Sound Designing with Winds, Part II
Wind sounds by time of day and season
Wind sounds begin late dawn with the distant roar of wind arriving from far away. The roar is barely perceptible at first, prompting the question: “What sound is that?” The answer soon becomes clear as the quiet roar comes closer and is joined by the rustle of broadleaves or the humming tone of needle-leaved plants and even the whipping sound of grasses--all usually faint.
Broadleaved deciduous plants are nearly silent in spring when their leaves are newly expanded, but as they mature and develop stiffening cells, they’ll rustle similar to tissue wrapping paper. And then, during autumn, as they change color and prepare to drop, they’ll clatter like parchment. If the leaves are on long petioles, leaf collisions can produce dramatic slaps. Leafless branches sound differently. They vibrate with a deep and defining roar.
When it comes to needle-leaved plants, the longer the needle the lower the pitch; the shorter the needle, the higher the pitch. Thus short-needle alpine spruce trees are higher pitched than their longer-leaved white pine cousins in the valleys. Pine trees also produce high pitched clicks under moderate to high winds due to the stiff, prickly-scaled cones.
Grasses fall under the needle-leaved rule, where length determines pitch, but also create characteristic, length-determined whipping sounds.
To demonstrate this principle, firmly grip a plant and wave it through the air. You’ll get a good sense of its wind-driven sound signature.
As wind intensities increase with the progression of the day, gusts are common. Leaf rustle or pine tone or grass whipping becomes more continuous, and are joined by branch clatter. Large shrubs and herbaceous perennial plants also chime in, along with new sounds, such as seedpod rattling.
Strangely, as wind intensities increase for all vegetation types, the acoustic mélange grows into a deepening roar. This is because the human ear is less sensitive to lower frequencies than most other frequencies and greater amplitudes are needed to reach audible levels, but then, once reached quickly dominate our perception.
As the sun approaches the horizon this sound sequence now plays in reverse.
Special considerations for deserts
As used here, desert winds include all areas where the land surface area is dominated by exposed soil, sand, or rock. The same daily cycle of wind occurs here, except the sound is fundamentally different. You hear a light, deep roar and an absence of plant details. Also, because some new deserts, such as lava fields or basalt rock outcroppings, have sharp edges yet unsmoothed by time, a whistling sound may occur. I encourage those interested in recording desert winds to move your microphones into rock enclosures to experience a myriad of whistles with surprising shrill sounds. The longer you listen, the more you’ll hear.
Nature whispers, Man shouts. Know your setting, pick the right wind, and don’t overplay your hand. Add just the right amount (not too much).
Outdoor sound propagation http://en.wikibooks.org/wiki/EngineeringAcoustics/OutdoorSound_Propagation
Wind speed and direction at various locations over time