Do The Doppler

[Updated for QLab 4.1 13JUL 17 ]

This workspace shows how a simple pan effect can be made more realistic, by the addition of a Doppler pitch shift. There are 2 cues. In cue 1 a police car with siren pans left to right. In cue 2 a Doppler effect is added half way through the pan, which simulates the drop in pitch when a vehicle passes an observer. The effect is quite subtle, but it is details like this that will subconsciously register with an audience, and give greater realism and verisimilitude to your designs.

Here’s a screen recording of the workspace:

How it Works.

All the cues are in fire all groups. This means when the cue is triggered, all the cues in the group will fire at the times entered in the pre-wait column.


In both cues the audio cue starts on the left output only (output 1)


I am using a Mono recording, for reasons we will discuss later. It is important to check that the mono recording is set on the matrix to send to both channels like this:

Mono Matrix

This won’t be the case if your default levels are set for normal stereo cues. On a mono cue, the default will only send mono audio to the odd-numbered slider, you will have to enter 0 in the input to cue output 2 crosspoint as in the picture above.

The audio starts a bit abruptly so I have used the fade envelope to create a fade in at the beginning of the cue.

Dopfade in

To create the pan I have added a fade cue, targeting the audio cue ,and set  a fade of 10s  duration, entered in the action column. The state when this cue completes is set in the Levels tab of the inspector. When the fade completes we want no audio coming from the left, so we set slider 1 to -inf, and we set slider 2 to 0 to give full output on the right.


It is likely that the fade with the default curve will give us a dip, so in the curve shape tab of the inspector we can switch to ‘custom curve’ and reshape the curves to something like this:


Finally, after 12 s pre-wait another fade cue, targeting the audio cue is triggered,  which fades the audio out, and because the stop target checkbox is ticked, stops the audio cue.


Cue 2 is similar to cue 1, with the addition of another fade targeting the audio. This fires at the midpoint of the pan cue, i.e. 5s and has a much shorter duration. Instead of altering the level it instead fades the playback rate, lowering the pitch of the sound in .8 secs, to simulate the Doppler effect.


It is important to select an appropriate recording to use for the effect. If you want to be in control of the pan then you probably want to use a mono recording. If you have a stereo recording, where the position of the vehicle is clear all the way through, and there is already a point in the recording where the Doppler shift occurs, you are not likely to have much success trying to alter the depth and perspective of the original recording. You can reverse the direction by altering the matrix, e.g.  left to right pan goes from right to left instead:


You can also exaggerate the pan in the stereo recording by starting hard in one channel and finishing hard in the other while retaining the additional depth that using a stereo cue will give you at the midpoint. To do this you just use the sliders in the same way as for the mono cue in the example.

You can download this workspace here.

Chapter Author: Mic Pool

Audio in Demo: recorded by MultiMax2121 public domain from