The NaO II RS Design objectives.
Contrary to other open baffle designs, the NaO II RS uses a unique, damped U-frame woofer system.
Correctly damped, the U-frame woofer approximates a cardioid radiation pattern. There is sound reasoning
behind this approach. As discussed in an AES article, Backman  examined dipole, monopole and
cardioid woofer systems in the modal region with regard to room interaction and sensitivity to
listening/system position. His results indicated that, at low frequency, in the sparsely populated modal
region of the response, which is around 100 Hz and below for a typical listening room, cardioid woofers
exhibit the lowest sensitivity to changes in speaker or listening position. Backman found dipole woofers to
be the most sensitive. Additionally, below the room fundamental Backman showed that the dipole response
drops off rapidly since a dipole source is incapable of room pressurization. These results are similar to
those discussed in the Music and Design articles on Room Response.
The careful integration of the NaO II RS dipole midrange/tweeter panel with th damped U-frame, acoustic
resistance woofer results in a high performance, tri-amplified speaker system which: 1) achieves nearly
ideal dipole radiation throughout the critical midrange, 2) has very uniform polar response over a wide
listening window, and 3) provides great flexibility in coupling the system to the room.
1) Low-frequency polar pattern control for improved in-room response. Juha Backman, Presented at the
115th Convention 2003 October 10–13
The NaO II RS is an open baffle design which attempts to maintain
constant directivity in the form of dipole radiation above the Schroeder
frequency. The Schroeder frequency is defined as the frequency
which designates the transition between the higher frequencies where
the room reverberant effects dominate and the lower frequencies
where discrete room modes dominate the response. For design
purposes a representative value in the range of 100 Hz is taken as
the lower limit for the Schroeder frequency in typical listening rooms.
Below the Schroeder frequency the NaO II RS panel is integrated with
a unique, U-frame woofer system. The current design of the NaO II RS
follows a digital, fully active approach which requires a minimum of 3
channels of amplification.
A narrow baffle design for the NaO II RS midrange/tweeter panel is
implemented to achieve dipole radiation. The MTM format was chosen
to allow a sufficiently narrow baffle to maintain dipole operation of the
midrange drivers from the panel low frequency limit to the crossover
point of 2.2 k Hz and provide sufficient radiating area for the targeted
maximum SPL of the system. Above 2.2k Hz the NaO II RS employs
front and rear tweeters. While the rear tweeter helps slightly in
controlling the radiation pattern above the crossover point its primary
function is to improve the spectral balance of the sound radiated to
the rear of the speaker. This aids in achieving a more even spectral
balance between the direct and reflected sound.