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NaO Design.....
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The NaO II Design objectives.
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The NaO II Design objectives.
A narrow, flat baffle design for the NaO II 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 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.
Contrary to other open baffle designs, the NaO II uses a unique damped, U-frame woofer system which can be
converted to a monopole through the addition of a removable rear panel. With the rear of the U-frame removed
the woofer approximates a cardioid radiation pattern. There is sound reasoning behind this approach. A recent
AES article by Backman[1] examined dipole, monopole and cardioid woofer systems in the modal region 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 lowest sensitivity to changes in speaker 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, as expected, since a dipole source is incapable of room pressurize. The behavior of cardioid and
monopole woofers is similar in nature below the room fundamental; both can generate room pressurization effects,
although 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. While the U-frame woofer with rear panel
removed operates in cardioid mode over its full frequency range, the required equalization is provided by the
active crossover. If higher SPL is required at low frequency the woofer system can be converted to a more
efficient monopole and a switch on the active crossover changes the woofer equalization appropriately. It should
be noted that when the woofer is placed in monopole mode the radiation pattern between the woofer and main
panel is intended to undergo a transition from dipole, to cardioid and then to monopole as the frequency
decreases through the crossover region as a 4th order Linkwitz/Rile crossover is implemented. Thus, a cardioid
response is still expected to be present through the transition centered at 120 Hz.
The result is a high performance, bi-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; 3) is integrated
with a woofer system which provides great flexibility is 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
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 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.
Contrary to other open baffle designs, the NaO II uses a unique damped, U-frame woofer system which can be
converted to a monopole through the addition of a removable rear panel. With the rear of the U-frame removed
the woofer approximates a cardioid radiation pattern. There is sound reasoning behind this approach. A recent
AES article by Backman[1] examined dipole, monopole and cardioid woofer systems in the modal region 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 lowest sensitivity to changes in speaker 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, as expected, since a dipole source is incapable of room pressurize. The behavior of cardioid and
monopole woofers is similar in nature below the room fundamental; both can generate room pressurization effects,
although 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. While the U-frame woofer with rear panel
removed operates in cardioid mode over its full frequency range, the required equalization is provided by the
active crossover. If higher SPL is required at low frequency the woofer system can be converted to a more
efficient monopole and a switch on the active crossover changes the woofer equalization appropriately. It should
be noted that when the woofer is placed in monopole mode the radiation pattern between the woofer and main
panel is intended to undergo a transition from dipole, to cardioid and then to monopole as the frequency
decreases through the crossover region as a 4th order Linkwitz/Rile crossover is implemented. Thus, a cardioid
response is still expected to be present through the transition centered at 120 Hz.
The result is a high performance, bi-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; 3) is integrated
with a woofer system which provides great flexibility is 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 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 panel is integrated with a unique, U-frame woofer system. The design of
the NaO II follows a hybrid approach. An active crossover 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
compensating for the gradient roll off of the woofer and midrange drivers is
addressed in the active circuit. In this manner very little amplifier headroom 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, bi-amplified system which has many of the benefits of a fully active
system without employing additional channels of amplification. Further discussion of
the Hybrid Design philosophy can be found in the Music and Design technical section
and application to the NaO II may be found under the NaO menu.
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 panel is integrated with a unique, U-frame woofer system. The design of
the NaO II follows a hybrid approach. An active crossover 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
compensating for the gradient roll off of the woofer and midrange drivers is
addressed in the active circuit. In this manner very little amplifier headroom 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, bi-amplified system which has many of the benefits of a fully active
system without employing additional channels of amplification. Further discussion of
the Hybrid Design philosophy can be found in the Music and Design technical section
and application to the NaO II may be found under the NaO menu.
NaO II by
Mike Hunt,
PA.
Mike Hunt,
PA.
