August 16, 2009

DIY Open Baffle speakers


It all started with a saturday afternoon experiment. A very quick and dirty chipboard baffle and H frame. Three years later I'm still hooked on the sound. The difference became most obvious when I tried to go back to box speakers. After listening for a few weeks then changing back, it became clear that open baffle speakers have a certain magic to them I find hard to live without.

What is an open baffle speaker?

An open baffle speaker is variant of a dipole where no box is used. Electrostatic loudspeakers are also an open baffle dipole, but I use the term here to refer to speakers based on conventional dynamic drivers.

The key attributes to an open baffle speaker are:
  1. No box - the output from the rear of the driver is used constructively, rather than attempt to absorb it's energy inside a box; this results in reduced box coloration and an improved interaction with the room.
  2. Dipole radiation - increased rear radiation and reduced side, floor and ceiling reflections
  3. Velocity source bass interacts differently with room modes.
A fair comparison

In changing between a box speaker and open baffle, I attempted to remove anything that might sway me either way. I used the same drivers and crossover, and used EQ to get the same frequency response.

How do they sound?

The biggest difference is in the sound stage and imaging. The sound stage is bigger - wider, deeper and more immersive. In switching from open baffle to dipole, the sound stage seems immediately small. I find the sound stage of open baffles to be more realistic and natural. I also find that the imaging is more stable. It's less likely that you will need a centre speaker (and more difficult to include if you do want one). The sweet spot is bigger and as you move sideways across the room, the sound stage is less inclined to collapse to one side.

The downside is that the imaging is not as tight and focused. When sitting in the sweet spot and where everything is just right, and you are listening to a recording where the imaging isn't confused, box speakers have the potential to create a tighter image where it's easier to pin down where the sound is coming from. Open baffle speakers seem to stretch the sound stage. It's like comparing a projector to an LCD TV. You can't expect the projected image to be equally sharp. It's a matter of personal preference. Which do you prefer? It's a decision you can't make without experiencing both options.

One of the more subtle qualities of an open baffle speaker is the absence of box coloration. High end speaker manufacturers work very hard to try to remove the impact of the rear wave, but it is never completely removed. As a result, budget drivers on an open baffle can compare very well to much more expensive drivers in a box.

Bass - monopole vs dipole

Open baffle purists insist on dipole bass as well. This is where one has to balance cost, size and output. To match the output of a conventional sub or woofer an open baffle needs to move much more air. Open baffle bass has a number of advantages:
  1. No box coloration
  2. Different room interaction
The first advantage is part of the appeal of infinite baffle subwoofers. The second is unique to dipoles. With open baffle room pressurisation doesn't occur. Room modes are excited differently.

There are two schools of thought regarding the room interaction of dipoles with the room. One is in favour of dipoles as a means of minimising the excitation of room modes. This view has been made popular by Siegfried Linkwitz. A different view is promoted by Dr Earl Geddes, in which it is considered more desirable to deliberately excite as many room modes as possible with monopole subwoofers in order to achieve a smooth in room bass response. My own preference is for the Geddes approach.

My initial listening tests showed open baffle bass can be more accurate with conventional subs. However, once I obtained some Rythmik Servo subs, I discovered this isn't always the case. I set up two Rythmik subs - one a monopole and the other in a H frame. Both were calibrated so that their output and response were identical. Then listening to one of my familiar reference tracks with acoustic double bass, I compared, switching from one to the other until. Which one was more accurate? They were so close, it's hard to say any differences were real or imaginary.

I believe there is a reason the Rythmik servo subs didn't reveal a difference. The servo provides extra damping which vastly reduces the impact of the rear sound wave. Box coloration is reduced to the point that you can't tell if you are listening to a monopole or open baffle. This is why I now run my subs as monopoles. There is no reason in this case to give up output.

Navigating complexity and performance

Open baffle speakers are normally considered expensive, complex and large yet with serious output limitations. This is not necessarily true, depending on certain choices.

My open baffle speakers are relatively simple and have high output without being large. The main panels have dipole roll off below about 300 Hz. This means eq is needed between this point and 80 Hz where the main panels cross to the subs. Midbass drivers can easily handle this eq and in real use I find the system sounds stressed before running out of excursion. The efficiency in the midrange is actually quite high at around 94 db 1w1m. Where high output is more important, an efficient woofer could be used as a monopole crossing where dipole roll off occurs.

Digging deeper

There is a lot more that can be said about open baffle loudspeakers. If you'd like to read more, then I recommend the following websites:

Linkwitz Lab - the best online source of information on open baffle loudspeakers
Music and Design


  1. Hi Paul, I have read a few of your articles and comments at the forum and found them all to be very well researched and in line with my own experiences. In regard to dipoles have you ever listen to the Oskar Heil Ess tweeter? Could be a good fit in your dipoles?

  2. Hi Sommar and thanks for the feedback. I've seen it mentioned many times but don't think I've actually heard one. I'm now wanting to try waveguides and compression drivers - not sure if this will fit with open baffle midrange just yet.

  3. This comment has been removed by a blog administrator.

  4. Hi Paul,
    what are the drivers & crossover for this open baffle speaker?

  5. Vifa P17 and D25AG with a passive 2nd order crossover at 3.5k

  6. Hi Paul,

    When I built my first 807 SE tube amplifier, I bought a few dirt cheap Visaton BG 20 full range units to build a TQWT with. But entertaining some friends and wanting to demonstrate my new amp, I made up a few very quick and very dirty open baffles like yours. One of the friends had Magnepans at home and was very impressed by the hardly run in Visaton's. I played with them for about two years, longing for more bass and more highs I switched to a closed box system. Although the bass of the O.B. was actually very good. But I was never satisfied with conventional box speakers. I now have a two way monitor with which I am very satisfied. But the fact that I am now commenting shows the longing for open baffle's. I miss the "live" sensation of the O.B.'s, they do not do everything right, but what they do is mesmerizing and a habit hard to kick.
    Thanks for the interesting piece of reading material.

    Best regards from the Netherlands,

  7. Still waiting for those open baffle with compression driver horns.
    How did they sound?

  8. Hi Lauge, it's not really on my radar now as I have my hands full. Andi (Gainphile) has done a lot of experiments with open baffles, some of them going in that direction. If you've not seen it already, you might want to check out his blog

  9. Hi Paul,
    Thanks for the link. Though not exactly what I was looking for, but I kept going and found this thread:
    Wich I think fullfills my demands for a passive x over with few elements.


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