Proceedings of the Institute of Acoustics
gle point measurement. A classical method to avoid this is to use a weighted average of responses
measured within a listening area. Such spatial averaging is often required when the listening area is
large; see examples described in the automotive industry
6
and cinema in the SMPTE Standard
202M
7
. Spatial averaging can reduce local variance in mid to high frequencies and can reduce prob-
lems caused by the fact that a listener perceives sound differently to a microphone, but typically re-
duces the accuracy of equalisation obtained at the primary listening location.
The room transfer function is position dependent, and this poses major problems for all equalisation
techniques. For a single loudspeaker in diffuse field no correction filter is capable of removing dif-
ferences between responses measured at two separate receiver points. At high frequencies a re-
quired high-resolution correction can become very position sensitive. Frequency dependent resolu-
tion change is then preferable and is typically applied
8,9
but with the expense of reduced equalisa-
tion accuracy. Perfect equalisation able to achieve precisely flat frequency response in a listening
room, even within a reasonably small listening area, appears not possible. An acceptable equalisa-
tion is typically a compromise to minimise the subjective coloration in audio due to room effects.
Typically electronic equalisation in active loudspeakers uses low order analogue minimum phase fil-
ters
10-12
. Since the loudspeaker-room transfer function is of substantially higher order than such
equaliser, the effect of filtering is to gently shape the response. Even with this limitation, in-situ
equalisers have the potential to significantly improve perceived sound quality. The practical chal-
lenge is the selection of the best settings for the low-order in-situ equaliser.
Despite advances in psychoacoustics, it is difficult to quantify what the listener actually perceives
the sound quality to be, or to optimise equalisation based on that evaluation
13-15
. Because of this, in-
situ equalisation typically attempts to obtain the best fit to some objectively measurable target, such
as a flat third-octave smoothed response, known to link to the perception of sound free from colora-
tion. Also, despite the widespread use of equalisation, it is still hard to provide exact timbre match-
ing between different environments.
Several methods have been proposed for more exact inversion of the frequency response to
achieve a close approximation of unity transfer function (no change to magnitude or phase) within a
certain bandwidth of interest
16-24
. Some researchers have also shown an interest to control selec-
tively the temporal decay characteristics of a listening space by active absorption or modification of
the primary sound
25-30
. If realisable, these are extremely attractive ideas because they imply that the
perceived sound could be modified with precision, to different target responses. Then, spatial varia-
tions in the frequency response can become far more difficult to handle than with low-order meth-
ods because the correction depends strongly on an exact match between the acoustic and equali-
sation transfer functions, and can therefore be highly local in space
25
.
2.2 Room
Response
Controls
The loudspeakers to be optimised have room response controls
1,32
. The smaller loudspeakers have
simpler controls than the larger systems but the philosophy of filtering is consistent across the range
(Tables 1-4).
The
treble tilt control
is used to reduce the high frequency energy. In the small two-way systems
and two-way systems it is a level control of the treble driver and has an effect down to about 4 kHz.
In large systems it has a noticeable effect only above 10 kHz and has a roll-off character.
The
driver level controls
can be used to shape the broadband response of a loudspeaker. They
control the output level of each driver with frequency ranges that are determined by the crossover
filters.
The
bass tilt control
compensates for a bass boost seen when the loudspeaker is loaded by large
nearby boundaries
33-36
. This typically happens when a loudspeaker is placed next to, or mounted
into, an acoustically hard wall. This filter is a first order shelving filter.
The
bass roll-off
control compensates for a bass boost often seen at the very lowest frequencies
the loudspeaker can reproduce. This typically happens when the loudspeaker is mounted in the
corner of a room where the loudspeaker is able to couple very efficiently to the room thereby exac-
erbating room mode effects that dominate this region of the frequency response. It is a notch filter
with a centre frequency set close to the low frequency cut-off of the loudspeaker.
