Proceedings of the Institute of Acoustics
systems (Table 9) and even relatively simple two-way systems show a substantial improvement
compared to the number of steps needed by direct search method (Table 5). There are two main
reasons for this improvement: the constraint of not allowing modification of all three driver level set-
tings simultaneously and breaking-up of optimisation into stages.
Table 9. Number of filter evaluations by the optimisation algorithm.
Type
of
loudspeaker
Optimisation stage
Large
3-way
2-way
Small 2-way
Preset
bass
roll-off 6 6 6 3
Find midrange/ treble ratio
49
49
-
-
Set bass tilt and level
35
35
5
5
Reset
bass
roll-off
6 6 6 3
Set treble tilt
5
-
4
2
Total 101
96
21
13
Total re. direct search
0.2%
1.1%
26%
81%
The run time on a PII 366 MHz computer for three-way and large systems is about 15 s (direct
search for three-way systems 3 minutes, large systems 15 minutes). The processing time is directly
proportional to the processor speed. A modern PIII 1200 MHz based computer takes about 4 s to
perform the same optimisation. Further improvements in the software have improved run times by
about 30%.
3.4 Algorithm
Features
3.4.1 Frequency Range of Equalisation
The default equalisation frequency range is from the loudspeaker low frequency –3 dB cut-off
f
LF
to
15 kHz. Manual readjustment of the design frequency range (indicated on the graphical output by
blue crosses, Figure 1) is needed in some special cases. Examples of these include a strong can-
cellation notch in the frequency response around
f
LF
, when off-axis loudspeaker location reduces
significantly the high frequency level, when a loudspeaker is positioned behind a screen, or when
the measuring distance is very long. It is naturally preferable to remove such causes of problems, if
possible.
3.4.2 Target for Optimisation
There are five target curves from which to select:
1. ‘Flat’ is the default setting for a studio monitor. The tolerance lines are set to ±2.5 dB.
2. ‘Slope’ allows the user to define a sloping target response. There are two user defined knee fre-
quencies and a dB drop/lift value. A positive slope can also be set but is normally not acousti-
cally desirable. The tolerance lines are set to ±2.5 dB. Some relevant slope settings include:
•
for large systems a –2 dB slope across the passband up to 15 kHz to reduce the aggressive-
ness of sound at very high output levels
•
–2 dB slope from 4 kHz to 15 kHz to reduce long-term usage listening fatigue
•
–3 dB slope from 100 Hz to 200 Hz for Home Theatre installations to increase low frequency
impact without affecting midrange intelligibility
3. ‘Another Measurement’ allows the user to optimise a loudspeaker’s frequency response magni-
tude to that of another loudspeaker. For example, measure the left loudspeaker and optimise it,
then measure the right loudspeaker and optimise this to the optimised left loudspeaker re-
sponse. This results in the closest match possible between the left-right loudspeaker pair ensur-
ing good stereo pair match and phantom imaging. Tolerance lines are set at ±2.5 dB.
4. ‘X Curve – Small Room’ approximates the X Curve for a small room (volume less than 5300 cu-
bic feet or 150 cubic meters) as defined in ANSI/SMPTE 202M-1998
7
. The curve is flat up to 2
