Q: “Does sound really travel the way that graphs and images in books indicate?”
A: We see graphs of wave forms in print so much that it’s easy to start thinking of sound as a two-dimensional, linear occurrence. Most often we see a plot that represents the motion of a vibrating string, which also happens to be a depiction of a sine wave. It could be misinterpreted to imply that the sound moves directionally from left to right. Even when we use the “ripples on a pond” visual example we aren’t accurately depicting sound wave propagation.
In fact, sound waves generally move in a spherical pattern, in all directions, including vertically. If you’re in doubt, climb a tree and note that you can still hear the sound of everything happening on the ground. In some cases steps can be taken to restrict the directionality, such as with a horn, but most sounds will “naturally” seek to disperse in all directions unless otherwise acted upon.
As a pressure wave moves through an increasingly larger space, it does change. First of all, as the energy of the wave is dispersed along an ever-expanding front, it diminishes accordingly until ultimately there is not enough energy to generate further sound. This is a consistent, measurable occurrence – if we consider loss of energy in terms of loudness, the sound pressure level (SPL) drops by approximately 6dB each time the distance from the source doubles if the sound is dispersing in a free space. In other circumstances the losses are less.
Second, the waveform (which is almost always much more complex than the simple sine wave we often use for reference) begins to be reshaped by merging with additional waves in the space. These can be from other instruments, wind noise, voices, machines – anything else that produces a sound.