## anonymous one year ago speed of sound.. If I am travelling at 1/2 the speed of sound, and there are sounds coming from behind me and in front of me. I understand they will sound at different frequencies to me, but will I still detect them as both travelling at the same speed?

1. arindameducationusc

Yes

2. Michele_Laino

hint: we have to consider the Doppler effect

3. anonymous

well I get that the doppler effect will change the frequency.. but the question is.. So if I were in a vehicle and had marked out say 1 meter distance, will I see the crests of each wave passing over that meter length in the same amount of time, regardless of how fast I am travelling? So the crests always travel x meters per second? And I am disregarding how sound travels at different speeds at different altitudes, lets just assume we maintain altitude for the experiment. Oh and that densities and pressures had somehow remained constant also. We are just moving relative to the speed of sound.

4. rajat97

no i don't think that we should apply the concept of relative speed or relative velocity here, because when we use the equation of Doppler effect, and if we are moving towards the sound source, we do not make any change to the speed of sound. We assume that the wave has shrunk and thus, we observe the increase in the observed frequency. We make changes to the speed of sound if and only if the medium is moving.

5. rajat97

what say? @hughfuve

6. anonymous

Thnx rajat, I have no idea really.. I was just wondering if I would see a change in the time it takes the crests to traverse a given distance, or if I just see the same velocity but with more crests coming into the picture. Although I guess I could travel at exactly the speed of sound, in the direction of the sound, and then I should see a standing wave. So that should tell me something about the perceived speed of the crest. The time for a crest to cross the meter stick must actually change. My ear will sense the rise and fall of those crests as frequency of sound, and things will get very quiet. Different source frequencies should add and remove the number of crests that can be observed to span the meter stick. A 3.4Khz signal should distribute, ah maybe about 10 crests per meter? 3400 crests / 340(m/s), as I slow down I will see them pick up speed across the stick and they will start to move ahead of me.

7. IrishBoy123

"So if I were in a vehicle and had marked out say 1 meter distance, will I see the crests of each wave passing over that meter length in the same amount of time, regardless of how fast I am travelling? So the crests always travel x meters per second?" No they will have a relative velocity.

8. anonymous

"Perceived" frequency changes. "Perceived" wavelength changes. The speed of the sound wave remains constant. The speed of a wave is dependent on the properties of the medium through which it travels, in this case, air. The speed of a sound wave is not affected by motion of either the sound emitter or the sound receiver. The quantities are related through the universal wave equation$v=f \lambda$where v is the speed of the wave, f is frequency, and \lambda is wavelength. For a given medium, v is constant. If there is relative motion between the source of the wave and the receiver of the wave, the perceived f and \lambda may change, but their product must remain constant.