What is the difference between "monochromatic" and "impulse" force? In a paper I am reading (linked below), the following is stated:

The transient motions of the sphere and the gas bubble in the elastic, incompressible, inviscous medium are investigated in response to a time-varying force. Both monochromatic and impulse forces are considered.

What exactly is the meaning of a "monochromatic force," and an "impulse force?" Is it as simple as a force modulated at a constant frequency vs. a force composed of multiple frequencies of modulation, or is it something else?

In case it is helpful, the paper in question is:
Y. A. Ilinskii, G. D. Meegan, E. A. Zabolotskaya, and S. Y. Emelianov, “Gas bubble and solid sphere motion in elastic media in response to acoustic radiation force,” J. Acoust. Soc. Am., vol. 117, no. 4, pp. 2338–2346, 2005.
https://doi.org/10.1121/1.1863672

 A: Those terms refer to the two simplest kinds of time dependences that the force can have.  The monochromatic force oscillates at a fixed frequency, $F=F_{0}\cos(\omega t+\phi)$ for some angular frequency $\omega$ and phase constant $\phi$.  This is called "monochromatic" because electromagnetic radiation with this kind of time dependence consists only of a single color.
The other kind of really simple time dependent force is one that acts instantaneously.  The force acts for a time $dt\rightarrow 0$, delivering a total momentum change $\Delta p$ in that time. This change in the momentum is known as the "impulse."  If a functional representation of the impulse force is required, you need to use a Heavyside $\delta$-function (which is not really a function), $F=(\Delta p)\delta(t-t_{0})$, where $t_{0}$ is the time at which the momentum transfer occurs.
A: What they also can refer to is that in the field of medical ultrasound you can have two types of excitation - continuous and pulsed-wave. Typically, nowadays pulsed-wave excitation is used in ultrasound elastography applications (which could be one of the applications for this seemingly fundamental discussion in the paper). Pulsed-wave excitation is achieved by applying a short-duration (up to 100-200 microseconds) acoustic radiation force by means of a focused ultrasound transducer, so here is your impulse force. However, initial ultrasound elastography techniques relied on external vibrators, as well MRI-based elastography (there you can't excite an acoustic radiation force with any other way), here goes the reason for considering a continuous (monochromatic) excitation.
Theoretically, both acoustic radiation forces can be excited from the same ultrasound transducer, emitting sinusoidal longitudinal waves of the same central frequency but different duration (long and short). The forces are excited as a result of the absorption of the acoustic energy of the transmitted longitudinal waves. The differences in the motions of the spheres in the field of the acoustic force will be caused by the duration of these signals and hence the duration of the acoustic radiation force application, hence leading to different frequency spectra when analyzing the motion of the spheres. Taking that into account is important in further research if you would for example want to know material properties of the underlying tissue. Without going into detail, then you want to be consider frequency-dependency of the sphere motion for many different reasons. Hence, you do need to take into account different types of excitation causing these differences.
