November 29 is the 205th birthday anniversary of Christian Doppler, the Austrian mathematician and physicist who hypothesized what’s now called the Doppler Effect. (You know, that effect when an ambulance or other fast-moving vehicle flies by and the perceived pitch changes.) That calls for Doppler trivia, astrophysics, audio software, and a drink.

In celebration, go check out the excellent Wikipedia page on the Doppler Effect, including one of my favorite Physics equations (while I wasn’t failing.) And if the idea isn’t sinking in, there are plenty of online demonstrations of why this effect occurs. (Science aside, I also recommend celebrating by imitating the sound of an English police car driving by in a movie. It works best if you simultaneously run by your significant other at high speeds.)

Because light can be a wave as well as a particle, the Doppler effect applies to light as well as sound. An increase in the observed wavelength of light emanating from a star is called a redshift. The principle is the same: as the source gets further from you, the wavelength (what it sound we perceive as pitch) shifts; in the case of a star, that translates to observed color.

Here’s the mind-bending caveat: there’s a misconception that Doppler-like redshifts are what allow astrophysicists to measure the expansion of the universe. Wrong! Why? Because it’s not the stars moving away from you (a la the Doppler ambulance); it’s the intervening space stretching, as per the understanding of General Relativity. There you go; if that hasn’t convinced you to go have a drink in honor of Christian Doppler, nothing will. (Heck, it’d probably convince him to have a few drinks, were he alive.)

Back to digital audio: If you want to reproduce the Doppler effect accurately, GRM Tools Classic has one of the best Doppler plug-ins I know of, available for both Pro Tools (RTAS/TDM) and VST. See the detailed review from Electronic Musician of a few years ago. GRM Tools is a great collection of plug-ins, but if you’re on Windows you can also opt for the much-cheaper GBP 15 a la carte option, Spacestation (VST). (Thanks, Afro!)

Or just go have that Dopplertini. (Anyone got a good recipe? I think it involves throwing the drink at high velocity . . .)

  • atomic_afro

    Oli Larkin's Spacestation


  • admin

    Thanks, Afro. I'll go try that out on my PC.

    Added the link to the story and, thanks to all Oli's other plugs, this calls for a separate item.

    Sorry I just deleted the link from your comment; those are still screwing up the template. One of the MANY things that will be fixed when we move the official site over to our in-development CDM 2.0 in WordPress! Can't wait!


  • enigmania

    As a physics technicality, in General relativity it's not really less 'correct' to talk about the stars moving away as to talk about the space in between stretching. The difference is just in how you lay down the coordinate system on the space. If you use coordinates that are expanding with the stars, you'd say space stretches. If you use cooordinates that maintain a fixed distance from some reference point, then the stars are moving with respect to those coordinates. GR tells us that the 'real' physics is the coordinate invariant stuff, and the two ways of thinking about it give the same redshift result.

  • You mentioned in the article that as the sound gets further away from you there is a drop in pitch, and the redshift isn't caused by the stars moving away from each other but rather the stretching of space. I understood what you meant, but I'm sure there are some people reading that who will think that means the changing distance between objects is what causes a doppler effect.

    To prevent people from thinking the doppler effect is based on distance rather than velocity I thought I might offer a clarification. A wave form will always travel at a constant rate based on the position it originated. If you're traveling at a different speed than the original source then when the wave hits you it will appear either longer or shorter than the true wave form actually is. That relationship between the speeds of the objects will cause a change in pitch for sound waves, or a red-shift for light waves.

    So the distance between objects will cause a observable difference in intensity of a wave, the the relative velocity of the objects will cause an observable difference in length of a wave. The doppler effect is what happens during the moments immediately before, during, and after the source of the wave form overtakes the observer.

    If an object is travelling faster than an observer then…

    As it approaches the observer the intesity will be constantly increasing, and the wavelength will be consistent based on it's relative speed. (sound will get louder and pitch will be higher than the actual sound should be, light will get brighter and be blue-shifted)

    As it passes by the observer, the intensity will peak and the wave length will equalize. (sound will reach it's peak volume based on the disance of the objects and pitch will be the correct pitch, light will reach it's peak brightness and will be it's true spetrum)

    As it recedes from the observer the intensity will be constantly decreasing, and the wavelength will be consistent based on it's relative speed. (sound will get quieter and pitch will be lower than the actual sound should be, light will become dimmer and become red-shifted)

    The doppler effect is caused by how those three things happen based on the relative trajectories of the two objects. The time length of the doppler effect is determined by the distance and angle of the two trajectory paths.


    relative distance determines observable intensity

    relative velocity determines observable wavelength

    relative tragectory, distance and velocity causes the doppler effect.

    It was an interesting note you made regarding the expansion of the universe is caused by space stretching rathe than the objects actually moving apart, but from an observation standpoint it doesn't really change anything. Even if it's caused by a stretching of space, it still results in relative distances, velocities and trajectories between objects in the universe. The red-shift is still caused by a difference in relative velocities between objects and still gives support to the theory of an expanding universe.