The NaO Mini Design objectives.
PLEASE NOTE: These pages are now for historical purposes. Music and Design is no longer offering these products for sale.
The design of the NaO Mini follows the hybrid approach used in the NaO Note and NaO II full range speaker systems. In the hybrid
approach an active crossover and equalization is used between the woofer and main panel, and a passive crossover is used between
the midrange and tweeter. The passive crossover is designed in such a manner that its only function is to control the response of the
midrange and tweeter through the crossover region. Irregularities in the midrange response due to the open baffle design and
matching the tweeter and midrange level are addressed in the active circuit. In this manner very little amplifier power is wasted in the
passive crossover compared to a design where the necessary response equalization is performed in the passive circuits. The hybrid
approach results in a cost effective, biamplified system which has many of the benefits of a fully active system without employing
additional channels of amplification. Further discussion on the Hybrid design philosophy can be found in the Music and Design
It is also noted that, contrary to other open baffle designs, the Mini uses a sealed box, monopole woofer system. There is sound
reasoning behind this. A recent AES article by Backman examined dipole, monopole and cardioid woofer systems with regard to
room interaction and sensitivity to listening/system position. His results indicate that in the sparsely populated modal region of the
response, which is centered around 100 Hz for a typical listening room, cardioid woofers exhibit the least sensitivity to changes in
system or listening position. Backman found dipole woofers to be the most sensitive. Additionally, below the room fundamental
Backman shows that the dipole response drops off rapidly due to the inability of a dipole source to pressurize a room. The behavior of
cardioid and monopole woofers are similar in nature below the room fundamental, however, the monopole woofer system benefits
from higher sensitivity. [This is a result of the 6dB/octave roll off of 1st order gradient type speakers; dipoles and cardioids.] These
results are similar to those discussed in the Music and Design articles on Room Response. Backman thus suggests that a woofer
system which operates in cardioid mode through the sparsely populated modal region and undergoes a transition to a monopole
response below the room fundamental could be optimum for low frequency reproduction.
Several excerpts are quoted from the Backman paper below:
“Below the first mode the sound pressure, especially for the monopole and the cardioid, first increases with the decreasing frequency,
and then reaches a flat level. The increase can be explained by the fact that below the first mode the sound pressure gradually starts to
obey the adiabatic equation of the state, which predicts the sound pressure to be proportional to the relative change in the enclosed
[room] volume...The flattening of the response at the very lowest frequency results from the room losses; in realistic rooms the losses
at very low frequencies are not frequency-independent.
“The dipole speaker does not produce a net change in the room volume, but due to the direct sound and the first reflections from the
boundaries, which are not totally cancelled by later reflections in a lossy room, some sound pressure is still produced.
“Although the change in room absorption appears to have about equal effect on all the speaker types ... the cardioid appears more
immune to even extreme changes in room absorption around the lowest modes and just below them. The dipole, as is expected, has
significantly less low-frequency output than the other types.
“The results from source placement simulation indicate that a cardioid speaker is more immune to source position effects in the
sparsely modal region than omnidirectional [monopole] or bidirectional [dipole] speakers, confirming the hypothesis of more even
“As a summary of the results presented above for the room-speaker interaction it can be stated that cardioid source has more immunity
against changes in source placement or room absorption in sparsely modal range. Below the lowest mode the cardioid speaker does
not have any advantage over the monopole source, but both exhibit both higher output and less source position dependence than the
dipole speaker. These results indicate that creating a loudspeaker that has a unidirectional polar pattern [cardioid] in the sparsely
modal region and omnidirectional below the lowest mode represents a good compromise between low frequency output capability and
avoiding room coloration effects. ”
1) Low-frequency polar pattern control for improved in-room response. Juha Backman, Presented at the 115th Convention 2003
The design of the NaO Mini employing dual woofers (one woofer for each channel) uses a novel approach to accomplish this result for
low frequency reproduction. Referring again to the Technical Studies area, and specifically to the technical article on Crossovers
between different sources, it is apparent that when a monopole source (Mini woofer) is crossed over to a dipole source (Mini panel)
using a 4th order acoustic crossover the polar response varies continuously from a dipole above the crossover, to a cardioid through
the crossover region and then finally to a monopole response well below the crossover point. Since the crossover between the dipole
panel and monopole woofer is centered in the sparsely modal region such a combination should result in a system which tends to
minimize low frequency room interaction effects and sensitivity to placement.
Along with the consideration of excitation of room modes and room response, consideration was also given to the power response of
the system at low frequency. Contrary to what may seem obvious, when a woofer located close to the floor is coupled to a midrange
several feet above the floor, with the same radiation pattern, be it monopole, cardioid, dipole or otherwise, the total acoustic power
radiated by the woofer will 3dB below that of the midrange. This is because the midrange, being located away from the floor, radiates
into full, 4π space. That is, it is free to radiate sound in all directions. However, a woofer sitting on a floor or ground plane can only
radiate sound into ½ space or 2 π space. The result is that the total power radiated into the listening environment is reduce by 3dB
when the anechoic on axis level is matched between the midrange and woofer. Thus, if the total power is to remain frequency
independent the woofer system must have a fuller radiation pattern than the midrange. Such is the case between dipole midrange and
monopole woofers. A monopole woofer radiating into 2 π radiates 1.8dB more acoustic power than a dipole midrange radiating into 4π
space. Not a perfect match, but closer than the -3dB drop in power which would occur with a dipole woofer. Further discussion of this
power matching can be found in the Music and Design article on Midrange to woofer power matching.
Consideration was also given to the maximum SPL capability of the different woofer systems. A single NaO Mini woofer using the 10”
Peerless XXLS driver is capable of producing an excursion limited SPL of 101dB at 32 Hz. An H dipole woofer system with 18” front to
back separation would require four 10” XXLS drivers to obtain the same SPL. An 18” U frame cardioid would require two drivers. Such
considerations were instrumental in the design of the NaO II woofer system where the U-frame woofer could be quickly converted to
sealed box woofers for extremely high SPL applications. For systems using dipole woofers, the situation is further complicated by the
reduction is SPL which occurs below the room fundamental. For that reason, dipole woofer systems are typically augmented by sealed
box subwoofer systems when high SPL, low frequency response is required. This inevitably leads to further degradation of the bass
response due to the introduction of an additional crossover with associated group delay and the additional cost of the subwoofer and
An additional consideration arises from anecdotal observations which suggest dipole bass just sounds more natural. Research at
Music and Design suggests that the primary reason for this observation lies precisely with the inability of a dipole woofer to
pressurize a room below the fundamental. However, monopole woofers may sound boomy or ill-defined at low frequency, even if
designed to have flat anechoic response, if the woofer’s cut off frequency lies significantly below the room fundamental. This is
particularly true in small rooms. To circumvent this problem the NaO Mini woofer response can be tuned by changing the value of a few
resistors in the active circuit. An Excel spreadsheet is included with the plans for calculating the suggested resistor values for you
Lastly, there are always situations where for any number of reasons the woofer system is best located at a position different from the
main speakers. The separation of the woofer system from the main panels in the NaO Mini makes it possible to alter the placement of
the woofers and panels independently should the need arise. While this does mitigate the benefit of the frequency dependent transition
between source types, it offers greater flexibility in system setup in problematic rooms.
It should be apparent that every consideration has been given to the design of the NaO Min in an effort to bring fourth the highest quality
sound reproduction in a small, flexible and economic package.