Different kinds of tree make different sounds. What is the physics behind this?
鈥� Any airflow disturbance, such as that caused by leaves, creates sounds of characteristic volume, frequency and oscillation. Trees鈥� songs change with wind speed and direction, and the type of leaves.
Needle-like leaves, or petioles, shed vortices as the wind oscillates round them, creating the high-pitched, romantic whisper of conifers. Flat leaves flap like flags, depending on thickness, firmness, edge outline and surface texture. This is commonly the main component of the rustling sound. Pointed, narrow willow leaves shed wind energy with whisperings.
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Colliding leaves suffer damage, so they grow in patterns to avoid touch. In high winds, though, impact is inevitable, causing another kind of rustling. Smooth, large, simple leaves tend to give low notes except when flapping vigorously; trees with small leaves, prominent veins, complex outlines, furry surfaces and rough bark seem quieter, but produce ultrasonic sounds.
Crisp autumn leaves act as rattles. Hollow leaves emptied by aphids, and acacia thorns hollowed by ants, may whistle. Dense foliage dampens high notes. Leaves on high branches differ in shape and texture, and encounter higher winds. The leaves of rushes scrape and vibrate like the reeds of wind instruments, giving rise to the Greek legend about their whispering: 鈥淜ing Midas has ass鈥檚 ears!鈥�
Jon Richfield, Somerset West, South Africa
鈥� As the wind blows it causes leaves to strike other leaves or twigs, which creates percussive sounds, and to flex or deform. As the leaves deform, energy is released which is dissipated as sound 鈥� in a similar way to how sound is created when wrapping paper is crumpled or flattened out (see 鈥�A crackling mystery鈥�, New 杏吧原创, 24 December 2005, p 64).
Apart from the speed of the wind, important factors determining the type of sound created include the stiffness of the stalks, proximity of a leaf to its neighbours, and the size, stiffness, weight, moisture content and shape of the leaves themselves. The impact of large, heavy leaves will be louder than that of small, light ones, in the same way that the sound created when you tap a postcard on a table is louder than if you tap with a piece of paper.
Dry leaves are stiff and brittle so more energy is required to flex them, which means that more energy is available for dissipation. Therefore the rustling of the dry, desiccated leaves of a beech hedge in spring will sound quite different to the quieter sound of its pliant summer leaves.
Richard Holroyd, Cambridge, UK