Specifications
15
Operating Manual - ne24.24M Matrix Processor
The steeper the slope, the more abruptly the “edges” of the pass band will drop off. There is no best lter slope for
every application, so experiment to see which one sounds most pleasing in a specic system. The Ashly default crossover l-
ter is 24dB/octave Linkwitz-Riley, but of course they can be changed to suit the application. In addition to the frequency and
slope, crossover lters can be selected as having Butterworth, Bessel, or Linkwitz- Riley response. These refer to the shape of
a lter’s slope at the cut-off frequency, affecting the way two adjacent pass bands interact at the crossover point. 24dB/octave
Linkwitz-Riley lters produce a at transition through the crossover region, assuming both overlapping lters are set to the same
frequency, slope, and response type. 24dB/oct Linkwitz- Riley lters are the industry standard, the easiest to use, and the lter
type recommended by Ashly. Other lter types are available, but may require polarity switching or other adjustments for proper
results. The following paragraphs offer a summary of the three lter types as used in the ne24.24M crossovers.
Butterworth
Butterworth lters individually are always -3dB at the displayed crossover frequency and are used because they have
a “maximally at” passband and sharpest transition to the stopband. When a Butterworth HPF and LPF of the same crossover
frequency are summed, the combined response is always +3dB. With 12dB per octave Butterworth crossover lters, one of the
outputs must be inverted or else the combined response will result in a large notch at the crossover frequency.
Bessel
These lters, as implemented on the ne24.24M, are always -3dB at the displayed crossover frequency. Bessel lters
are used because they have a maximally at group delay. Stated another way, Bessel lters have the most linear phase response.
When a Bessel HPF and LPF of the same crossover frequency are summed, the combined response is +3dB for 12dB/oct, 18dB/
oct, and 48dB/oct Bessel lters, and -2dB for 24dB/oct Bessel lters. One of the outputs must be inverted when using either
12dB/oct or 18dB/oct Bessel crossover lters or else the combined response will have a large notch.
Linkwitz-Riley
The 12 dB/oct, 24dB/oct, an 48dB/oct Linkwitz-Riley lters individually are always -6dB at the displayed crossover
frequency, however the 18dB/oct Linkwitz lters individually are always -3dB at the displayed crossover frequency. The reason
for this is that Linkwitz-Riley lters are dened in terms of performance criterion on the summing of two adjacent crossover
HPF and LPF lters, rather than dened in terms of the pole-zero characteristics of individual lters. The 18dB/oct Linkwitz-
Riley individually are 18dB/oct Butterworth lters in that they have Butterworth polezero characteristics and also satisfy the
criterion for Linkwitz-Riley lters. When a Linkwitz-Riley HPF and LPF of the same crossover frequency are summed, the
combined response is always at. With 12dB/oct Linkwitz-Riley crossover lters, one of the outputs must be inverted or else
the combined response will have a large notch at the crossover frequency.
8.2c Output Delay
Output delay can be used to time align discrete drivers within a cabinet or
cluster using short delay times, or align multiple drivers in different locations using
longer delay times. The following example illustrates a use of short delay to time align
speakers within a group: A typical three way speaker cluster has low end, midrange,
and high frequency drivers all located near one another. The different drivers for each
frequency band are not necessarily the same physical depth with respect to the front of
the loudspeaker cluster, so there exists the problem of the same signals (at the cross-
over points) arriving at the cluster “wavefront” at different times, creating undesirable
wave interaction such as frequency peaks or cancellation. The solution in this case,
rather than xing the frequency anomalies with EQ, is to slightly delay the signal to
the drivers closest to the cluster front. Using the location of the driver diaphragm
farthest back as a reference point, measure the distance to other drivers in the cluster,
and set the output delay for each accordingly, with the driver diaphragm closest to the
front getting the longest delay and the driver at the very back getting no delay at all.
The minimum adjustment is 0.02 milliseconds, or about 1/4 inch. When appropriate,
always time align the loudspeakers before applying EQ to the outputs of the ne24.24M.