Learners might have learned the basic concepts of the acoustics from the ‘Introduction to Acoustics (Part 1).’ Now it is time to apply to the real situation and develop their own acoustical application. Learners will analyze the radiation, scattering, and diffraction phenomenon with the Kirchhoff –Helmholtz Equation. Then learners will design their own reverberation room or ducts that fulfill the condition they have set up.
Lecture 5-2 Part 2. Duct Acoustics 1
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Del curso dictado por Korea Advanced Institute of Science and Technology
Introduction to Acoustics (Part 2)
7 calificaciones
Korea Advanced Institute of Science and Technology
7 calificaciones
De la lección
WAVE PROPAGATION IN SPACE / DUCT ACOUSTICS
For the last, you will learn Helmholtz Resonator and the phenomenon about the duct acoustics
Conoce a los instructores
Yang-Hann KimProfessor
Mechanical Engineering
Okay, duct acoustics.
[SOUND].
[COUGH] Okay, when we have, like, an engine over here.
[SOUND] It will discharge.
You know, the, a lot of noise say,
if noise spectrum measured over here,
look like, like that.
And maybe you say this is, 100 dB.
And then you know this is frequency 1, and
this is frequency 2, and this is frequency 3, okay.
And what you want is to control.
This noise coming from engine or sometimes a fan.
You may want to devise
some muffler over here.
Or you may employ some resonator, okay, that has this frequency.
Maybe you put resonator over here, or muffler.
Sometimes called
silencer.
Every case it uses impedance mismatch.
Utilize.
Okay, let me ask you then.
What kind of impedance mismatch you can use for, having appropriate muff, muffler.
So back to the very basic definition.
Impedance.
Okay, has 2 parts, right,
real part plus imaginary part.
Okay, real part is in phase with the velocity.
An imaginary part does not, it has out of phase with velocity.
Right?
For example, if you have, you know.
Sound observing material, like polyurethane or whatever.
Then the impedance of such,
sound absorbing material, has good real part.
Yeah, the sound absorbing material.
When you push the sound absorbing material can be easily pushed.
That means the,
the material has the property that moves
in phase with the velocity. But some material,
when I push the material will resist to your force or motion.
That means, you have something that has, imaginary part.
So what would be the best muffler, in this case?
If you have the impedance that has very strong
imaginary parts then every wave will be reflected.
What would be the best one?
If you don't consider, you know, other factor.
That's the one obviously is to, block this, okay?
Then you have a very large impedance over there.
and you will not get any noise coming out but
the problem is you have a large
back pressure to the engine or fan.
In fact the engine could be stopped.
Oh.
Yeah. That's the best solution because
engine is stopped.
There's nothing, noise radiating from the engine.
Or you can, if you block this, the fan will stop.
Of course, there is no noise.
You are very successful noise control engineer, but
you would not be hired by the company anymore.
You will be fired.
So, from this, observation.
The best silencer, or
resonator, or muffler is, first,
reduce noise as well as keep
back pressure as
small as possible, right.
I mean, if you don't have any back pressure by putting the muffler.
That means you keep the performance of engine, because engine does not really,
experience, experience back pressure, so,
[SOUND] the muffler has to pursue these two rather contradictory
requirement.
That's why designing muffler is, is difficult, because if you just
want to reduce the noise, you just have a very high impedance mismatch.
Then of course the noise will be reduced.
OKay, it's like
designing the silencer that follows mass law.
You can reduce noise but you increase the weight of silencer.
And again you would be fired because you use, if you say that,
oh I can reduce the noise by putting a lot of big,
you know, impedance mismatch by having a large mass,
mass difference, that means your mass, your mass of muffler is.
Is, is, is, comparable size with the mass of the total car then,
it's nonsense, right?
So, there are a lot of things we can play with.
So, what would be the
candidate for the muffler.
One candidate, would be,
the muffler that can, absorb sound energy.
Absorptive type.
Like putting a,
sound absorbing,
material in the muffler.
Okay, then sound will be, attenuated and then dissipated into heat.
And then we can, have certain, reduction of noise.
But, the property, of this kind of
muffler is, as you can expect,
is good, in high frequency.
The typical sound absorbing material has good characteristics up to over 800 hertz.
Or 700 hertz, 1 kilohertz, 2 kilohertz.
1 kilohertz [SOUND].
Meaning that the material is oscillating 1,000 times per, per second.
So, it moves very fast.
That means that a lot of sound, sound can be dissipated very effectively, okay.
[SOUND] 1000 times, there is a lot of heat, is,
it has to be generate, right?
[SOUND].
And you have a lot of oscillation then, you have,
you have, I mean the heat can be dissipated.
Okay, another candidate,
second candidate is what?
Using this, this actually use the real part.
Using imaginary part, what will be?
Okay.
This kind of chamber.
Okay?
Then, there is a big impedance mismatch over here, and
a big impedance mismatch over there.
Then the sound entering in, will be reflected back and
forth due to the impedance mismatch.
Certain frequency of wave cannot.
Come out, okay.
Therefore we can, control some of this noise component by using reactive element.
So that is, reactive type.
Okay?
So if you look at the, muffler, the automobile muffler,
then most of the automobile muffler, use the combination of, of absorptive type and
the combina, combination of absorptive type and the reactive type.
Reason is very simple.
This one is very good for, high frequency, so you may use,
control to, to control this one.
But cannot
use the absorptive type to control this, this one.
So he use the, reactive type,
for controlling this low frequency noise.
And use absorptive type.
To control the high frequency component, all right?
So in the coming, next lecture I will try to,
explain how to use this, and how to use that.
And how to design this.
And how to design them.
So in fact, if you look at the muffler,
the automotive muffler especially,
it look like expansion chamber, they have expansion chamber.
So the, there's a big impedance mismatch and there is a engine.
[SOUND].
And, this length is very important, right?
So determining this length, I mean will control the certain
frequency of sound because there is a big impedance mismatch,
and also the size of the tail is also very important because after
the noise coming out from this resonate from this muffler using
this impedance mismatch we can control also certain frequency.
So that's why you can see some of the muffler has this kind of,
shape, okay.
Because that shape would change
the characteristic length of muffler over there that can cope,
to control the certain frequency, okay.
And the inside of this, it usually look like it has some,
you know, chamber inside.
Because each chamber can control some of the wavelengths, and
the total chamber will control some of the wavelengths.
Also combining of these two chamber control some of the wavelength and
of course this chamber will control the some of the wavelengths.
So that hopefully.
Control the, some of the frequency.
For example, if the noise look like, [SOUND] like this,
this one sometime controlled by this length, okay.
Some of the noise controlled by this length and the some of the noise controlled
by that length and the some of the noise controlled by this length and
the control by this length and the control by that length.
OKay?
And also, they use some of the sound absorbing material over there.
And the problem with sound absorbing material
is that it is not good for, heat.
And usually for automobile case the heat
coming out from the engine is, well above 300 degrees sometimes.
So it can burn this under serving material over there.
That's why the idea, they had, they use,sss so called perforated pipe.
In other words, they put a lot of small holes, so that air can coming I,
in and out, and that produce some friction and it will dissipate energy.
There are various types of, perforation, okay,
available in, in, in, in, in in practice.
So, this is also the absorptive type.
So basically this means that, this is the noise coming from engine.
And for example, if you have, say
oh I would like to have, noise like that.
Exhauste noise I, I want noise like that, exhauste noise.
I want noise to look like that, okay?
For example, this is a very big issue.
When you start your engine, what you hear is like.
When you, start.
Accelerate your engine.
This is some noise coming out.
And you want to design the noise like that.
Okay?
Nowadays car company want to have their own style of noise?
Exhauste noise.
So coloring, coloring the noise.
That means you want to have this noise, but you do know the noise characteristics
coming out from the engine, but then what is the solution, of this?
OKay.
There's no unique solution, of course.
Therefore it is a design problem.
Okay, you invite many different component of absorption coeff,
absorbing material or perforation.
Or other, I mean, available absorbing, component.
And you can design also the chamber.
That's why when you look at the, muffler,
automobile car's muffler, there's a, there's a.
Each company has their own style because there
is no unique solution, okay.
So let me continue to talk about designing muffler, absorptive,
absorptive type design in muffler.
At the reactive type designing of, of muffler has been coming.
coming, next lecture.
That would be also very interesting.
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