[Paul Spencer]

I've used a number of freeware speaker design programs for subwoofers;
I consider WinISD to be the best currently available as it is the only
free program which allows you to see the relatively complex interactions
between input power, alignment, cone excursion, group delay and filters.

Getting started

Firstly, get the program here:
https://www.linearteam.dk/default.aspx?download=winisdpro

In order to follow this tutorial you need the pro version.
The simplified version is not sophisticated enough to
account for all you should consider in your design.

Introduction

This is a basic guide to using WinISD to design vented and sealed subs / woofers.
This page may evolve as others contribute, but for now this is how I do it.
Discussion of sealed vs vented is beyond the scope of this tute.

Primary Considerations

Before you get started, you should consider:

1. Does your listening room have solid construction?
If so, an enclosed room will have room gain. If the room is large and/or open or
has light construction then room gain may be negligable.

2. Do you have an intended location to place it?
If you have a lot of room gain and plan to place it in a corner, this will have
design implications. Either vented or sealed subs can be designed to work with
room gain.

3. What is your target SPL and extension?
You can't readily estimate in-room SPL accurately as there are too many variables
which no available programs can account for, however, you can get some indication.

4. At which point do you intend to cross to your main speakers
and how do you intend to integrate them?
You may wish to add on a single sub to take off where your mains have no output.
Or you might have dual mono or stereo subs crossed as high as 120 Hz with a
highpass filter on the mains to limit their bass reproduction.

You may notice I haven't asked if the sub is for music or home theatre. This is
because the differences are really performance differences. Some will consider 40 Hz
extension and 90db SPL to be adequate for home theatre, and others will consider
110 db @ 16 Hz to be essential for accurate music reproduction!

For beginners I suggest using the following performance targets at least as a starting point:

1. Accurate music reproduction Extension to at least 40 Hz without room gain
with clean output of 90 db at your listening position, 105 db peaks if you like loud music.
If you like to reach live sound levels, you may set your target considerably higher.

2. Home theatre Extension to at least 25 Hz with 105db peak SPL at this point in your
listening position. You may consider setting your target at or below 20 Hz.
If you intend to aim for higher SPL, a typical hifi floorstander isn't likely to keep up.
If you have a high SPL system which migh include a line array, horns or large direct radiators,
then design for higher SPL levels may be appropriate.

Design process

1. Start a new project - if the driver isn't on the WinISD database, you will need to enter parameters.

Note: WinISD won't accept TS parameters which conflict. What you have to do
is enter in the most basic parameters, and let the program work out some of them. This might
result in slightly different TS parameters to those published. This is not a problem unless
they are significant differences. Refer "Entering new driver data" in the WinISD help contents.
It mentions specific order you should enter data to avoid errors.

2. Choose alignment (sealed/vented)
Don't worry too much about which alignment to choose. It is not necessary to stick with a
particular alignment and you will normally get better results tweaking.

3. Box tab
Try different values of vent tuning and box volume and see the impact on F3

try a larger box and lower tuning
try a smaller box and higher tuning

Making the box larger increases efficiency at the tuning point. If you have a peak at tuning,
the box is too big, or the tuning is too high, or both. Conversely, making the box smaller
decreases the efficiency at tuning, resulting in some roll-off above tuning. Try different values
until you find a compromise you are happy with.

4. EQ/filter tab
Add a low pass filter to cross to your mains. What you are trying to do is achieve a crossover point
which is the same as your mains, which has the same slope. To achieve this you may use a
low pass filter on the sub which matches the roll-off of your mains, or you may also add a high pass
filter to your mains if they are sealed. Both mains and sub should have matching slopes to get a
good integration.

Start with a low pass filter with a corner frequency which matches the F3 of your mains.

For a sealed sub you may wish to add some parametric eq or a Linkwitz transform.

For a vented sub a high pass filter is advisable. This is also referred to as "low cut" or
a sub sonic filter. It is a good idea for any sub, especially for vented alignments and/or home theatre
use. Start with a high pass with a corner frequency at 20 Hz with at least a 3rd order slope.

Now it't time to do some juggling until you reach your target F3 and upper crossover point.
You need to get both right, as all the parameters you are changing are inter-related.
You will need to focus on these:

box volume and tuning
low pass settings

5. Check max SPL
In the "signal" tab, put in a value. View the cone excursion chart and increase the signal
input power until xmax is exceeded, the thermal power handling of the driver is exceeded,
or you reach the power limit of the amp you have in mind. If you have used a lot of filters
which decrease the gain, then you may use up all the amp power without coming close to the
limits of the driver or amp. In this case, you can add some static gain to compensate.
Check the gain (third last chart on the list - trasnfer function under Filter/EQ)

Now look at the max SPL chart. This is how much output you can achieve theoretically without
room gain. In practice, you may achieve more or less output than this depending on your room.
This is possible to predict.

6. Vent design
Now look at the vent diameter and length. Choose a diameter you actually can use.
Check air velocity. The air velocity which is acceptable varies depending on the aerodynamics
of your vent. Too much air velocity on a vent without flared ends will result in audible
"chuffing."

Highly recommended: See experiments conducted by Bill (Collo) on this forum:
[vent flare testing]

[discussion of Bill's results]

Note: Many sources will suggest anything from 17 m/s to 34 m/s but in many cases this will cause problems!
Acceptable vent velocity varies greatly with the design of the vent and its flare.
Unflared vents aren't suitable for any driver that could be called a subwoofer.

Some important points to consider:

a rumble filter reduces vent velocity
lower tuning also reduces vent velocity
vent chuffing can be masked by other program material
a larger flare allows a smaller vent
you may need to make the box a little larger - the vent won't have to be as long to achieve the desired tuning point and it will fit in easier
resist the urge to use a slot port as it will need to occupy much more volume due to the difficulty in flaring it and the thickness of the walls
if you aren't able to achieve a suitable compromise, consider a different driver or a passive radiator

[How to get a vent that doesn't chuff]

Rumble filter comments

There are many that consider a rumble filter to be unnecessary.
If you wish to achieve the maximum possible SPL from your driver then you need one.
Good engineering is about allowing for the worst likely situation. A rumble filter
ensures that cone excursion will never get out of control due to unexpected low frequency
signals. Have a look at the cone excursion with and without the rumble filter.
With a vented subwoofer in particular, the reasons for using a rumble filter are obvious.

[What is a rumble filter & do I really need one?] This wiki page goes into more detail about rumble filters.

In a nutshell: Neglecting a rumble filter is a bad idea
(unless you have a very large amount of spare displacement)

Conclusion