Why is the pitch shift not symmetric?

Because the two multipliers are not mirror images. Approaching, the arriving waves are compressed by a factor of one over one minus the speed ratio; receding, they are stretched by one over one plus it. The mathematics produces a bigger rise than fall, and the report shows both figures rather than averaging them.

What does the closest approach distance change?

How abrupt the pass feels and how far the source travels across the image. A close approach makes the level swell more dramatic and the crossing faster, while a distant one spreads the whole event out and leaves the pitch change as the main cue.

Is this the same as a pitch shifter?

No. A pitch shifter moves the pitch by a fixed interval; here the pitch is continuously changing because the distance to the source is changing, and the rate of change is derived from the speed. The result is a pass rather than a transposition.

Why is thirty metres per second a lot for a road vehicle?

It is about a hundred and eight kilometres per hour, which is fast for a car. The range goes higher because the model is a physical one and the effect is meant to be usable for aircraft and other fast sources, but most convincing road effects sit at the lower end of the range.

Can I make the source pass in the other direction?

The panning is fixed to run from left to right. To reverse it, swap the two output channels by processing the file again with a channel tool that exchanges them, or convert to mono first and pan the result afterwards.

Doppler Effect Simulator

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Doppler Effect Simulator sends a recording past the listener at a chosen speed and distance, so the pitch rises as it approaches and falls as it recedes. Everything you hear comes from one physical model rather than from a set of effects.

What is Doppler Effect Simulator?

Drop a file, set the speed and the closest approach, and simulate; the outcome is the Doppler shift, a level swell and a stereo flyby that crosses from left to right. The model is driven by the file's first channel, so a stereo source is reduced to one side before it is sent past the listener.

The controls:

A speed from five to eighty metres per second, defaulting to thirty, shown alongside the equivalent in kilometres per hour.
A closest approach distance from two to eighty metres, defaulting to twenty, which is how near the source passes.
A fixed panning spread of thirty units, which is what carries the source from one side to the other as it passes.
A report giving the speed, the distance, and the peak pitch shift in cents in each direction, with the arithmetic shown.

Three things happen at once, and all of them fall out of the same geometry. The pitch shift comes from the change in distance between source and listener over time: while the source is approaching, each successive wavefront has a shorter distance to travel than the one before it, so the waves arrive closer together and the pitch rises; once it is receding the reverse happens and the pitch falls. The shift is asymmetric, and that asymmetry is a property of the physics rather than an adjustment. Approaching, the factor is one over one minus the ratio of speed to the speed of sound; receding, it is one over one plus the same ratio. Those two multipliers are not mirror images of each other, so a source passing at thirty metres per second rises further than it falls — the panel reports both figures separately for that reason. The level swells toward the moment of closest approach and falls away afterwards, following the inverse of distance, and the panning carries the source from one side to the other as it crosses. The speed of sound is taken as three hundred and forty-three metres per second, which is the standard figure at ordinary room temperature. Amounts here are gentle compared with what a real pass-by at speed would produce, since eighty metres per second is already faster than any road vehicle, and the effect is meant to read as movement rather than as a stunt.

Where the model is simplified:

The source travels in a straight line at constant speed.
No reflection, absorption or environment is modelled.
No acceleration, so there is no rise in pitch from a revving engine.
The speed of sound is fixed rather than adjusted for temperature.
The output is always stereo, whatever the input was.
Output is sixteen bit WAV.

Use it to give a passing vehicle, a flyby or a moving sound source the sense of travel, or to hear how a passage sounds when its pitch is pulled through a rise and fall. When the pitch movement should be an oscillation rather than a pass, the vibrato does that with a delay line, and when the movement should be one of position rather than pitch, the 8D audio converter rotates a source around the listener instead of past them.

How to use Doppler Effect Simulator

Drop an audio file onto the panel; the flyby is built from its first channel.
Set the speed, remembering that thirty metres per second is already a fast road pass.
Set the closest approach distance, which controls how dramatic the pass is.
Simulate the flyby, check the reported pitch shifts, and download the stereo WAV.

When to use Doppler Effect Simulator vs related tools

Doppler Effect Simulator is the tool when a sound has to travel past the listener, which is what makes it useful for vehicles, flybys and moving effects. 8D Audio Converter rotates a source around the listener instead of past them, and Stereo Widener changes the width without any movement. When the pitch movement is a steady oscillation rather than a pass, Vibrato Effect is the closer match.

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