“How large an effect does wind have on sound at an outdoor show? At some shows it seems like the sound drops in and out quite a bit.”
Wind effects are divided into two classes: velocity effects and gradient effects. Wind velocity – specifically in the form of a cross wind – can shift the direction of the sound, making it appear to come from a different location. Wind gradient effects occur when one air layer is moving at a different speed than an adjacent layer; usually one layer is above the other. Such a gradient might be encountered when the audience is shielded from the wind by a barrier (such as a group of trees or a wall). The effect of this would cause the sound to be refracted upward and downward respectively. Now, having said all of this, the actual effect of wind is usually minimal due to the speed of wind versus the speed of sound – sound being drastically faster. Certainly, the size of your outdoor show and the speed of wind would be deciding factors, but it usually take a pretty strong wind to have a significant effect. Of course, the effect will be cumulative, so the sonic changes will be proportional to how far you are from the loudspeakers. Further, the effect will usually be more pronounced at higher frequencies due to their small wavelengths – longer wavelength sounds aren’t as easily effected by obstacles, wind or otherwise. Sometimes wind can have the appearance of having a large effect on sound, but that is often due to temperature gradients that accompany wind. We’ll talk about this in a later inSync column.