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RE: What is the purpose of the rear wave in dipole speakers? Can you just put planars in a box?

Posted by josh358 (P) on August 25, 2015 at 07:27:52

In Reply to: What is the purpose of the rear wave in dipole speakers? Can you just put planars in a box? posted by jodaboda@gmail.com on August 19, 2015 at 14:54:19:

Off the top of my head, there are several advantages to a dipole radiation pattern:

1. Less energy gets dumped into the room, because the dipole radiation pattern has a "figure 8" characteristic with no sound at the sides. This means you need less acoustical treatment of room reflections.

2. No box resonances, which color the sound and are very expensive to minimize.

3. The rigid, non-resonant, furniture-quality enclosure adds significantly to the cost of a box speaker.

4. Less excitation of room modes for smoother bass.

5. No baffle step. This is the point at which a box speaker switches from an omnidirectional to the cardioid pattern as the wavelength becomes small with respect to the baffle and stops diffusing around it. This makes the power response right, which is impossible with an omni if you want to maintain on-axis response -- and research indicates that good power *and* on axis response is critical to sound quality.

Disadvantages:

1. Dipole cancellation reduces bass output, requiring a 6 dB/octave bass boost (in many dipoles, the boost is accomplished acoustically). This increases the size and cost of the speaker and reduces bass output and extension.

2. At higher frequencies, rather than a 6 dB/octave rolloff, you get comb filtering. This can be minimized e.g., with trapezoidal baffles, which alter the path length continuously.

3. Directionality is great if you don't want to disturb the neighbors or add lots of acoustical treatment but it makes dipoles poor party speakers, since the sound level diminishes as you move off axis.

4. In the right position, dipoles achieve better results acoustically, but by the same token they're more sensitive to room placement. You can't just throw them on a shelf. Often, inches matter to the sound. However, it should be noted that box speakers are sensitive to placement, too. You won't get good imaging if you put box speakers against a wall -- they need to be out, just like dipoles.

The reason for this is that the time delay of reflection off the front and side walls is what tells the brain how large a space is. In effect, it's artificial reverb. So if the path length from the speaker to the front wall and back to the speaker again is less than the distance from the musician to the wall of the studio or concert hall, the sense of depth and acoustical space will be diminished. This effect can be minimized by adding diffusion on the front wall (and sides, for a box), which makes the space acoustically larger.

In general, 5' is the minimum for reasonable imaging with most speakers because 10 ms is the point at which the brain starts to interpret the reflections as ambiance rather than part of the direct sound. But I've heard reports that depth keeps improving until the speakers are 15' from the front wall (something I've never been able to test, because I didn't have a large enough room!).

5. Since dipoles don't radiate much to the side, they don't have the width expansion that omnis get from the side reflections. However, in most setups, I've found that the sound does tend to wrap around the corners because of the second reflection (speaker to front wall to side wall to ear).

6. When out from the wall, both dipoles and omnis suffer from midbass cancellation from the reflected rear wave. But the effect is worse with dipoles because with omnis the midbass is being reflected off multiple surfaces, typically diluting the effect.

Whew! I'm sure I've left something out. But I think the bottom line is that while nothing is perfect dipoles sound more natural to me than boxes.

This post is made possible by the generous support of people like you and our sponsors:

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