GOLDBERG AND MÄKIVIRTA
AUTOMATED IN-SITU EQUALISATION
AES 114TH CONVENTION, AMSTERDAM, THE NETHERLANDS, 2003 MARCH 22-25
6
Table 9. Number of filter evaluations needed 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%
3.4. Algorithm
Features
3.4.1. Frequency Range of Equalisation
The default frequency range of equalisation is from
the low frequency
–
3 dB cut-off of the loudspeaker
f
LF
to 15 kHz. If there is a wide band cancellation in the
frequency response around
f
LF
, or the high frequency
level is decreased strongly due to an off-axis location
or the loudspeaker is positioned behind a screen or
due to very long measuring distance, manual read-
justment of the design frequency range (indicated on
the graphical output by the blue crosses, Figure 1) is
needed. Naturally it is preferable to remove the causes
of such problems, if possible.
Figure 1. Typical graphical output of the optimiser
software. Original response
x
(
f
), target response
x
0
(
f
)
and final response
y
(
f
). Also, –3 dB cut-off frequen-
cies (triangles), optimisation range (crosses) and target
tolerance (dotted).
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
’ gives a user defined 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 generally not desirable. The
tolerance lines are set to ±2.5 dB. Some relevant
slope settings include:
•
–2 dB slope from low frequency –3 dB cut-off
to 15 kHz for the large systems to reduce the
aggressiveness 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 intelligibil-
ity
3. ‘
Another Measurement
’ allows the user to
optimise a loudspeaker’s frequency response mag-
nitude to that of another loudspeaker. For example,
measure the left loudspeaker and optimise it, then
measure the right speaker and optimise this to the
optimised left speaker response. The result will be
the closest match possible between the left and
right speaker pair ensuring a good stereo pair
match and phantom imaging. Tolerance lines are
set at ±2.5 dB.
4. ‘
X Curve – Small Room
’ will give the closest
approximation to the X Curve for a small room as
defined in ANSI/SMPTE 202M-1998 [10]. This is
a target response commonly used in the movie
industry. A small room is defined as having a
volume less than 5300 cubic feet or 150 cubic
meters. The curve is flat up to 2 kHz and rolls off
1.5 dB per octave above 2 kHz. Tolerance lines are
set to ±3 dB.
1
5. ‘
X Curve – Large Room
’ will give the closest
approximation to the X Curve for a large room as
defined in ANSI/SMPTE 202M-1998 [10]. The
curve is flat from 63 Hz to 2 kHz and then rolls off
at 3 dB per octave above 2 kHz. Below 63 Hz
there is also a 3 dB roll off, with 50 Hz being
down by 1 dB and 40 Hz by 2 dB. Tolerance lines
are set to ±3 dB with additional leeway at low and
high frequencies.
1
An example of the room equaliser settings output for
the large system optimised in Figure 1 is shown in
Figure 2. The optimised result is displayed in green
and dark grey boxes. The green boxes are room
1
The room response controls do not directly support
the X Curves but it may be possible to achieve X
Curves in a room due to particular acoustic circum-
stances. This is also a good way to check how close
the response is to the selected X Curve.
x
(
f
)
y
(
f
)
x
0
(
f
)
