Dismiss Notice

Psst... Ready to join TalkBass and start posting, make new friends, sell your gear, and more?  Register your free account in 30 seconds.

The Doppler effect

Discussion in 'Miscellaneous [BG]' started by Davidoc, May 1, 2003.


  1. I've been learning about the doplar effect in physics, and it has me a little confused. I've been wondering about this for a while. Like for example on a vibraphone, when you spin the disks. Is the pitch actually changing due to the dopplar effect? It sounds like the fundamental stays the same, yet it sounds like it's getting higher/lower. I checked with a tuner, and it didn't show a change.

    Where can I find more information on this subject? Does it actually change the pitch? I don't understand. If this were the case, then wouldn't it make it impossible to play in tune with anybody? I don't get it. Can someone explain?
     
  2. Nick Gann

    Nick Gann Talkbass' Tubist in Residence

    Mar 24, 2002
    Silver Spring, MD
    With a vibraphone, it almost sounds to me like the wah equivalent for percussive instruments. And with a wah, the pitch is the same, it is just a very rapid EQ shift. When it is the W sound, it is all lows, the AH sound is all highs. I don't know if that is it, but that is what it sounds like to me, not a tremelo-like effect.

    Then again, the tuner you use might have been slow tracking *shrugs*

    I don't think the doppler effect is the thing that makes vibes sound they way they do.

    A good example of doppler effect is race cars. The sound coming towards you increases in pitch as the sound waves are compressed by the incoming car. The sound decreases in pitch as it goes as the soundwaves get farther apart.
     
  3. Nick Gann

    Nick Gann Talkbass' Tubist in Residence

    Mar 24, 2002
    Silver Spring, MD
    And the doppler effect does not change the actual pitch of the sound. It changes the percieved pitch, percieved by a stationary position (e.g. you).

    Another example is the Red Shift of planets, which also proves that the universe is expanding and planets are drifting apart. Red is a lower frequency lightwave, and as the planet moves away, the actual color frequency is lowered until it is percieved[/] as red, but in reality is blue, green, yellow, or purple.



    I did really well in physics :smug:
     
  4. Nick Gann

    Nick Gann Talkbass' Tubist in Residence

    Mar 24, 2002
    Silver Spring, MD
  5. SuperDuck

    SuperDuck

    Sep 26, 2000
    Wisconsin
    The Doppler effect is only caused by an object that is in motion, in relation to another stationary object.

    The actual pitch of the object doesn't change, just the listener's perception. If you want to figure it out, here's the formula:

    f0/V-v0 = fs/V-vs

    Where f0 is the frequency observed by the stationary object, fs is the actual frequency at the source, and the velocities are plugged in accordingly. (EDIT: V is the velocity of the sound. My bad for leaving that out.)

    As far as I know, a vibraphone doesn't have much to do with the doppler effect. Instead of a true vibrato, I think it's more of a tremolo. (With a vibrato being a change in pitch and tremolo a repetitive change in volume over time. A whammy bar is a vibrato, not a tremolo for example.)
     
  6. Thor

    Thor Moderator Staff Member Gold Supporting Member

    The classic example of the doppler [ not doplar ]
    effect is the train moving toward you and then passing you on the track and heading away.
    |
    eeeeeeeeeeEEEEEEEEAAAAAAWWWWaaaaaawww

    So we know the train whistle is the same,
    we know your ear did not change in its perception in that 30 seconds, what we know is that the time the sound waves took less time to reach you while the source was accelerating towards you at a high rate of speed relative to the speed sounds waves are normally propagated at, compared to when the train receded from your ear.

    The waves bunched up and you heard a higher tone, [ than you would have if you heard the train whistle travelling next to the car at the same speed ].

    Ain't acoustics fun? Before you know it you'll want to know about LESLIE speakers, redshift in astronomy, 7 string basses and weird hair.
    Lookout, TB transmogrification here!
    [Quote}
    I don't understand. If this were the case, then wouldn't it make it impossible to play in tune with anybody? I don't get it. [/Quote]

    In the microscopic universe, that would be true, but in the universe within the range of our perceivable aural senses, it is a not a factor, as if you were on the train, we wouldn't be playing together, would we? And if I was on the train with you, we would be at the same relative speed as you, same for if we were both travelling at 99% of the speed of light.

    Boost
    O
     
  7. Saetia

    Saetia

    Mar 27, 2003
    Wisconsin
    Props to Nick Gann on knowing what Red Shift is!!!

    Im glad im not the only physics buff in here:)
    it also allows us to graph and mark where the universe started, the POO(haha Poin of origin) crazy stuff!!!
    Peace
    -Ben
     
  8. moley

    moley

    Sep 5, 2002
    Hampshire, UK
    I've always thought the police car with the siren on is the classic example. Y'know how it goes flat as it speeds away from you...
     
  9. Nick Gann

    Nick Gann Talkbass' Tubist in Residence

    Mar 24, 2002
    Silver Spring, MD
    Ain't physics great?:)

    It's the only class I remember from 9th grade :D
     
  10. geshel

    geshel

    Oct 2, 2001
    Seattle
    SuperDuck: if the source is moving, the pitch of the sound is actually different (than the frequency of whatever is vibrating on the source to create the sound.

    If the observer is moving, the pitch of the sound does not change, they merely have a skewed perception of it.

    Funny thing, the doppler effect for light isn't like this since there's no "air" equivalent for the light to travel through - light always moves at the speed of light (thank you Einstein). So the doppler effect for light is the same no matter if source or observer is moving - it's only the relative velocity between the two that matters. Which is good, because as relativity states, nobody knows which one is really moving! (in some cases at least). Which reminds me,

    Saetia, the point of origin of the universe is right here (whoops, there it went! just kidding). Seriously though, everywhere is the center of the universe. All points are expanding away from all other points as the universe expands. Though I think my cosmology is a bit out of date so this might not be quite right anymore. . .;)

    I don't think the vibraphone has anything to do with the doppler effect.

    http://www.netspace.net.au/~pertout/vibraphone.htm

    Sounds to me like, when the discs cover the top of the resonator bars, it makes the sound lower in volume, and when they uncover them it increases. So, what SuperDuck said. (I always get vibrato and tremelo mixed up).
     
  11. SuperDuck

    SuperDuck

    Sep 26, 2000
    Wisconsin
    Here's how I understood it. The source is moving, say a train, right? The person at the station is hearing the difference in pitch. If you were at the source, running alongside (in a car or something) the pitch would still be the same. The perceived pitch is different to the observer, but never changes at the source, not matter if the source is moving or not. But I could be wrong. ;)
     
  12. Nick Gann

    Nick Gann Talkbass' Tubist in Residence

    Mar 24, 2002
    Silver Spring, MD
    Speed and motion is relative. It can only be measured between two objects. If you are driving at 30 mph, and a guy passes you at 50 mph, he would be moving away from you at the same rate if you were parked and he passed at 20 mph. Its all relative.

    In reality, the only stationary object in the universe is the very center of the universe. But then again, who's to say what's outside out universe? ;)






    *Light has properties of both matter and waves, but the perception of light is based on waves.
     
  13. thrash_jazz

    thrash_jazz

    Jan 11, 2002
    Ottawa, Ontario, Canada
    Artist: JAF Basses, Circle K Strings
    Red shift is NOT the same as the Doppler effect.

    Doppler effect is a result of relative speeds of the source, combined with the speed of sound. You cannot add relativistic (that is, close to speed of light c) velocities in the same way.

    For instance, if I was traveling on a space ship at 0.75c and shot a projectile forward at 0.5c, the speed would obviously not be 1.25c! (don't ask me to work out the actual answer please). Redshift is based on the expansion of the universe and the resulting change in the wavelength of light, IIRC.

    I don't understand Nick - you say that nothing is really stationary but how is that relevant?
     
  14. Lots of good explanations of doppler here. Let’s try and visualize what’s really happening.

    Ex A:
    Think of dropping a bowling ball into a calm pool of water. The bowling ball just set a series of waves into motion in the surface of the water ( the wavy ripples that you see). Now, if you were in the pool and in the wavy ripples were going past you, but you remained in one place (not moving)… the waves will pass you (the crests/troughs) at a certain speed (or frequency ).

    Ex B:
    Now, if the bowling ball was dropped in the pool, setting the waves in motion, and you were in the pool and walking toward the source of the waves, the waves (crests / troughs) would pass you at a higher rate than Ex A, because of relative motion.

    Ex C:
    If the bowling ball was dropped in the pool, setting the waves in motion, and you were in the pool and walked away from the source of the waves, the waves would pass you at a slower rate than Ex A OR Ex B, because of relative motion.

    Sound does the same thing. With the water examples, we observe the crests and troughs of the waves. In sound, we observe what is called cyclic compression and rarefaction (think ‘crests and troughs’ of air pressure) of the air. But, the relative motion between the observer and the source is what causes the Doppler effect.

    Fact:
    With the ‘train whistle coming toward you’ example, the pitch is not only heard by the stationary observer, but measurable by a stationary pitch/frequency-sensing device. I.e. if something was right beside you and measuring the frequency of the train whistle, it would read high until the train passed and was going away, then it would read lower.

    I was a sonar technician in the Navy and I used doppler to track submarines. It was fascinating and fun....
    Hope my explanation makes sense.
     
  15. Killdar

    Killdar

    Dec 16, 2002
    Portland Maine
     
  16. Nick Gann

    Nick Gann Talkbass' Tubist in Residence

    Mar 24, 2002
    Silver Spring, MD


    I was refering to this part of Geshel's post. The part that I italicized is the part I am directly refering to.

    I was saying that movement and speed is relative to emphasize that the doppler effect of sound is the same as what he said the doppler effect of light is.

    Does that clear up what I said? Sorry for the confusion :)
     
  17. moley

    moley

    Sep 5, 2002
    Hampshire, UK
    Ordinarily, you're right - but what thrash actually said was 1.25c, and c is the speed of light. So, it can't be moving at 1.25c, because nothing can move faster than c, right?
     
  18. Saetia

    Saetia

    Mar 27, 2003
    Wisconsin
    THe difference i can see between the sound and the light waves are as i believe act the same in theory, but in actuality(sp?) it comes to some variants of other forces acting upon them, if there is a lilly pad in the pond, the waves disperse around the lilly pad, sound bounces off of walls, ect, and light can be bent, through water vapor and other variouse things to be seen around coners, a fact of this would be a mirage in the desert, the allusion of water or a paridise thats not there, or looking at a hill as you drive up it on a hot day, and you see the cars top before it actually comes over the hill, they each loose part of the wave or are weakened by what is acted upon them, with light it gets bent and contorted.
    as i go on and on i bet i sound stupid, but they are similar in aspect
    as for the doppler effect, i remember watching a thing about it when i was younger on bill nye the science guy, the sound is more accute infront of the train or what not because it is being compressed and is actually being pushed at you, as it moves past you the waves disperse and are longer, kinda like S and P waves in an earth quake to an extent( im in no way saying they are exatally the same)
    as for the Red Shift, the origin, not nessesarily the center(which there would be 2 of because the universe is an elipse like out galexy and our planetary revolution around the sun) of the universe has been triangulated, and for the longest time was thought to be this star that had collapsed, but not releasing a supernova i cannot remember the name ill look into it, but now with higher powered telescopes we can see that this imploded star is not the origin of the universe, but actually one that we can only see for one day a year, that is directally hidden behind it, a massive black hole,
    anyway, this has gotten way from music, if anyone wants to contiune this on a non-musical stand point IM me, i know some break through stuff on teh actual make up of black wholes they arnt really sphears, they are actually Doughnut shapped, anywho
    peace
    -Ben
     
  19. Nick Gann

    Nick Gann Talkbass' Tubist in Residence

    Mar 24, 2002
    Silver Spring, MD
    What about the Millenium Falcon?


    Seriously though, is there any way to really prove that light is the fastest thing in existance? (honest question, not sarcasm or rhetorical). How can that be proved? It is all theory, right?
     
  20. Saetia

    Saetia

    Mar 27, 2003
    Wisconsin
    Serius B
    thats the name of the star that imploded and sparked the universe.!
    peace
    -Ben