What happens when you put two speakers in isobaric?

Let’s dive into the DIY isobaric speaker design and see what happens. We are going to build some speaker boxes and measure them up and see what’s up. Basically we are going to test the theory. First of all, if you prefer watching instead of reading, you can check out this video over here :

If not, you can stick around and keep reading this article.

What’s the isobaric theory

Well, basically, you have 2 speakers which are identical. You place these speakers either cone to magnet, or cone to cone, also called clam shell. In the clam shell design you have to wire one speaker in reverse polarity so that the speakers move into the same direction. If you want more information about the theory of isobaric compound loading, you can check out this article.

Anyway, let me give you the short version. When you place 2 speakers in isobaric, the pocket of air between the speakers binds them together. Since only one speaker cone emits sound (the other is tucked away inside the box), you can imagine that the 2 speaker combine into one speaker with different characteristics.

Parameters that change

You will be surprised to hear that most of the parameters remain unchanged. Well, at least the important ones. So we have to add the moving mass of both speaker. Therefore, the moving mass is doubled. The suspension characteristics of both speakers needs to be added as well. In conclusion, “this speaker” is twice as rigid, therefore compliance is halved. Since Mms is doubled and Cms is halved, Fs will remain the same. So here is the short version of what happens to the T/S parameters in a DIY isobaric speaker design :

  • Fs (resonant frequency) stays the same
  • Qts (quality factor) stays the same
  • Vas (equivalent compliance) is halved

For this reason, if you have 2 speakers in isobaric, you can make a box twice as small, with the same result as if you would have only one speaker in full size box. In addition, if you wire the speakers in parallel and your amp can handle half the loud of one speaker, in theory, you will get the same amount of power.

DIY speaker design build

What are we building? Well, first of all, I have 2 Scanspeak 26W/8534G00. I’m going to make 4 different boxes:

  • 50 liter sealed box with 1 speaker
  • 25 liter isobaric cone-to-cone (clam shell)
  • 25 liter isobaric magnet to cone – small as possible chamber protruding outside the baffle
  • Same magnet to cone but with the isobaric chamber same size as the baffle

In theory, these enclosures should match in terms of frequency response and resonant frequency.

Build log

Here are some pictures with the enclosures :

sealed box

So this is the 50 liter sealed box with only one speaker. This will be the reference enclosure. All the other boxes should measure exactly like this one.

diy isobaric speaker design clamshell

This is the push pull design. To mount the second speaker I used an additional panel. In this case I can remove the speaker with no hassle. Otherwise the inside speaker is kinda trapped inside the box.

diy isobaric speaker design magnet to cone

This is the DIY isobaric speaker design with the indenting chamber. Keeping the volume to a minimum. However, it looks pretty awkward.

Isobaric compound design

This is exactly as the enclosure beforehand. The only difference is that the isobaric chamber is extended to be the same size as the baffle.

Results

Let’s check the results :

results frequency response

You want to focus on the red curve. That is the frequency response of the sealed 50 liter box. The rest of the curves should overlap that one. We can see that they pretty much overlap. Except for the clam shell design. I don’t really know why that is. The explanation I came up with is that it’s a measurement error. Since the magnet is sticking out. I cannot get a good nearfield measurement. Can’t place the microphone as close as possible to the middle of the speaker cone.

Impedance results

If we look at the impedance results. I won’t post all the pictures, just the numbers. So the results are pretty much 1-2 Hz off. Which is within margin of error, I guess. It’s actually interesting that the resonant frequency goes down as the isobaric chamber gets larger and larger. Check out the numbers.

Conclusion

So when you make a DIY isobaric speaker design, you really must care about enclosure size. To be honest, I don’t think it’s worth the extra speaker. You can buy the appropriate speaker in the first place. However, if you plan to build a 4th order or a 6th order bandpass enclosure, this might be handy. Using a clam shell design which is hidden inside the box. And since these boxes can get pretty large, the fact that you make them smaller seems worth it.