Secrets Q & A

Speakers with No Box Enclosure

Question
I heard some speakers at a local HiFi show and they were mounted on a single board with no box enclosure, so you could look behind the board and see all of the speakers, wiring, and cross over network. What is supposed to be the desired effect of mounting them this way?

- Steve G
Location: Atlanta, GA

Answer:
These types of speakers are known as open baffles (OB) or sometimes referred to as dipolar, which means they radiate sound waves in 2 directions, front and back. Compared to conventional box speakers, some find that the sound stage is bigger and creates a more realistic sense of spaciousness. The sound can also be more immersive and the “sweet spot” much bigger.

It is less critical to sit in dead center of the room and the sound will seem to coming from more than just speakers on the front wall. The down side to this is the imaging can be less precise and not as tightly focused as a conventional boxed speaker, though some find this trade off to be acceptable. An open baffle speaker has a more controlled interaction with the room. Placement can be important, as being too close to the front wall can cause cancellation of some frequencies as the primary and secondary reflected waves reach the listeners ears out of phase.

As you can see above, the polar response of the open baffle speaker has a more controlled dispersion pattern. There is a null on both sides of the speakers - this means less side wall reflections and less ceiling reflections. The only increase is in the radiation from the rear of the speaker in the midrange which if set up correctly, can increase the ambience and enhance the sense of spaciousness. It's also possible to eliminate completely the first reflection from the front and rear walls, if certain placement guidelines are followed. There are many variations of OB speakers and several websites dedicated to helping people build them from scratch, if you are a DIY’er. Only you can decide if the sound of an OB is “right” for your room.

So, just to reiterate, 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 its 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 and doesn’t pressurize the room like a sealed woofer.