THERE is music in the woods if you care to listen for it. Their medley ranges from the friendly rustle of the leaves on a summer's day to the often plaintiff murmur of a great oak; from the sibilant sigh of a single conifer to the loud wailing dirge of a pine forest. All these sounds have one thing in common: they are rooted in the wind.
The noise of the wind as it whistles through the trees is best understood by recalling, first a sound that nowadays we seldom hear - that of a telephone wire humming in the breeze. As children we used to think that the wire sang when messages were passing through it, but this, of course, was nonsense. A wire stretched before the wind like this disturbs the flow of air, causing it to become unstable and to break up into a sequence of little eddies that are carried on downstream. These "flutterings" of the air are sometimes fast enough to be heard as a musical note.
The flutter frequency increases with the speed of the wind, thus increasing the pitch of the audible note, and decreases the greater the diameter of the wire. Sometimes, if wind is strong enough and the eddy frequency is close to that of the natural frequency of the wire itself - the rate at which it would oscillate on its own if plucked and then released - the alternating variations in pressure above and below the wire cause it to vibrate; this vibration is not necessary for sound to be produced but when present it enhances the volume and makes the sound more persistent by prolonging it during lulls in the wind when the hum might otherwise momentarily die away.
Now, let's go down to the woods again. As the wind passes around a myriad of twigs and branches, the resulting eddies produce notes that vary widely in their pitch and volume the large twigs of a mature oak, bare in winter time, provide a low rumble, while the tiny needles of a conifer produce a high pitched hiss. The blend of sound we ultimately hear comprises, as it were, the average pitch of all the individual notes, and the loudness of the noise is the combined volume of the sum of individual sounds.
When we hear a swarm of bees, the aggregate buzz is pitched to that of the "average" bee. And the group can be heard far beyond the audible range of any one of the individual insects. In the same way, the pitch of the forest murmur is essentially that of the average twig, and although the note of a single conifer needle may be inaudible even at very close range, the forest as a whole is often heard a mile or more away.