For this new article on the blog, I decided to go into a new domain of engineering I never talked about until now:
Acoustics and Sound Wave Propagation!
It’s always exciting to enter and discover a new type of physics because I start to see more possibilities around me and it allows me to understand the world better!
As usual, let’s break down the most complex things to the simplest principles, and then let’s see how to simulate all that with FEA ;-)
Ambitious, isn’t it?
First things first
What is a sound wave and how sound propagates?
I already gave you an important information here…
Sound is a wave!
When I heard the word « Wave », it does make me think about that:
A position in space emitting an electro-magnetic field which is able to transport energy to another point in space…
It’s a nice engineering representation of things, but it remains a bit unclear of what a wave is physically.
Electro-magnetic waves (such as light) are just one type of waves… there are many other types!
Let’s take a simpler example:
A drop of water which falls into a static pan of water creates ripples which are also a certain type of physical waves
To understand this, we have to see the water as a medium of particles rather than a continuous medium.
At t= 0s, when the drop falls into the water, the particles of the water are at rest
At t=timpact, the particles at the impact point start to move and impact the other particles around. Water isn’t still anymore and this energy for the impacting particles start to propagate in the water.
After that, we can see a ripple forming at the top of the water surface, reflecting the particles which are moving at a position higher than the water surface.
Why is it important to remember?
Physical Waves are created by a microscopic movement of particles
The shape of the wave you get depends on the type of excitation that you provide to your particles
If you excite them in a periodical manner, you get a periodical wave too.
What about Sound waves?
Think about a device which makes sound such as a speaker:
Speakers make sound by moving rapidly its front part called its diaphragm
The diaphragm moves in a back and forth motion… which is usually called a mechanical oscillation:
We got our periodical exciting load :)
Note that it’s oscillating too fast and the human eye can’t see the movement so it looks static (but it’s not ;-) )
Why is this back and forth movement important?
This movement of the speaker causes the air in front of the diaphragm to oscillate as well
But let’s be clear the air oscillate, but doesn’t actually move!
Moving air would be called « Wind » rather than « Sound » ;-)
What happens is that the oscillation of particles cause other particles next to them to oscillate as well.
Air is in an oscillation movement, so there is kinetic energy!
And Energy can be transmitted from the speaker to your ear without air particle actually moving
How about other medium, such as steel for example?
It’s exactly the same!
Steel is a solid, but it is also constituted of particles!
A wave doesn’t cause the particles of the medium to actually move, it transfers energy through oscillation of the particles.
Because of that a wave can propagate even in a steel medium.
By the way, a medium is just a name we use to call the material through which a wave is travelling
What is Acoustic Pressure?
As we are going into simulation, you will often see colored plot like that:
Here’s a good definition from Wikipedia that explains what acoustic pressure means:
Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. (Wikipedia article is here)
Ok, now…
How can I simulate that with FEA?
There are many codes that can do acoustic simulation in the market, but I decided to show you the fastest of all of them, a software called OnScale.
OnScale integrates with Amazon Cloud allowing you to run million degrees of freedom analysis very quickly. It has also a top notch multi-physics solver and an easy to use graphical interface which allows you to run acoustics and various electro-acoustics analysis very smoothly.
Here’s the video of the tutorial:
What you will learn in this video:
- How to create a simple geometry model in OnScale
- How to assign time-varying pressure loads and boundary conditions
- How to set up a material and setup your grid mesh
- How to simulate the propagation of a 2D Wave and obtain acoustic pressure results
You can download the software OnScale here:
Ok, that’s all for today ;-)
I think that’s already a good introduction to the domain of acoustics…
In the future, I’ll prepare more detailed tutorials for you so makes sure that you are subscribed to the newsletter to receive the newest articles!
In case, you have questions, things you want to simulate, things you would like to see, leave a comment in this article to let me know, I read every comment I receive through the blog
Thank you :)
— Cyprien “Getting into Acoustics” Rusu
Rama Krishna says
This is really great explanation that helps beginners like me in acoustics. Thank you very much. By the way, I keep reading that rapid expansion of a medium (maybe both solid and liquid) due to thermal effects (i.e. thermoelastic expansion) will result in stress waves (and also shock waves?) to be generated. Can you tell me how the particles in the medium are moving in its case? Are they still going back and forth like in the case of this diaphragm? Also, how are stress waves and shock waves in this thermoelastic case different from the acoustic waves you described?
Thank you so much in advance,
Rama Krishna
Cyprien says
Great questions Rama! I will try to explain those in the next articles ;-)
Rama Krishna says
Thank you Cyprien. I will look forward to those articles. Meanwhile, do you mind to give me at least a short intro to that topic, just a line or two about how the particles are moving in the thermoelastic expansion case? Thanks in advance.
Cyprien says
Hi Rama,
Actually it works exactly in the same way in the case of a shock wave in a fluid, the only difference is that the medium becomes compressible only over a certain limit of temperature/pression. You can understand that water particles are more mobile than inside a solid so if the temperature/pressure are too low, the vibration will be absorbed by the movement of the particles, but if you increase temperature/ pressure, your particles become less mobile and the medium becomes better for the propagation of waves. That’s how I see things now by I am not 100% sure either so I wanted to check in some books before writing it out there on the blog ;-)
Rama Krishna says
Thanks Cyprien. The way I have understood ithermoelastic expansion s that, when a portion of a solid is exposed to intense heat (e.g., maybe through a laser), the solid expands locally but rapidly. That rapid local expansion of the solid creates a stress wave that travels at the speed of sound. However, if the heating is above a certain threshold, the local expansion of solid may create a shock wave (instead of the usual stress wave) that travels at a speed faster than sound. If all of this makes sense, what I am wondering is about the particle motion. Are the particles in the solid moving back and forth in this case? If yes, is it because the portion of solid that is receiving the heat is expanding and then compressing and then expanding…something like the diaphragm of the speaker you talked about?
Cyprien says
The back and forth movement I described is for a sinusoidal type of excitation. If your input is a laser, It is more like a pulse applied during a very short time, not a back and forth movement. This pulse causes a very rapid movement of the particles, further accelerated by extreme temperature, creating even higher momentum. Heat creates also local change in pressure and density. Over a certain temperature and pressure condition, there is indeed creation of a shock wave (you have to calculate the Mach Number if I remember well). As you are in adiabatic conditions, you can also use the equation PV**gamma = cst, this gives you the relation between pressure and volume for an ideal gas.
Eash says
Hi
Instead of acoustic,how to make a transducer excite and make those wave?
Can you do it in this
Cyprien says
Check this new article: http://feaforall.com/how-to-simulate-a-pzt-disc-in-onscale/
That’s how you can excite a transducer.
Instead of a simple PZT Disc, you can model a full transducer and simulate it.
Rama says
Hello Cyprien,
I was wondering about how thunder is able to produce sound. This site (https://www.loc.gov/rr/scitech/mysteries/thunder.html) states that it does so by rapid expansion of air. Based on your article, sound requires the air particles to be in an oscillatory motion i.e. they must go back and forth. I guess ‘rapid expansion’ does NOT mean that the motion of air particles is oscillatory. If this is correct, how come sound is generated?
I would appreciate any response.
Thanks in advance,
Rama Krishna
Selim says
Hi it was very helpful for me. Thank you very much! I want to ask that, how I can add a pressure middle of the metal or anywhere of the metal for see signals two edges of the metal.