Difficulty in understanding non-oscillatory waves

[tsienni]

I understand that a wave is most often oscillatory in character. That said, it does not have to be the case, for a wave is simply not the same as an oscillation: the former refers to a spatial pattern whereas the latter to a variation in time. We may think of a wave formed by an infinite number of motions (typically oscillatory), one at every point in space and all generally different. Now, for a non-oscillatory wave, there's just a single big disturbance that passes anyone point for merely a short time.

Examples of a non-oscillatory wave:
a) the wave thrown off by the bow of a speedboat
b) the sonic boom from a supersonic plane
c) the sound wave emitted from a single gunshot.

If you take a snapshot at any given time, a non-oscillatory wave pattern consists of only one localized disturbance plus tiny motions seen anywhere else. How could the wave, then, be properly propagated in time? A moment later when you do an observation yet again, you see the point where the last disturbance takes place is now virtually motionless. How could this happen at all?

 

[Claude Bile]

The term wave applies to all solutions of the wave equation.

Solutions to the wave equation are all in the form f(x-vt), i.e. solutions to the wave equation all have one thing in common, they propagate with a velocity, v.

These propagating solutions include, but a certainly not limited to sinusoidal wave solutions, however it is possible, via Fourier theory to express any waveform as a linear combination of sinusoidal waves. Even aperiodic waveforms can be expressed in this fashion.

The wave equation is not a fundamental equation, it is derived from other, more fundamental laws. Asking why waves behave as they do is like asking why the laws of mechanics are as they are, or why Maxwell's equations are true.

Claude.

 

[sir-pinski]

It is understandable that non-oscillatory waves can be difficult to understand, as they do not follow the typical pattern of oscillatory waves that we are used to seeing. As the content mentions, a wave is simply a spatial pattern, and it can be formed by an infinite number of motions, not just oscillations. This means that a non-oscillatory wave can still have a defined shape and direction, even though it may not have the same repetitive motion as an oscillatory wave.

The examples provided (the wave from a speedboat, sonic boom, and gunshot) are all great illustrations of non-oscillatory waves. These waves are caused by a sudden disturbance or change in the environment, which creates a single, localized disturbance that propagates through space. This is why, when taking a snapshot at any given time, we only see one disturbance and very little motion elsewhere. This can be confusing because we are used to seeing waves with multiple oscillations at different points in space.

However, the key to understanding non-oscillatory waves is to remember that they are still waves, just with a different type of motion. The disturbance may be short-lived, but it still has the ability to propagate through space and time. And as the content mentions, when we observe the wave again, the point of the disturbance may now be motionless, but the wave itself has already moved on and affected other points in space.

In conclusion, while non-oscillatory waves may be challenging to understand at first, they are still an important part of understanding wave behavior. They may not follow the same patterns as oscillatory waves, but they still have the ability to transfer energy and information through space, making them a crucial concept in the study of waves.