Why does this loudspeaker have only one drive unit?
This single loudspeaker is up for sale on Ebay and, at the time of writing, the bidding stands at $2125 with four days to go! Clearly there must be more to this loudspeaker than meets the eye, to command such a high price.
The reason for the price is that inside this loudspeaker is a vintage Tannoy drive unit and the cabinet is made to their 'Canterbury' design. You can see a plan for the similar York cabinet here.
Tannoy drive units are incredibly collectable these days. One reason is that they have a certain special sound quality that is hard to find these days. It isn't exactly a natural sound quality to my ears, but very pleasant to listen to, and very effective for studio monitoring.
The other reason is their famous dual-concentric design. Although it may seem that this loudspeaker has only one drive unit, the Tannoy Monitor Silver drive unit is actually two drive units in one - the tweeter is mounted concentrically inside the woofer's cone.
The brilliance of this idea is that the low frequencies and high frequencies both come from the same point source in space. Thus you can walk around the room, stand up and sit down, and hear a much more consistent balance of frequencies wherever you go.
That isn't the case with loudspeakers that have multiple drive units. Not only might you be closer to one drive unit than the other(s), the different distances from the drive units to your ears when you are not directly in front can cause phase cancellations at certain frequencies.
If you have conventional near-field monitors mounted vertically, then the difference in sound is quite noticeable whether you sit or stand. With Tannoy dual-concentrics, you can mix standing on your head if you like!
Although this would be an expensive buy even if you do plan on only ever monitoring in mono (the auction is for a single loudspeaker), I would love to see the dual-concentric design return to the mainstream. It is a valuable monitoring option that should be much more widely available - hopefully at a reasonable cost!
As noted in the comments there are other coaxial drive units. However, it is important not to confuse a device that has two drive units, like the Tannoy described in the article, with a drive unit that has two cones, where the central cone is a passive 'whizzer cone'. Whizzer cones can be useful, but it isn't the same as a true dual concentric.