Hitting a wall of sound

by Jim Barbour

Sound may be invisible, but it’s physical impact can be very real. On a recent holiday in the north of Victoria, we were sitting in a National Park by the Murray River, more than twenty kilometres from the nearest town. The Murray is an old, slow moving river, winding its way through dry scrub and forest. The land around is flat and arid, and only green where irrigation has brought the land to life. Little rain falls there and the river forms the lifeblood of the region, for flora and fauna. The eucalypts are healthy enough, surviving on the moisture from the river and growing to maybe ten metres in height, on average, though the mighty river red gums often double that, with a substantial girth. There is little groundcover and the local animals, including kangaroos, emus and wombats, have adapted to this environment.

The bird life is flourishing though, with many species thriving around the river and small local streams. Eagles are perched high in the gums, ducks and swans on and around the water, parrots and other smaller birds throughout the forests and scrub. The screeching of the cockatoos, in particular, reverberates through the trees and bounces back from across the river. Listening intently reveals a wealth of calls and the immersion in the soundscape is an experience well worth the journey. At this point, the river follows a long slow arc, bending away from us in both directions. We sat enjoying our picnic about ten metres above the waterline and had a clear view in all directions. This is a place where human made sounds are infrequent, and in the hours we spent, two motor boats passed our spot. We saw them long before we heard them, and I focused on the fine detail of the onset of the sound, its volume and timbral changes as the boat approached, the Doppler shift as it passed and the subtle changes as the boat left us and the soundscape reverted to the bird calls again. A couple of planes passed some distance away; the sounds were very quiet and the spatial movement was subtle but fascinating.

After several hours quietly enjoying the river, I noticed a flock of ducks, maybe about a dozen, flying a few metres above water headed towards us from downstream. They glided along in a loose formation, as had several previous flocks, moving from one feeding ground to another, with small talk amongst themselves the main sound heard above the flapping of their wings. Just as they passed directly in front of us, we were assaulted by the din of two or three trail bikes, a couple of kilometres upstream in the park, roaring down to the rivers edge and off into the forest again. It certainly startled us, but the effect on the ducks was astonishing! They hit the wall of sound generated by the bikes and broke flight as one, tumbling onto each other for a second, literally as if they had hit a solid wall. They fell vertically a metre or two before they regained their composure and flight sense and veered off towards the far bank. They wheeled up and over the trees and were last seen heading towards distant forest and safety.

We also regained our composure, but the peace and quiet of the place was gone now, as the bikes proceeded to career around the park, closer to our position until they passed nearby. We decided to leave them to it, and summoning our flight response, headed back into town. The image in my mind of that wall of sound moving down the river and through the forest still gives me pleasure and I easily imagine now the waves of sound radiating out from even the quietest sound source, interacting with the environment.