Open baffle: a loudspeaker without a box

An open baffle is a driver on a board with no box. The sound from the back of the cone is not absorbed but radiates freely backwards. The speaker therefore plays forwards and backwards in opposite phase, and almost nothing to the sides. This kind of radiation is called a dipole.

Open baffle: a driver on a bare board. On the left the front wave, on the right the rear wave in opposite phase. At low frequencies the rear wave wraps around the edge of the board and cancels the front one.

How it works

At high frequencies the board is large enough to keep the front and rear waves apart. At low frequencies the rear wave wraps around the edge and cancels the front one. Below the frequency set by the path around the board the bass therefore falls at 6 dB per octave, and lower still, below the driver's resonance, at 18 dB per octave.

Siegfried Linkwitz showed that a dipole excites the room less: the side walls get less sound and the critical distance is about 1.7 times larger. The sound therefore feels spacious and the reflections colour it less. The price is bass equalisation and drivers with a lot of excursion.

What the calculation shows

An open baffle needs a different woofer from the other types: one with a high Qts and a low resonance. The chart therefore uses a 15" Monacor SP-382PA (Fs 25 Hz, Qts 0.75) on a baffle with a 33 cm path around the edge.

TotalThe same driver without the rear wave

Open baffle with a 15" driver. Without equalisation the bass at 50 Hz is 10 dB and at 30 Hz 16 dB below the midrange. The dotted line is the same driver with no effect from its rear wave.

The difference between the lines is the loss caused by the rear wave. It is made up with equalisation or a bass boost in an active crossover, which takes power and excursion. That is why dipole bass is built with large drivers, often two per side.

The impedance of an open baffle has a single peak at the driver's own resonance, here 29 Ω at 25 Hz. No box shifts it.

Pros and cons

Pros: no box to resonate, spacious sound, less room excitation and fewer standing waves. An excellent stereo image.
Cons: weak bass without equalisation, it needs large drivers with a lot of excursion and a powerful amplifier. The speaker must stand well away from the rear wall, at least 1 m, otherwise the rear wave reflects back.

Which driver and which room

Drivers with a Qts of 0.6 to 1.2 and a low resonance, for example 12" to 15" PA drivers or dedicated dipole woofers. For larger rooms where the speakers can stand free.

Well-known designs

Siegfried Linkwitz: ORION and PHOENIX with two woofers in an H-frame, an active crossover and equalisation. Linkwitz also described the dipole models that Loudsmith is based on.
Martin J. King: a comparison of a flat baffle, a U-frame and an H-frame with a 15" Eminence Alpha 15A. A frame tunes the same driver lower: the F3 is 41 Hz on a flat baffle and 28 Hz in an H-frame.

In Loudsmith

The open baffle is one of the box types. Loudsmith calculates the rear wave around each edge of the baffle following Linkwitz's models, and warns you if a driver's Qts is too low for a dipole. The project library includes the ORION-3 and several H-frames by Martin J. King.

Enclosure types series: sealed · bass reflex · passive radiator · aperiodic · transmission line · TQWT · ML-TL · open baffle · back-loaded horn. You will find the formulas in the article Speaker box calculations step by step.