BSS Audio FDS366T OMNIDRIVE User Manual page 21

Crossover Shapes and Frequencies
WHISEWORKS-NTM
Crossover
Thiele 4 rolloff
10
0
10
20
30
40
50
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1 . 10
100
Hz
The WHISEWORKS-Neville Thiele Method (NTM) crossover filter is a new
type of electrical/acoustical filter offering significant performance advantages
over all previous crossover filter types in audio applications. The filter was
developed by Neville Thiele (pronounced "Teel").
This filter uses a unique notched response to achieve a very steep roll-off rate
outside the passband. There are two choices of NTM crossover included in
the FDS-366T; a 4th order (NTM 36) and an 8th order (NTM52). The 4th order
shape offers exceptional group delay flatness with a roll-off of at least 36dB/
octave. The 8th order version offers the steepest roll-off rate of all.
WHISEWORKS-NTM Crossover Filter advantages:
1. It sums to give constant output against frequency.
2. It has a well defined phase response.
3. It keeps the phase difference between adjacent drivers at zero degrees
throughout the crossover region to prevent beam shifting.
4. It has the highest known rate of attenuation in the stop band of all
conventional analogue or analogue-equivalent filters of similar order.
4
1 . 10
The faster roll-off rates realised by the NTM filter shape enable lower
distortion that is particularly valuable in applications where space is at a
premium as smaller drivers may be used. Driver and cabinet design
constraints are also eased since designers can work 'closer to the edge' where
resonance, breakup, or other anomalies may normally occur with more gentle
slopes.
The new Neville Thiele Method crossover shapes now represent the optimal
combination of characteristics for most applications and listening tests seem
to bear this out! We feel that this shape could topple the Linkwitz-Riley as
the new 'industry standard', particularly as the nearest Linkwitz-Riley
equivalent (L-R 48) has attained a reputation for a "hard sound", this doesn't
appear to be the case using the NTM shapes.
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FDS-366T