November 8, 2010

Open baffle bass

What's so special about open baffle bass? I ask this question every time I see someone planning a diy open baffle bass speaker. But first, what is it?

Here is one example. It's called a W frame dipole. In this case, it's also push pull mounted. One driver has the polarity reversed so that while one moves out of the magnet gap, the other moves into it. It's a method to reduce 2nd order harmonic distortion. 
A dipole does not contain the rear wave from the driver and since it is 180 degrees out of phase, they both cancel. The result is a null to the sides and much less bass. An open baffle dipole has some unique traits:
  • no box spring to limit excursion, so less power is required to reach xmax and there is a high risk of bottoming drivers
  • dipole roll-off due to the acoustic cancellation typically adds 6db/octave loss below 120 Hz (frequency depends on baffle size)
  • velocity source - the room is not pressurised
  • dipoles are a low Q bass source
The ultimate bass?

Many believe that open baffle bass is the best you can get. It's claimed that they are less impacted by room modes and therefore have a smoother in-room result and better sound quality than the alternatives. If this were the case, then it would be worth the extra expense. However, I contend that open baffle bass falls short on both counts.

Smoother in-room bass

The side nulls are said to cause less room interaction. This sounds logical enough, and they do certainly measure differently in a room. However, simulations and measurements have shown that they don't necessarily result in a flatter response in room. It is in fact highly system dependant. You might find a dipole measures better, it might measure worse.

Better sound quality

I have had mixed results here. There are some drivers that sound better in a dipole arrangement, including those not intended for the application. However, in one particular comparison, I found no advantage. My listening comparison used two Rythmik Servo kits, each in a sealed box. Both had EQ to ensure I was comparing the same response. I used a familiar track well suited to subjective comparison of bass. Side by side, trying to pick a winner was like comparing many electronics components. I would have struggled to pick a difference in a blind test. Even testing sighted, expecting the dipole to have an edge, it was neck and neck. At that point, I abandoned open baffle bass. I had one sealed box at the time and one open baffle. When I converted the dipole to sealed, I noticed an immediate increase in low frequency authority. That might sound like a contradiction until you consider that the test was set up to level the playing field. When going back to a range of content, including music with more demanding bass and movies, the improvement was clear.

Still considering open baffle bass?

If you still want open baffle bass, then I suggest doing a test for yourself. You'll need to measure and EQ to match fairly. In many cases you are most likely to find that one modest sealed box will match four drivers in a dipole arrangement.


  1. Aha! You compared to sealed woofers and concluded the sealed one was better than the open baffle? I'm a bit confused as to what is meant here. Also, one modest sealed box may match four OB drivers in SPL, but so does a kitchen machine. The four OB will give you high fidelity, the sealed box will give you a box. Exception is the Rythmik Audio, since their totally genious system all but eradicates traditional boxed woofer drawbacks. With any other woofer fidelity will be better when one coloration (box) is lifted and the other (room) much improved. There is just no way around the fact a musical instrument is a (quasi)omnidirectional source, like a no baffle dynamic driver with adequate Fxo. A box is a directional source with diffractions, combfiltering issues, exaggerated LF behind the box (or exaggerated HF in front, whichever way you wanna put it), etc. If you're after a totally flat characteristic speaker, OB is as close as you'll get.

  2. Let me put it this way. Let's say we have two restaurants next to each other in competition. One of them serves generous meals at a reasonable price. The other serves skimpy meals at four times the price. Both have a nice interior, service and atmosphere. The expensive restaurant is counting on the quality of their food. But now let's say you do a blind test and find that they both taste the same. What do you do when you find the only advantage of the expensive restaurant actually turns out to be an illusion that you wanted to believe?

    Have you actually put your ideas to the test?

    I have investigated and experienced everything that I consider critical to getting the best bass reproduction possible. There are many things necessary to achieve this, most of them are covered in my bass integration guide. The only one I left out is group delay correction, which is possible with DSP. I find no real evidence that open baffle dipole bass brings anything to the table that can't be done better in other ways, all of which with much higher output and efficiency. There are some poor quality woofers that sound better on open baffle, but if you start with something decent, I highly doubt you will find any real improvement, the kind that a group of people would pick in a blind test as clearly better.

  3. Interesting, i'm using DSP'ed GPA Altec Lansing clones and they sound much better in OB than in closed box. They also connect well with fullranger in OB, not so well in box. For under 50Hz it may work, but over it OB bass is simply more natural and clean.


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