Design Your Own Subwoofer Box - Part 1: Box Design


Uploaded by rumreich on 01.02.2010

Transcript:
This video is about how to design your own subwoofer box.
The first step is to choose the
subwoofer driver that you’d like to use. I’ve chosen
The Infinity
1252W 12-inch sub. It's a
top seller right now.
So once you pick the driver, you want to find a specsheet for that driver.
You’ll find the Thiele/Small parameters and there are three numbers that you’ll need.
The first is called fs
and here it is free air resonance, fs,
for this driver.
It's 24.23 Hz and I’ll write that down.
The second number
that you need is called Qts, total Q. Qts for
this driver 0.48. I’ll write that down also.
The last number
You need
is Vas,
compliance volume. On this specsheet it’s shown in
cubic feet and liters.
I will
write down the cubic feet. I prefer to work in cubic feet but
the software will handle either one, you just need to keep track of
which you've chosen, so 3.48 cubic feet.
The next step in doing the design is to use some
speaker design software and
I like to use the Subwoofer Design Toolbox for this.
So the first thing we need to do is enter those three numbers into the software. So fs
was
24.23
Qts
0.48
Vas in cubic feet,
(we could have chosen liters,
we chose cubic feet)
is 3.48 cubic feet
Now the first thing we're going to do is see
how this
driver acts in a sealed box,
and the software lets you choose sealed, ported and other types of boxes too.
We’re going to start with sealed
and I’m just going to choose a
half cubic foot,
0.5 cubic foot box and see what happens.
So update the plot.
Now if you're not used to looking at these frequency response plots
they show you the bass response.
Think of it as the loudness of the bass
versus frequency, so here at the higher frequencies we’re
near zero dB, but as you start to get lower and lower,
there's a point where it just starts to fall off
and so
for this speaker
we’re
-3 dB
at about
49.6 Hz. Up here it shows
it's called data tracking you can see
where this cursor is so
it looks like about 49.6 Hz.
Well, our goal is
40 Hz
or maybe even better.
So let’s see if we can do a little bit better.
I'm going to put
a data description in here.
We’ll call this
sealed box
and it’s
0.5 cubic feet.
So the next thing I want to do
I’d like to keep all these curves up here I’ll just add a data set
and
the next one I want to show will also be a sealed box.
But let's make the box one cubic foot.
So I’ll
put the box volume of one.
I’ll put
1.0 up here.
I’ll update the plot, so now this plot shows
the red line is the sealed
.5 cubic foot,
the green line is the sealed one cubic foot. Well you can see what happens here,
we got an improvement. Now we’re -3 dB
At about 41 Hz compared to
about 50 Hz before, so that's a good improvement.
Let’s try making it even bigger, let's try a 2 cubic foot sealed box.
I’ll add a new data set,
we’ll call that sealed 2.0,
make the box volume two cubic feet,
update the plot.
Well now, something interesting happens which is, it’s a little bit better
than one cubic feet,
but not much.
And so we've kind of hit the point with this speaker where making the box
bigger
doesn't really do much for us and so when that happens
it means it's time to look at a ported box.
So what I'm going to do
is
I’ll leave the sealed .5 and sealed 1 up there, but I’m going to
now play a little bit with some ported boxes.
Let's
have a ported box with a volume of
two cubic feet.
Now with a ported box
there's one more number that you need to
add. With a sealed box, the only number
you need to worry about is the volume of the box. With a ported box
there's this other thing here
called port frequency.
That has to do with how you tune your port.
Well, the software recommends a value
to use and were going to take its recommendation. I’ll just
double click here,
it’ll put that value in there,
we will update the plot
and see what happens.
So the yellow line is showing us a properly tuned
port in a
two cubic foot box. Well
now we’re
30 Hz at the
-3 dB frequency
and we were not able to get that out of a sealed box. Only by going to ported
were we able to squeeze that extra 10 Hz
out of the system.
This shows one of the
rules of box design, which is you always have a
small box that’s sealed, and a large box
that's ported.
I’ll just fix up the description here, this should say ported.
Let’s
add one more data set.
let's try a ported
four cubic feet.
Well you can see a similar thing that
happened before, which is we're starting to run out of steam.
Now here we have -3 dB at
about 23 Hz, so yes it's an improvement but
you can't just keep increasing the box size forever and keep seeing those improvements.
The design process is you usually start out with a sealed box,
increase it until you get
your design goal of 40 Hz and then
if you can't get there,
switch to ported
and once again keep increasing the box size and hopefully you'll be able to
hit your
35 or 40 Hz goal.
In later installations of this,
I’ll talk about how to use the
port design tool,
the enclosure design tool
and the woofer selection tool.
If all you're doing is a sealed box, you don't need the port design tool.
The woofer selection tool helps you choose woofers, we just
knew we wanted that Infinity and didn't have to mess with that here. But if you're
shopping for woofers, you will want to use that tool.
And the enclosure design tool helps you
build the wood
cabinet, to make sure that you've got the volume that you need.