changed, but P never seemed to change.

2. Doppler effect. I had a problem where there was a stationary horn and a person running towards it and then away from it. If the person wanted to hear double the frequency then they would have to run very fast, but if the person wanted to hear half the original frequency then apparently they don't have to run as fast as the speed t

Answer 1

they don't have to fun as fast as the speed they needed to double the frequency. So is why does this happen. Running towards the sound source must be faster to double than to run away from the sound source and reduce the frequency by 1/2???

Answer 2

In Q1 - What is I and r ??

Answer 3

@Fellowroot ^^

Answer 4

Q1. Power can be called the rate of propagation of energy which is physically related to the velocity of the wave.

As the velocity of a mechanical wave depends only on the properties of the medium, Power also depends only on the medium (once the wave is generated).

Q2. I cant give you a physical interpretation as I myself never understood Doppler for observer. Maybe @Vincent-Lyon.Fr and @Mashy may help here.

Answer 5

P is the power (in watts) emitted by the sound-source.
I is the sound intensity (in watts per m²) at a distance r from the source if the source emits equally in all directions.

The further away, the lower the intensity. This phenomenon is a form of attenuation due to geometry and not due to damping.

Answer 6

@Vincent-Lyon.Fr
In Doppler's effect - when the source is stationary and the observer is moving..
From the frame of reference of observer, the velocity of sound increases (or decreases). Then why do we get the illusion of pitch ??

I couldn't find a reason as to why we talk about apparent frequency instead of apparent velocity..

Answer 7

This is not a good idea to use the observer as a reference frame, it makes all equations more complicated.

The reason of the initial question is that the equation is not symmetrical for moving source or moving observer.
Only for small velocities wrt sound is the change in frequency symmetrical.

Answer 8

On the other hand, with Fizeau effect for light, you can choose whichever frame you want, since light travels in a vacuum at constant velocity.

Answer 9

Q: "I couldn't find a reason as to why we talk about apparent frequency instead of apparent velocity.."

A: Well, our detectors (ears, microphones) are not sensible to sound celerity but to pitch, since they work at a given position.

Answer 10

Thank you :)

Answer 11

"This is not a good idea to use the observer as a reference frame, it makes all equations more complicated."

no it doesnt.. its infact very easy to derive the apparent frequency that the listener hears considering HIS frame of reference

from his frame of reference, the velocity of sound increases (in case he is running towards the source) but he STILL sees the waves having the same wavelength .. thus he registers it to be of higher frequency....

its like you have to imagine.. sound is coming towards u at a particular frequency (like tomatos being shot at you at a particular rate).. but if you also go towards the tomatos then u ll get hit more number of times per second.. won't you? thats precisely cause frm your reference frame the tomatos are moving faster ..

Answer 12

@Fellowroot

For your second question ..


in order to double the frequency.. speed of sound WITH RESPECT TO YOU must be double..
so for that to happen, you need to move towards the source AT SPEED of sound..

ir order to half the frequency, the speed of sound WITH RESPECT to you must also be HALF, so for that to happen, you need to move away from the source at ... half the speed of sound :D.. (if you move AT the speed of sound from the source.. u ll hear nothing.. cause sound can't catch you.. you ll hear zero frequency)

Answer 13

I wrote "This is not a good idea to use the observer as a reference frame, it makes all equations more complicated."

Well, after reading Mashy's contribution, I must admit he proved me wrong! ;-)

Answer 14

@Vincent-Lyon.Fr WOW.. this is sooo rare.. proving u wrong.. :D :D: D

Answer 15

No problem ! If someone proves me wrong, it just means I will learn a new interesting thing! thanks :)