Proceedings of the Institute of Acoustics
where
x(f)
is the smoothed magnitude of the in-situ frequency response of the system,
a
m
(f)
is the
midrange and treble level control combination
m
currently being tested,
x
0
(f)
is the target response,
f
1
and
f
2
define the ‘midrange and treble driver band’ (Table 7). The lower frequency bound is fixed
at 500 Hz but a user selectable high frequency value is permitted. The default value is 15 kHz.
The midrange-to-treble level ratio is saved for performing the third stage of the optimisation proc-
ess. The reason for this is to reduce the number of room response control combinations to be
tested in the next stage. This stage of the optimisation algorithm takes 49 filtering steps and is not
required for two-way models or small two-way models.
3.2.3 Bass Tilt and Bass Level
This stage of the optimiser algorithm filters using all possible combinations of bass tilt and bass
level controls for a given midrange/treble level difference. By fixing this difference the total number
of filter combinations can be reduced substantially.
A constraint imposed in this stage is that only two of the driver level controls can be set at any one
time. If three of the level controls are simultaneously set the net effect is a loss of overall system
sensitivity. Table 8 shows an example of incorrect and correct setting of the driver level controls.
Table 8. Driver level control settings.
Control
Incorrect Setting
Correct Setting
Bass level
–4 dB
–2 dB
Midrange level
–3 dB
–1 dB
Treble level
–2 dB
0 dB
Input sensitivity
–6 dBu
–4 dBu
The least squares type objective function to be minimised is the same as shown in Equation 3.
However,
a
m
(f)
is the bass tilt and bass level combination
m
currently being tested together with the
fixed midrange and treble level ratio setting found in the previous stage. Also,
f
1
and
f
2
now define
the ‘loudspeaker pass band’ (Table 7). The user can select both values. The default values are the
–3 dB lower cut-off frequency of the loudspeaker and 15 kHz.
This part of the optimisation algorithm takes 35 filtering steps. There are no driver level controls in
two-way or small two way systems so these virtual controls are set to 0 dB. The bass tilt control can
then be optimised using the same objective function. Only five filtering steps are required for two-
way and small two-way systems.
3.2.4 Reset Bass Roll-off
Firstly, the bass roll-off control is reset to 0 dB. Then the method used to set the bass roll-off earlier
is repeated, but without modifying upwards the final setting. The same objective function as pre-
sented in Section 3.2.1 is used.
3.2.5 Set
Treble
Tilt
The least squares type objective function to be minimised is the same as shown in Equation 3.
However,
f
1
and
f
2
now define the ‘loudspeaker pass band’ (Table 7). The user can select both val-
ues. The defaults are the –3 dB lower cut-off frequency of the loudspeaker and 15 kHz. This part of
the algorithm requires five filtering steps for two way and large models (three for small two way
models), and it is skipped for three ways because having no such control.
3.3
Reduction of Computational Load
The optimiser algorithm reduces the computational load by exploiting the heuristics of experienced
calibration engineers. As a result, the number of filtering steps has dramatically reduced for larger
