The Doppler Effect and sound?

[flakko 5,000+ posts]

i was searching through the internet trying to expand my knowledge of sound (how it is produced and stuff like that) when i came across this:

http://demo.physics.uiuc.edu/lectdemo/scripts/demo_descript.idc?DemoID=417

now i learned in my astronomy class about the doppler effect and light (blue shift/red shift) how does that relate to sound? does it relate to the volume of the noise as it comes closer or gets further away?

Click to read more...

 

[implodein]

im quite certain that your astronomy class does not deal with sound, why would they considering that light travels a **** load faster than it which is why there is the red shift blue shift and not 10db 20db. Sound is produced by osciliating pressure waves, the sound is louder as something is coming towards you because the waves are pushed together, perhaps that has something to do with light as it is coming towards you which makes it a certain color to astronomers, but they are not likely related in any other way.

 

[flakko 5,000+ posts]

i was just making a comparison because i always thought that the doppler effect had to deal with light.

you are correct on the light changing as an object is getting closer/further away, so red/blue : loud/soft ? thats wat confused me.

 

[T3mpest 10+ year member]

haha, 2 science based threads in 2 days, this forums finally getting interesting! I have no problem explaining.

First thing, dont' think of it as loud/soft, it's the frequency that is changing, not the intesity. (techically it's both, but we are in a perfect world for these examples, so disregard any volume changes that would occur as it moves away or toward you)

The dopplar effect involves frequency. All the frequency of any wave is, is the distance between crests, essentially. A speaker moves back and forth creating high and low pressure areas, I think that part makes enough sense on it's own without much needed explanation? The trick is understading exactly how frequency works.

Think about it this way, if a frequency is high, it is repeating very quickly, A 20k tone reproduces itself 20,000 times in 1 second. A 10hz tone does so 10 times. Now let's add in the fact that sound waves move. If both waves move at the same speed, which wave would have more space between each crest? Well a 20k tone reproduces itself alot, meaning that the wave really wont' get very far before another one is produces following just behind it. A 10hz tone will get quite a bit further before the cone moves forward agian to produce another wave. Try and wrap your head around that, just imagine real o-scope type waves being produces and moving.

Now let's add in what happens if the object producing the wave is moving.

Ok, let's say a car is moving away from you playing a 30hz test tone. The very first speaker pulse is sent out. It heads toward you. By the time the first tone reaches your ear you the car is now further away than before right? Meaning the very next wave that is released by the speaker will be much further away than it would have been had the car not been moving. Because of this, the distance between each pulse becomes longer. Since the waves all travel at the same speed, the amount of time between each wave repeating is increased. The amount of time between the waves is exactlywhat frequency is! Think about what you learned in the first paragraph. Because the waves are further apart in reference to you, you percieve that note to be lower than 30hz. The first wave has to travel let's say, 20 meters to reach you. By the time the next wafve is sent out it needs to travel 25, then 30 then so on, since the sound isn't moving any faster toward you it takes longer to reach you, more time between each crest. wave 1...wave2.....wave3.........etc.

Consequently, if the car had been stationary and playing a 20hz tone, the time between the waves may have been just the same as the 30hz tone the moving car was playing. Ala, each wave might have been like wave1.....wave2.....wave3..... Since there is no physical differences between how the air is moving, you end up hearing the same notes, essentially. Basically for all intents and purposes that car is producing a 20hz tone relative to you, any attempts you make to measure it would show that at least.

Light works the same way, our brains interperet as color is nothing more than the changing freqency of light. Light is a wave, just like moving air particles, but they go much faster and have no mass. If a star is moving toward us, the light waves have less distance to go as each pulse is sent out, that makes them closer together. Our brains percieve higher frequencies of light to look more blue.

edit: Just realized you didnt' quite get my last explanation on subs playing 2 notes at once. I hope you can wrap your mind around this one, I'm trying to make it easy. i checked my physics books and it was purely a mathmatical explanation, no help. I guess if you have questions feel free to holla back.

 

[CBFryman2 5,000+ posts]

I dont know if you are talking about doppler distortion or the dopper effect in sound waves comming from a travleing source.

If you are talking about dopler distortion, it is almost undetectable to the human ear with music.

If you are talking about the later you need to watch more malcome in the middle. In other words, yes it does. If the source is moving towards you the frequincy will be higher because as the source moves it compreses the sound waves infront of it (beause it is constantly trying to catch up to the waves it is producing). As it moves away the waves get stretched out as it is running away from the sound waves it is producing. You can actually calculate the frequincy change with a basic equasion but I'm too lazy to remember it from physics.

If you are talking about something else I have no idea. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[Bolognablake 10+ year member]

Good thread, though I think the effect is negligible for most aspects of car audio.

 

[flakko 5,000+ posts]

t3mpest... you cleared up what frequency means to me, i thank you for that, so you are saying as the object making the noise is moving further away from me, i perceive the frequency to be lower than it really is and as it is moving closer, i percieve the frequency to be higher?

 

[CBFryman2 5,000+ posts]

You got it Mr. Flakko. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

Next time Mr. Cool is tryign to show off his motorcycle pay close attention. Sounds high pitch comming towards you and as it goes by you doing 80+mph it sounds much lower.

Works with police sirens as well. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[flakko 5,000+ posts]

cool that makes total sense to me now!!! its just has to do with different senses, i wonder if there is one for smell or touch.

 

[squeak9798 5,000+ posts]

The age old race car example. The engine sounds higher pitched as it coming towards you, and lower pitched as it's driving away from you. Doppler Effect.

 

[yacob.naif 10+ year member]

so would your subs have a lower Fs if you installed them on trailer, and then disconnected the hitch on the highway?

 

[Fast1one 10+ year member]

I have always been a visual person...so heres a few pictures that are similar to the ones that helped me when I took physics...

This first one deals with sound...

And the second with light...

Hope that helps any that still dont understand it //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[wilzyx 5,000+ posts]

just watch fantasia, thats where i learned all i need to know about frequencies.

 

[cool9 10+ year member]

The example they gave us in physics was a boat travelling thru the water at some speed. The waves it creates in the front become closer together because the boat is moving forward and pushing the waves closer together. The waves behind the boat are moving farther apart because the boat is travelling away from them.

This is analogous to anything that moves and is making sound: The definition of high frequency sound are waves that are close together and low frequency sound are waves that are far apart.

Doppler effect pops into my head everytime I hear an ambulance or fire engine drive by. You can easily hear the tone drop as the vehicle passes you by. The faster the vehicle is moving the greater the tone drop.

Sound is created when air is being "pushed" or moved.

Grab a physics book, it's one of the easiest physics principles to understand.

 

[T3mpest 10+ year member]

cool that makes total sense to me now!!! its just has to do with different senses, i wonder if there is one for smell or touch.

Good, you've discovered a slightly higher truth than what you were looking for. You caught the correlation between physical things happening and your brain interperting it as a essentially non-existent "feeling". Anyway, I don't believe any exist for smell or touch. Smell is the physical particles being caught in your nose and interperted as a smell, it's very sensative! Touch is nerve endings being stimulated by contact. Not much room for that kind of phenomenon.