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WinISD Pro Tutorial
By Andrew Vaserfirer​

What is WinISD?

WinISD is a freeware program that allows one to use driver specifications to model how this driver will act in a specific alignment [sealed, ported, passive radiator etc..)] given a certain volume and/or port size.

Differing drivers have different specifications requiring variations in box and/or port design. Use of WinISD Pro is a relatively easy way to ensure that the optimal cabinet is designed for the specific subwoofer in use.

You can download WinISD Pro via this link.


To use this program you will need a computer that can run WinISD [virtually any PC will do] and the driver T/S specifications [coming soon]. Please see the Home Theater Shacks database of T/S specifications [coming soon].

Starting with the Program

Before starting there is one extremely important word of warning: While modeling at low frequencies is fairly accurate there is one large, erroneous, assumption made. That is that the driver being modeled will act in a linear fashion at moderate to high SPL/excursion. This simply is not the case for the majority of the drivers on the market. Of course, one specifically chooses a well designed, high quality unit, that has been proven via credible measurement to have these attributes that is a different case.

Open WinISD. Your screen should look like this:

Click the 'New Project' button to get started. This pop-up will display and give you some options:

Now you have some choices:
  • You can click the drop down arrow and look through the database supplied to see if WinISD already has the driver you want entered.
  • You can go to the Home Theater Shack T/S data base [coming soon] and see if we have the driver already in a WinISD Pro file for you.
  • You can enter the driver data yourself.
If you fall under the first two options you can skip the next section as it will go over entering driver data in WinISD as well as what all the T/S specifications mean.

Meaning of Thiele/Small Parameters

T/S specifications are electromechanical properties that essentially define how a driver will perform. A quick overview of the more common T/S parameters and their meaning follow. Please note this is not a comprehensive list or definitions.

Fs: Resonance frequency measured in Hz. is the frequency that the moving mass and suspension of the driver is reinforced by cone motion to the maximum amount.

Bl: Force factor (magnetic strength) measured in Tesla-meters.

Mms: Mass of everything that moves within the driver, measured in grams.

Rms: Measurement of the drivers losses in its suspension/moving system.

Cms: Inverse stiffness measurement measured in meters per Newtons.

Qts: Combined dampening of the driver - both mechanical and electric.

Qms: Mechanical dampening of the driver.

Qes: Electrical dampening of the driver.

Vas: Free air stiffness, of sorts, of the driver - measured in liters.

Sd: Effective area of cone diaphragm measured in meters squared.

Vd: Volume of the displacement measured in liters.

Re: D/C resistance of the coil measured in Ohms.

Le: Inductance of the voice coil.

For more detailed information on TS parameters... see our TS Parameters thread.

Inputting Parameters in WinISD Pro

From the new project pop-up click the 'New' button beside the drop down arrow. Once this is done a new screen will pop-up. This screen is fairly straight forward - Just enter the brand, make and model of the driver whose parameters you plan on entering. Then click the 'Parameters' tab and you will see something like this:

Ensure that 'Auto calculate unknowns' is on even if you have all the data. This is recommended because WinISD Pro can be very picky about the data and if it is even 1/1000th off it will reject the data so it is just easier to let it do the work. You might notice some small differences in the values from the calculated and the provided, but very rarely are these large enough to substantially effect modeling.

This is the recommended method for entering driver parameters [if at any point you need to change the unit of a parameter simply click on the displayed unit and it will change]:

  1. Enter Mmm and Cms which results in Fs being calculated. If Mmm and Cms are not known enter Fs.
  2. Enter Sd, Bl and Re. This will result in more auto calculations being made, but Qms and Qts will be blank.
  3. Enter either Rms or Qms.
  4. Enter Hc, Hg and Pe.
  5. Enter the number of voice coils
  6. Correct Znom if needed. Please note that drivers with several voice coils can cause some issues with autocalculation since Bl is often calculated with voice coils in series. This will result in a doubling of the Bl value relative to use of a parallel connection.
  7. Enter Xmax and any other missing parameters
If you want you can also tab over to the dimensions page and enter 'Dimensions'. Don't worry about the 'Advanced parameters' tab.

Time to Start Modeling

Once all your data has been entered hit the 'Save' button and save the file with a representative [of the driver] file name. Now close the driver editor and click 'New Project' again. Go to the drop down menu and look for the driver you just entered. Highlight it and click next. You will see a screen like this. Choose the number of drivers you plan on having sharing the same airspace in a single enclosure [this can be changed later] and you will probably want the 'normal' bullet chosen.

Click next. You will now see the next tab that will let you choose the alignment. If you already know what type of cabinet you want to build choose one. If not its time to research the merits of each style. A quick run down:

Sealed: Typically result in smaller cabinets that are easier to build. The cost is of low end extension, more distortion and less overall SPL capabilities compared to ported/PR.
Ported: Typically larger cabinets that are harder to build and design properly. These usually have more extension, SPL and lower distortion than sealed cabinets.
Passive Radiator: These have the capabilities of a ported enclosure, but can be built smaller due to use of the passive radiator. These typically cost more because high quality passive radiators are rare. A little more build work than sealed and possible more than ported depending on the type of port used.

Please note this guide will primarily focus on the sealed and ported alignments as they are the most common.

Click next once you have decided on what style cabinet you want to build. After this you will see a screen that asks what type of alignment you want. Just go with whatever is the default as it will rarely be the optimal choice for your application. You will now see something that looks like this:

From this point there are essentially two point of control. On one portion you can control aspects of the driver and from the other you can see the different graphs that model is performance.

Under the 'Box' tab you can control the size of the box as well as the tuning frequency if you are using a ported. The box size will directly effect performance in a number of ways. If you are using a ported alignment the next tab will be the 'Vent' tab this allows you to control the number, size and shape of the vent(s) used. The next tab of interest is the 'Signal' tab, it is notable because it allows you to adjust the amount of power being input to the sub. Enter the amount you plan on feeding the sub to get accurate modeling of certain parameters. The last tab of interest is the EQ/Filter tab. This tab allows you to model how certain filters will effect the response of the sub as well as other areas of performance.

Now if you click the other source of control you will see this:

There are some graphs available here that will be very important to you and others that will be meaningless this will depend mostly on what alignment you have chosen.

For a sealed alignment the main graphs are:

Transfer Function: this is essentially the modeled frequency response of the subwoofer
SPL: This is the SPL that the subwoofer is modeled to be capable of with the amount of power specified in the previously discussed signal tab.
Cone Excursion: This is the total movement of the subwoofers cone. You typically want to keep this within the boundary listed by the manufacture.
Impedance: This is the resistance to current flow of the driver. Depending on the amplifier you have it could be an issue if it drops to low.

Along side the previously listed if you are using a ported alignment the 'Rear Port - Air Velocity' tab will be of importance to you. This will show you how fast the air will be moving into and out of the port. The lower the better. To decrease this number the port surface area must be increased, but as you do this you also decrease the level at which the port resonates. Due to this a balance act must be had where you minimize port air velocity [at its fastest 20-25m/s should be accepted] and keeping first port resonance out of audibility range. If you plan on crossing your subwoofer over at 80Hz with a 4th order crossover [typical of modern receivers] having first port resonance modeled at or above 120Hz is safe an explanation can be found here of why this is 'safe'. There are two reasons why port air speed is important. If it gets too high it will create port chuffing [audible and annoying noise from the port] as well as port compression [there is too much impedance to airflow caused by the port resulting in nonlinear performance at high SPL].

How to prevent a sub from bottoming out

This aspect of designing a cabinet is very important. Not only can bottoming out damage your driver it just sounds bad. Proper use of an infrasonic filter is the best way to do this. Fortunately, you have the tools to model this in WinISD Pro under the 'EQ/Filter' tab. Click the 'Add' button and choose a highpass Butterworth filter. Cutoff frequency is the frequency at which the filter starts [you will typically want it at or slightly below tuning in a ported enclosure] and Q is meaningless in this case. Order refers to how steep the slope is:

1st order 6dB/oct
2nd order 12dB/oct
3rd order 18dB/oct
4th order 24dB/oct

The following are graphs of a modeled subwoofers transfer magnitude, cone excursion and port velocity with [gray] and without [yellow] a highpass 2nd order Butterworth filter at 20Hz:

Now that you have an understanding of what is going on try and play with the program seeing how changing the box/port size effects modeled tuning and response.

If you have any questions or need any help feel free to create a thread and ask questions. There are many members here who are willing to help the inexperienced learn to design and build subwoofers.
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