My interest in omni speakers was sparked recently at an audio gathering I hosted. We featured three sets of speakers, my own open baffle speakers and two pairs from Gainphile, one of the other attendees. He presented his own open baffle speaker as well as a Linkwitz Pluto clone.
Listening to a familiar track on Gainphile's omni, I was struck by the imaging! On the one hand, it had the spacious sound stage I enjoyed from my open baffle speakers, yet the imaging was sharper! It gained my immediate interest as a result.
My omni experiment
I decided I had to try this for myself, and have more time to experience omni speakers. I built a simple prototype using my existing drivers and crossover, and using Ultracurve to eq to the same target house curve that I'm accustomed to. The tweeter fires forwards, while one midbass fires up to the ceiling. The crossover point is 3.5k.
How does it sound?
Unfortunately, I didn't manage to achieve the same focused imaging of Gainphile's Pluto clone. I did, however, get a chance to experience the sound and appreciate some other benefits. Firstly, the sound stage is large and spacious like an open baffle speaker. It's clearly a bigger sound than you can achieve with a box speaker. It also has a very large sweet spot, even bigger than open baffle. The entire vocal range is covered with one driver which is radiates in an omnidirectional pattern on the horizontal axis from 80 - 3.5k. As a result, the sound changes much less across the room. When hosting a dinner party I had people sitting in places that I normally wouldn't when listening to the system, yet the sound didn't change much. No other type of speaker could achieve this result as well. Most would fail terribly.
I don't feel the sound is quite as transparent as an open baffle speaker. While the sealed box is lightly stuffed with dacron, I get the impression that there is a small amount of box coloration that isn't there with open baffle.
Omni speaker theory
For a long time I felt omni speakers had the wrong idea. It didn't make much sense to me to radiate sound the same way in all directions, but instead to direct the sound where you want it. And yet the legendary Siegfried Linkwitz recommends omni speakers, so one has to at least give them some serious consideration. The idea is that it has an even polar response in all directions. This means that sounds which are reflected around the room will be closer in character to the direct source. Less coloration of reflected sounds results. The increased reflected sound creates a larger sound stage and creates a spacious sound.
In order for a driver to operate as an omnidirectional source, there is a relationship between the size of the driver, and the wavelength. A 6.5" driver will be omnidirectional below 1k in all directions. The Linkwitz Pluto therefore crosses at this point to the tweeter. Most tweeters aren't suitable for such a low crossover point, so a larger driver with high excursion is used. The 2" Aura driver used in the Pluto operates as an omni source up to 3k.
It becomes clear that my prototype is a compromise. The midbass should ideally be crossed at 1k, but since it is crossed at 3.5k it is increasingly directional above 1k. This means more sound is directed towards the ceiling. For permanent use, this could be addressed with acoustic treatment. From a horizontal point of view it behaves as an omni through it's entire range.
More information, as well as kits are available at Linkwitz Lab
Pros and Cons
+ small and unique form factor possible
+ very good DIY kits available
+ huge sweet spot allowing them to be appreciated over a much larger area
+ spacious sound stage of open baffles yet with potential for sharper imaging
+ even polar response results in less coloured room reflections
- difficult to achieve high efficiency or high output
- very low crossover point required for tweeter
Overall omni speakers are a very attractive option which are rarely considered at all.
The ideal application for omni speakers is as a music system in a typical living room. High output won't be required and their huge sweet spot will mean they are appreciated more by all. Their novel form factor and modest size is also a bonus in this application. This isn't to suggest they aren't suitable for serious use in a dedicated room - indeed they are well suited to this. However, my personal preference is for speakers with higher output capability.
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