In this course students learn the basic concepts of acoustics and electronics and how they can applied to understand musical sound and make music with electronic instruments. Topics include: sound waves, musical sound, basic electronics, and applications of these basic principles in amplifiers and speaker design.
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From the course by University of Rochester
Fundamentals of Audio and Music Engineering: Part 1 Musical Sound & Electronics
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University of Rochester
275 ratings
From the lesson
Week 5 - Closed Box Loudspeaker Design RC & Ported Loudspeaker Design
Designing speaker enclosures, basic principles, interaction of speaker driver with the cabinet, why a cabinet at all? Speaker designs for home stereo, crossover networks, guitar amplifier (open) speaker enclosures, bass amplifier (closed and vented) speaker enclosures.
Meet the Instructors
Robert ClarkProvost and Senior Vice President for Research and Professor
Mechanical Engineering
Mark BockoDistinguished Professor and Chair
Electrical and Computer Engineering
This week, we're going to talk a little bit about speaker cabinet design.
we'll finish the lecture today with the closed-box design.
but before we get into the specifics of how you actually design the enclosure,
I'm going to spend a little bit of time talking about some of the basics.
Even, even the sim, simple components of construction.
and building a solid box will make a big difference in the quality of the sound.
both in how it's constructed and how it's sealed.
but I'd like to talk a little bit about the differences between, you know, a
closed box design and ported design. And then, spend a little time providing
you with the background to derive the design equations that are used in
designing closed-box speakers. some of the math there's going to be
beyond the requirements for this particular course.
Don't get caught-up in it. I would just use that as an opportunity
to think about, maybe some other courses you might like to take later.
If you'd like to be able to do, to derive the design equations yourself.
But not to worry, you'll be able to do, execute the designs independent of
whether you can derive the equations or not.
Okay. this week we're going to focus a bit on
speaker cabinet design. And before we get into the design of
closed-box speakers, which is what we'll consider first, I'd like to briefly
discuss two of the primary cabinets. one being the closed-box speaker and the
other being the ported or bass reflex speaker.
you see, the difference between them is basically the port that's in the, the
cabinet of the speaker. It allows the volume of air in the port
to act in similar ways that the Helmholtz resonator did earlier.
The mass of the air in that port oscillates and can enhance the sound
radiation. But these are the two basic cabinet
designs. there are other designs that include
labyrinths and transmission lines. there's speakers that are far more
complex to construct and tune and, and basically we're just not going to cover
those in the context of this course. But you're more than welcome to look them
up and do a little more reading on them. So, for now, we're going to focus on
these two primary designs. the closed-box and the ported or bass
reflex. Let's talk about the enclosure shape and
design. Now, you know, we spent a good bit of
time talking about room acoustics. and acoustics in three dimensions.
this is an interesting case, because in this time, we're talking about the
construction of a box that we're going to put our speaker in.
we'd set that speaker box in a room. So but the, the box basically, the design
of the box we can use some of what we learned in room acoustics to think about,
how we choose, choose the enclosure shape.
you'd never, you know, if we look at this top box here.
You'd never want to pick an enclosure shape or design where the dimension,
where one dimension is significantly greater than, than the other.
so if, if you look at the, at, at the case here, actually, we have actually,
it's Ly that's, three times Lx so this should be a y.
I guess this should be x. But we wouldn't want to pick a design
with a, with a an enclosure shape, like that.
Because it'd behave like a resonant duct. And we already saw the the basic
equations that describe that. But it at low frequency, it had very
dominant standing waves, you know, along the length of the duct itself.
And, and of course, that would give a, a, a very spiky response to the to the
enclosure as a result. The other thing that to, to mention would
be that we don't want to have a cube. so we really don't want an enclosure
where, you know, Lx, Ly, and Lz are all the same.
Now, why would that be the case, why do you think that would be the case?
well, what happens is, is if you have a cube, the standing waves are going to be
the exact same dimension in for each, the x y z dimensions.
And my sketches here aren't great, but you get the point.
So, and the resonant frequency associated with the standing wave in each of these
predominant directions are all going to be identical.
and that means they don't line up and they make the frequency response to the
enclosure really peaky. Okay.
So it turns out in enclosure design, a well established and a preferred
dimension for the enclosure itself is based upon the golden ratio.
And so a ratio of 0.618 to 1 to 1.618. and for the golden ratio, you can notice
that 1 plus 0.618 is also equal to 1 divided by 0.618.
basically, it provides a uniform distribtuion of the resonances in the
enclosure. so there's the, the, the, the choice of
that dimension actually, or the ratios of those dimensions ensures that you have,
that the resonant peaks don't line up specifically to enhance each other at
particular frequencies. It basically spreads them or distributes
them across the the bandwidth more uniformly.
All right. So, it's the least peaky of the design
and you know, as I said before, the case where the, the, the natural frequencies
you know, the 100, the 010 and the 001 modes, they don't equal each other.
as an example. so, you really need to choose an
enclosure that's based upon the parameters of the transducers that you
use. as well as the desired frequency response
of the speaker itself. and mainly the enclosure design is
going to have the greatest impact at low frequency, that's where, you know, for
the woofer or the low frequency driver. The box enclosures going to define some
of the stiffness characteristics and really affect the response.
The tweeters themselves, the higher frequency drivers relatively unaffected
by the the box size or design. so before we discuss or focus on the
options, it's probably worth providing a few guidelines for construction,
generally. you, you know, I realize some of you may
choose to build your own loudspeakers. I can tell you that it's that tuning of
the speakers is far more tedious than you might anticipate when you first start the
project. It's rewarding, I mean, and it's fun if
you enjoy doing it, but the trick is, is recognizing that, you know, you're not
going to just specify a few numbers in an equation and get something that sound
phenomenal. If that were the case there wouldn't be
any need to buy speakers. and there's typically some fine tuning
that's required you know, the parameters of the speakers that you buy or the
transducers aren't always perfect. And so, there's a lot of, you know, fine
tuning and tweaking associated with that as well.
but before we get to actually laying out the the speaker design, it's, it's really
worth talking about a few things that are good guidelines in general construction.
And one is, is that, you know, we put our high frequency drivers at the top of the
cabinet. And our low frequency drivers at the
bottom of the cabinet. the high frequency driver or transducer
is typically referred to as the tweeter. And that should be placed closer to the
plane or the ear. in terms of general construction rules
too, I would also avoid the use of nails. screws are preferred over nails, for
sure. And of course, glue and caulk are also
frequently used to seal the cabinet. Here, I've shown, you know, basically a
lap joint. in connecting the two sides of the
cabinet. You could miter this.
there's a lot of ways you can do it. You can strengthen it structurally.
by putting a block here in the corner that you can attach, you can, you know,
screw through to the to the block itself. You can glue that in the corner, but you
generally want a really strong joint. And you want to make sure they're sealed,
so that there's no air gaps. Otherwise, when the speakers vibrating,
you end up with kind of a wheezing sound because you'll push air in and out of the
cabinet, if there are any cracks or places where the air can leak.
And that won't be and that would certainly won't enhance your sound
radiation characteristics. so many options you can use to join the
wood of the enclosure, and but, but again make sure it's a very rigid a rigid
structure. It's also good advice to place some
material in the box such as a foam or an insulation material, something that can
absorb some of the reverberant sound in the box.
you know, as I mentioned before the, the modes or the, the acoustic modes in the
box or the enclosure itself inside will be fairly peaky.
And if you if you if you add some absorptive material, it will cause that
to decay out more quickly. And so, instead of having sorry, let me
sketch on my sheet here. Instead of inside the box having a lot of
frequencies that are, you know, that are peaky inside.
You know, if I'm looking at the frequency response of the box.
If you put some damping material in, some foam or something those peaks will round
out. And so, they'll end up being flatter.
not so sharp in terms of the in the in the enclosure themselves.
But, you know, it's also important don't stuff the box full of materials because
that will change the volume of the enclosure, and then it will effect the
design and performance. So you know, just a small bit of lining,
and maybe you can incorporate that into your estimate of the the volume of the
box enclosure.
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