DUP's system is based on Legacy's Whisper speaker, an unusual design, that uses four 15" woofers mounted in two pairs with one woofer behind the other and open to the speaker's rear. The rear cover has been removed in Jeff Wong's photo, so you can see the backs of the woofers. The gap between each woofer pair is open to the air, not sealed as in an Isobarik design, which Legacy's Bill Dudleston says increases the array's directivity.