This is a high performance base station speaker that, although designed for the K3 series, is equally at home with almost any make of base station transceiver. If you take a look at a typical base station extension speaker you will see that it comprises a speaker at the front and a mass of empty space. This is NOT the way to design a speaker!
Elecraft have employed a professional audio engineer to design this new speaker - only Elecraft would take such trouble! The result shows that it has been worth while. All loudspeakers are intended to be mounted on some kind of sound board and thereafter into a cabinet. It is the enclosure that often makes the big difference. An empty cabinet will have a very obvious series of resonances, the very last thing that you want. Elecraft have used accoustic material to fill and dampen these resonances. This results in a very smoth and predictable frequent resonse. The simple test that we ourselves carried out was to tune a hetrodyne from zero to 5kHz to see what the response was. There was absolutely no change in level that we could hear. Swapping over to another 'popular' design that essentially was an empty box, resulted in a very marked series of resonances and cabinet rattles. The SP3 is in a different class.
Another feature of the SP3 is that it permits the selection of two outputs, so you can use it for two different radios.
NOTE: The SP3 is supplied complete with interface lead for connection to your radio.