Infinity Rabos Brochure

Room
Adaptive
Bass
Optimization
System

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©1999 Infinity Systems, I n c.

R . A . B. O . S .

™

Matching the Loudspeaker to the Room –

“Room Friendly” Loudspeakers and the

Room Adaptive Bass Optimization System

(R.A.B.O.S.)

By Floyd E.Toole

3 Introduction 4 The Loudspeaker/Room System – Low Frequencies 5 R.A.B.O.S.–A Better Equalization

I n t ro d u cti o n

One of the many complicating fa c t o rs in the world of audio, for both

O

consumer and manufa c t u re r, is that the listening room has such a
powerful influence on what is heard . For the consumer, it means that
opinions fo rmed about a loudspeaker in one room may not translate to
a n o t h e r. For manufa c t u re rs, it means that the perfo rmance of their
l o u d s p e a k e rs will be significantly changed, for better or for wors e, b y
the choice of listening ro o m .

ROOM ADAPTIVE BASS OPTIMIZATION SYSTEM

3

O ver the years, many have attempted to control this powerful
i n fl u e n c e, so that the listening ex p e rience is both better and
m o re consistent (as we move from room to room and from system
to system). It was once thought that this could be done simply by
setting up a micro p h o n e,m e a s u ring a “ room curve ”a n d ,with a
h a n dy - d a n dy equalize r, adjusting the frequency response curve
until it looked “ ri g h t .”Not so easy. For eve ry system that wa s
i m p roved by such exe rc i s e s, t h e re were many that were adve rs e l y
a f fected because:

These cri t e ria are considered throughout the design of Infi n i t y
l o u d s p e a k e rs – during the initial design pro c e s s, d u ring the
e n g i n e e ring process and, of cours e, d u ring the ri g o ro u s
listening eva l u a t i o n s.We know from our own ex p e ri e n c e, as well
as from published results of psychoacoustic inve s t i g a t i o n s, t h a t
the benefits of “ room fri e n d l y ” design are substantial.A l t h o u g h
we cannot prevent these nondirect sounds from reaching the
l i s t e n e r,we can design loudspeakers so that dire c t ,e a rl y - re fl e c t e d ,
and reve r b e rant sounds all exhibit good timbral “ s i g n a t u re s.”

1 .A simple “ room curve ” is not a reliable correlate of perc e ive d
sound quality at eve ry fre q u e n c y.

The Loudspeaker/Room System – Low Fre q u e n c i e s

2 . Equalization can only “ fi x ”c e rtain kinds of loudspeaker ro o m
p ro b l e m s ; it is essential to know what can and cannot be
e q u a l i ze d .
3 .The kinds of measurements and equalize rs that have been
used have not always been the ones that were needed to eva l u a t e
or to solve the pro b l e m s.
4 . Some problems can only be solved with a better loudspeaker.

At low fre q u e n c i e s, the wavelengths are sufficiently long so
acoustical problems are pri m a rily caused by room re s o n a n c e s
(also called modes, eigentones and standing wave s ) .S o u n d s
re flecting back and fo rt h , or from surface to surfa c e, c a n
i n t e ract at certain fre q u e n c i e s, resulting in acoustic re s o n a n c e s.
The specific frequencies at which these resonances occur are
d e t e rmined by the shape and dimensions of the ro o m .Th e

The loudspeakers and the room together fo rm a system in which
what we hear,in terms of sound quality and spatial imag i n g ,i s

s t rength of these modes depends on how much low-fre q u e n c y
a b s o rption there is in the ro o m .

ve ry much a result of a complex combination of the two. In the
following analysis of the situation, we will divide the fre q u e n c y
range into two domains: below approximately 400Hz – the bass
re g i o n , and above approximately 400Hz – the middle and high­f requency re g i o n .

If the room is well damped, the modes may not be pro b l e m a t i c.
At ve ry low fre q u e n c i e s, a b s o rption is not provided by common
m a t e rials such as fi b e rg l a s s, acoustic foam sheets or dra p e ry.
I n s t e a d , it is provided by the walls and windows themselve s, w h e n
they move in response to powerful bass sounds. S o m e t i m e s, w h e n

At middle and high fre q u e n c i e s,b roadband re flections from ro o m
b o u n d a ries – wa l l s, floor and ceiling – are dominant fa c t o rs.
L o u d s p e a k e rs radiate sound in all dire c t i o n s, not just towa rd the
l i s t e n e r.These re fl e c t i o n s,as well as re flections of these
re fl e c t i o n s, all contribute to the sound that ultimately re a c h e s

building a “ s p e c i a l ”listening room or home theater,ex t ra effo rt
and expense will be put into making the walls ve ry heavy and
s t i f f.That is good from the point of vi ew of keeping the sound
contained within the room – sound isolation – but it is bad fro m
the point of vi ew of damping the room re s o n a n c e s. Such ro o m s

the listener.

Figures 1–3 show that in the design of a loudspeaker it is necessary to consider all of a sound ’s componen ts.

h ave a tendency to be “ b o o m y.”The exact nature of the pro b l e m , a n d
its seve ri t y,will be determined by how effe c t ively the woofe rs couple
their energy to the room resonances – i.e. , the locations of the
w o o fe rs in the room – and how effe c t ively the resonant energy is
coupled to the listener’s ears – i.e. , the location of the listener in
the ro o m .

In the real worl d , within the constraints of good stereo or
multichannel imaging and the demands of appeara n c e, t h e re are
s eve re limitations on where we can put loudspeakers and listeners in
a ro o m .Not many of us are willing or able to have a custom ro o m
designed ex cl u s ively for our audio pleasure. So what do we do if we
find we have a problem? We equalize !

R . A . B. O. S .– A Bet ter Equalizati o n

We call our fo rm of equalization, R . A . B . O. S. – Room A d a p t ive Bass
Optimization System.The diffe rence between this equalization and
the kinds that have given equalization a bad name is that here we
a re not going to allow it to ve n t u re into areas that are likely to give
us more tro u b l e. R . A . B . O. S. consists of a single para m e t ric fi l t e r
built into the electronics of the powered subwoofe rs.As such, it is
re s t ricted to addressing only those problems within the range of the
s u b w o o fer – up to about 100Hz.It is designed as an attenuation­only fi l t e r,so it cannot be used to attempt to fill frequency re s p o n s e
dips caused by acoustic cancellations, usually a futile task, re s u l t i n g
only in dy n a m i c limitations and distort i o n . It is para m e t ri c, so that
with care it can be tuned to the specific frequency of the most
s e rious problem and adjusted to match the bandwidth of the
re s o n a n c e. Fi g u res 4 and 5 are an example of the improve m e n t
R . A . B . O. S.o f fe rs.

Figure 4. Measuremen ts in a room before and a fter a
single band of parametric equalization was us ed to
attenuate an extremely power ful room resonance. The
improvement in sound quality was no thing less than
dramatic, with the bass being transformed from boomy,
fat and flabby to clean, tight and articulate.

Figure 5. Waterfall diagrams for the before (shown
above) and after (shown below) conditions, with the
resonance indic ated by the arrows. The attenuated
“ ringing” is obvious in the diagram above, confirming
that the time - domain problem is also corrected.

Figure 1. The direct sounds, the first sound
from the loudspeakers to reach the
listener. Usually desc ribed by the on - axis
frequency response of the loudspeaker.

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ROOM ADAPTIVE BASS OPTIMIZATION SYSTEM

Figure 2. The early - reflected sounds,
which reach the listener after a single
reflection from the room boundaries –
the floor, walls and ceiling. These are
desc ribed by measuremen ts made at
many, sometimes large, angles off axis.

Figure 3. The reverberant sounds: all of the
sounds that arrive at the ears after multiple
reflections. The technic al measurement of
sound power is the best correlate of this
component of room sound.

In most rooms, the quality of bass
that we hear is determined more by
the room than by the loudspeaker
itself !

Fi g u res 6 and 7 present another ex a m p l e ; this time we isolate

the subwoofer in the measure m e n t s.

Note that in both of these examples and in any other
application of room equalization at low fre q u e n c i e s, t h e

i m p rovement exists in both the frequency and time domains.Th e

audio fo l k l o re, that equalization is inherently “ b a d ”because it
“adds phase shift,”is based on old-fashioned ideas.E q u a l i ze rs

that change the frequency response do, i n d e e d , add phase shift.

What is not always re a l i zed is that the resonances being
c o rrected by the equalization

M e a s u rements must be done in such a way that offe n d i n g
resonances can be isolated vi s u a l l y, a n d
the para m e t ric filter of the equalize r
can be adjusted to match the shape of

the re s o n a n c e s.When this is done and

the equalizer is then adjusted to fl a t t e n
the frequency re s p o n s e, the phase

a b e rration is also compensated fo r.

The end result is that erro rs in the

t ransient response are also corre c t e d . For those with a scientifi c

b e n t ,this is all explained by the Fo u rier tra n s fo rm , which defi n e s

the relationships between what happens in the time domain and

what we observe in the frequency domain,i . e. ,between tra n s i e n t
p e r fo rmance and frequency re s p o n s e. H o w eve r, the final proof is

in what the measurements tell us and in what we

hear – it work s !

The audible improvement in each of these cases was not

i n s i g n i ficant yet the re m e dy – equalization – was simple and

s t ra i g h t fo r wa rd .Th e re are two re a l - w o rld altern a t ive solutions

that can address these pro b l e m s.The fi rst invo l ves a major

also

h ave phase shift.

prop er

I nfinity loudspeakers

are designed to be

listened to in ro o m s .

acoustical tre a t m e n t ,d ramatically changing the appearance of
the room and significantly impacting on the bank account.Th e
second invo l ves a major re a rrangement of the loudspeaker
and/or listener positions – again with significant visual and,
p o s s i b l y,p ractical consequences.We think that equalization is
cl e a rly pre fe rable to either.

A reasonable question at this point is whether one para m e t ri c
filter is enough. In our ex p e rience with subwoofe rs, the majori t y
of installations can benefit from equalization, and the majority of
those would achieve most of the potential benefit from a single
fi l t e r.Th e re are situations where adding more fi l t e rs could

result in a re finement in perfo rmance
but doing so would re q u i re many more
m e a s u re m e n t s, as well as an in-depth
technical understanding of fi l t e rs.Th i s
implementation of R.A.B.O. S. aims to
d e l iver the maximum perfo rm a n c e
enhancement to the customer, with a
minimum of installation diffi c u l t y.

Another reasonable question is if equalization is not
re c o m m e n d e d , what then
f requency response? Usually,these result from the specifi c
locations of the listener and/or loudspeakers ;dips are
position dependent.B e fo re ve n t u ring into the equalization
exe rc i s e, we there fo re suggest that, guided by measure m e n t s,
u s e rs ex p l o re the effects of va rious small positional changes to
see what improvements can be made.M ovements of just a few
inches often can make huge diffe rences in system perfo rm a n c e.
All of these helpful hints, and more, will be included in the
i n s t ruction manuals.

can

be done about major dips in the

very

Figure 6. Frequency response measuremen ts of a subwoofer,
before and after a single band of parametric equalization.

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ROOM ADAPTIVE BASS OPTIMIZATION SYSTEM

Figure 7. Time - domain behavior (transient response) of
the subwoofer, before and after EQ. The energetic ringing
( light line) is the “before” condition. The well - damped ringing
( dark line) is after equalization.

“ R

oom Fri e n d l y ” loudspeaker system designs ensure that, re g a rdless of
the ro o m , l i s t e n e rs will be treated to the best possible sound quality
and spatial imag i n g . R . A . B . O. S. is the icing on the cake, e n s u ring that
an otherwise elegant listening ex p e rience will not be corrupted by
a drone of monotonal,“ J o h n n y - o n e - n o t e ”b a s s, and fa t , flabby dru m s.
Our job is not done until the listener hears the right sound.