# What is the main difference between the acoustic and electromagnetic waves?

For simulation purpose, what will be the difference between using acoustic and electromagnetic waves? Just the carrier frequency only??

Reciprocity, particularly in Air acoustics. If you are down wind from from an acoustic source like an animal and you hear it, you are less likely to be heard by it. The same effect is true with currents in underwater acoustics but to a lesser extent because sound velocity is relatively greater than fluid velocity.

The variation of sound speed tends to be more important in simulations. There are specializations of the wave equation, actually more typically the Helmholtz equation to account for things like $c(z)$, depth dependence.

Acoustic waves do not have polarization, but do have vector particle velocity.

At the other extreme, the speed of light can not be exceeded while the speed of sound isn’t a universal constant.

The Friis equation is not used in acoustics. Isotropic lossless propagation in acoustics is usually inverse R squared. The term “path loss” have different conventions. The Friis equation implicitly includes the aperture of the receiver, so for isotropic lossless propagation, the path loss is proportional to frequency. Radio engineers often (stubbornly) claim that low frequencies propagate further because of the Friis equation. Lower frequencies do often propagate further but typically it is because refraction is related to wave length, so in terrestrial point to point communications, the curve of the earth, scatter by objects like houses, penetration into buildings, and mode coupling between the earth’s surface and ionosphere favor low frequency propagation. High frequencies tend toward line of sight. Friis isn’t why.

In acoustics, performance is typically ambient noise limited while in radio, the limit is usually related to the antenna temperature. When an array is moving through water, flow noise can exceed ambient noise. The calculation of array gain differs accordingly.

Acoustic waves are often encountered in combination with sheer waves. There are also slow waves in EM propagation but sheer wave propagation is essential in seismology.

In biological systems, first principle bulk properties like the Young’s modulus of living lung tissue tend to be avoided because they are very complex to model and directly measure, particularly if you don’t want to kill the subject.

There is also a distinct area of research relating acoustics to perception. Humans (and animals) are better at localizing sounds than classic array theory predicts.

No not just the carrier frequency of course. There are differences between those two because of the differences in their physical nature.

Nevertheless they are both expressed by quite similar mathematical equations (namely the wave equation which is an example of a partial differential equation) reflecting their common wave propagation nature.

However, in line with their nature their interactions with the medium is different and should be modeled accordingly. In addition basic EM wave analysis makes no regardence of thermodynamic laws but the acoustical waves may do but usually ignored for simplicity. And again these kind of physical differences may result in differences in their mathematical modeling and simulation.

Furthermore electromagnetic waves has a huge range of spectrum including Radio (RF) waves, milimeter waves, micrometer waves, thermal waves, optical waves, and X-rays and beyond. One last thing, (optical) EMWs has a quantum theoretic side as well, reflected in the photonic interpretetation of them. That does not exist for the acoustical pressure waves.

• I know that each of acoustic and elctromagnetic wave has its own model and equations, I means the main difference between them is in the wave speed and the center frequency – user24907 Jan 13 '18 at 18:23
• the main difference, user, is that EM waves are based on the physical fact that like charges repel and unlike charges attract. and there is an inverse-square relationship and special relativity applies (from that, a physicist can derive something similar to Maxwell's equations from which a wave equation can be derived). the fundamental physical facts that acoustic waves come from is the effect of compressibility of fluids (it's the gas law for gasses), Newton's second law, and the continuity equations. put those together and you can get a different wave equation than that of EM. – robert bristow-johnson Jan 13 '18 at 22:48