Design a passive radiator enclosure

Let’s find out how to tune a passive radiator. First of all, you should know all the basic stuff about passive radiators. How they are similar with bass reflex design, and so on. More info in this article. In the previous mentioned article, we talked about the principles behind the PR design, but also about different alignments, when the active driver is identical with the passive driver (without the motor, of course).

While in this article we will use matched speakers (active and passive driver), we are not going to use alignments. Instead, we shall make use of software, like WinISD.

Enclosure example

To serve as an example for our passive radiator enclosure design, I devised a small box, for a portable speaker. Don’t focus too much on it, it’s just a test enclosure, only one channel (that’s why there is only one tweeter).

front portable speaker
back portable speaker

Regardless off that, let’s focus on how to design the enclosure. After that, let’s switch our attention on how to tune a passive radiator. The drivers under test :

They are both 3.5″ drivers. And for the purposes of this test, the tweeter is not connected to anything. We are focusing on the drivers which matter to our project. In this case, since I’m trying to make a portable speaker, passive radiator is a great choice. Since the enclosure is so small, the port length would be too much to place inside the enclosure. As a result, the passive radiator solves this issue.

To help us in our enclosure design we are going to use WinISD.

Enclosure design using WinISD

Let’s go right in WinISD. First thing you want to do is click “Create new project” and select “Add New”. Never use anything from existing list. You cannot trust that those parameters are accurate. Sometimes the list contains the driver you want, but it might be old versions of the same driver. As a result, always manually input your speaker parameters. After you entered the name and model, go to “Parameters” tab.

In this section, make sure you check “Auto calculate unknowns” and don’t enter every parameter. Let it calculate the rest automatically. Otherwise, when you load this driver, you will get a parameter inconsistency error. So, for example, if you enter Qes and Qms, let it calculate Qts. Enter values and use “Tab” to switch between parameters. It will calculate automatically where it can. I entered only these parameters :

  • Qes
  • Qms
  • Fs
  • Vas
  • Mms
  • Le
  • Xmax

The parameter list looked like this :

active driver parameters

After you save the driver, you want to “Create a new project”. Select your newly created driver and click “Next”. We have 2 drivers into a normal configuration. After that, select “Passive Radiator”. Now we have to enter the parameters of the passive radiator. Don’t worry that we have 2 radiators, we will change that when we design the enclosure. You find these parameters in the spec sheet provided by the manufacturer. Pay close attention to the units :

  • Vas = 1.5 liters = 0.0015 m³
  • Qms = 6.15
  • Fs = 53.8 Hz
  • Sd = 31.2 cm² = 0.00312 m²
  • Xmax = 9 mm = 0.009 m
passive radiator parameters

Then you simply select no alignment, give the project a name and create the new project.

Enclosure tuning frequency

When it comes to tuning the resonant frequency of the box we have few options :

  • Alter the volume of the box. Increasing the volume lowers the resonant frequency.
  • Changing the moving mass of the passive radiator. We will see how to tune a passive radiator in the next section. Adding mass lowers the resonant frequency.
  • Adding extra passive radiators. If you want to add mass to the cone, it’s easy. However, what if you want to subtract and there is nothing to subtract? You simply add another passive radiator. For each extra passive radiator the tuning frequency goes up.

Before we start doing anything, let’s change some numbers that are clearly wrong. The default enclosure volume is set to 50 liters. Since we are designing a small portable speaker, let’s dial that down to 1.5 liters.

passive radiator box volume

As you can see, now we got a curve that looks much more appropriate. Next we got to change the number of passive radiators. The default value is set to 1, however, we have 2 pieces.

winisd passive radiator

If you go back to the box section, you can see that our resonant frequency has gone up. By both actions : reducing the volume and adding and extra passive radiator. The resonant frequency of the box went from 54.6 Hz to 93.18 Hz. This is a good thing. Since we have small drivers that have little output at low frequencies, it’s not a good idea to tune the box too low. The passive radiators cannot produce low frequencies if they are not present to begin with. In this particular case, aiming for an even high tuning frequency would be best (120 – 140 Hz).

How to tune a passive radiator

Our little text box measures at 1.42 liters, so let’s change that in WinISD.

1.4 liter box passive radiator

Since we reduced the volume, the resonant frequency has gone up to 95.38 Hz. This case is with no weight added on both passive radiators. Let’s do a quick impedance sweep to validate our results.

tuning frequency passive radiator enclosure

The tuning frequency of the box is given by the lowest point between the peaks. So, 94.88 Hz. Quite surprised that the accuracy is so good on modeling software. Now let’s see what happens when we add mass to the passive radiators. In our case, we don’t really want to decrease the tuning frequency, but we are doing this just for the sake of having an example. In a subwoofer setup, you will probably want to reduce the resonant frequency.

Now let’s add some weight to the passive radiator, to decrease the resonant frequency of the box.

how to tune a passive radiator

I’m going to add one disc (2 are available) plus the nut which you can tighten with your hand. In the picture above you can the the PR with just one disc, which is not screwed in all the way. If I bring up the scale :

added mass passive radiator

We can see that they add up to 10.7 grams. Let’s input this into WinISD :

add mass to cone

And now let’s check the resonant frequency of the box.

how to tune a passive radiator new resonant frequency box

It has gone down to 62.51 Hz. For our small speaker, you never want to do this. It is tuned too low. We only want to check the effects. Let’s take a look at the impedance plot.

new box resonant frequency with added mass

The measurement indicates 62.58 Hz. Impressive accuracy of this WinISD application.

Increasing the tuning frequency

Like I said before, if you get rid of all the weights, you cannot increase the resonant frequency. You can if you use more passive radiators. However, there are other creative ways on how to tune a passive radiator.

cut part of passive radiator

I used a Dremel to cut off the threaded rod.

passive radiator weight loss

By doing this we will lose an additional 1.1 grams from the moving mass, so the resonant frequency will go up by a bit.

how to tune a passive radiator

Here is how the passive radiator looks without the rod. By doing this, I eliminated some clearance issues also, because I had to make the enclosure deeper than it had to be. Otherwise, the passive radiator rod might hit the active drivers in the front.


Now we know how to tune a passive radiator. Adding weight is simpler compared to losing weight, but you can also add more radiators. However, Dayton Audio has a neat implementation to add weight. With some manufacturers you might not be so lucky. You will have to find some creative solutions on how to stick something to the cone, to increase it’s weight and reduce the resonant frequency.