JBL LSR Brochure

White Paper

JBL’s LSR Principle, RMC™ (Room Mode Correction)

and the Monitoring Environment

by John Eargle

Introduction and Background

Although a loudspeaker may measure
flat on-axis under anechoic conditions,
what you hear at the mix position may
be inaccurate. The room’s boundaries,
geometry, and surface treatment
contribute significantly to the response
you hear at the mix position, and as a
result the balances and spectral
content may be off-target. JBL’s
solution to this problem starts with an
accurate loudspeaker system and
incorporates the necessary equalization
to correct the response at the mixer’s
position.

In all likelihood, the room in which you
monitor your recording or mixing
activities is reflective enough so that
what you hear in the midrange at the
mix position consists just about equally
of direct sound and reflected sound
(Augspurger, 1990). You may be
unaware of the reflected sound
component as such, but it is an
essential element for comfortable and
extended listening. If both direct and
reflected sound fields are uniform and
free from excessive peaks and dips,
what you hear will convey an accurate

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impression of your mix. On the other
hand, if your loudspeakers are flat on­axis – but the reflected sound field is
not flat – what you hear will be
aberrated.

Many loudspeakers are designed to
deliver fairly flat on-axis response – but
may at the same time have irregular
off-axis response. This disparity
between on- and off-axis response
shows up in
measurements of the system. Power
response presents a picture of the
relative output of the loudspeaker
summed over all directions as compared
with what the on-axis listener hears.

As far back as 1983, JBL first
addressed this problem in the design of
the 4400-series monitors which made
use of the Bi-Radial® horn. This design
philosophy is shown in Figure 1.

At A we show the on-axis and power
response of a well-designed monitor.
Both the room response (reflected
sound field) and on-axis response
(direct field) can be adjusted for

power response

optimum response simultaneously, as
shown at B.

If the loudspeaker has smooth on-axis
response but irregular power response,
as shown at C, then any attempt to
make the reverberant response
uniform will result in irregular on-axis
response – or vice-versa. This is shown
at D (Smith, et al., 1983).

The LSR Principle:

More recently, the desire to extend the
on- off-axis response matching to its
highest degree has led to JBL’s LSR, or

Linear Spatial Reference

monitors. These systems make use of
proper choice of crossover
frequencies, dividing network
adjustments, and specific baffle
boundary details to ensure three very
important performance features:

1. Flat on-axis frequency response.

2. Flat forward listening angle response

(±30° horizontal; ±15° vertical).

3. Uniform, gradually diminishing power
response with rising frequency.

The degree to which the LSR goals
have been met can be summed up in
the single composite graph shown in
Figure 2. Here, we have plotted six
response curves on the LSR6328P
powered monitor:

, series of

Figure 1. The effect of uniform on-axis
response and uniform power response.

Figure 2. Response data for JBL LSR6328P monitor system.

1. Curves 1 and 2 show the forward
only response components. Note that
the averaged response over the
forward listening angle is virtually the
same as the on-axis response.

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2. Curves 3 and 4 show the radiated
power (power response) of the
system, based on both first reflections
and on total radiated power.

3. Curves 5 and 6 are derived by
subtracting the radiated power curves
from the on-axis curve, giving an
overview of the system’s DI (directivity
index).

Figure 3 shows the same family of
response curves for a competitive
monitor which has not been designed
according to these principles.
Specifically, the power response
(Curves 3 and 4) show significant
peaks and dips in the upper midrange,
which indicate that there will be an
uneven reflected sound field in the
room.

paying for is symmetry of design, bass
traps, and a high degree of acoustical
isolation, all of which may require
extensive structural alterations. The
control room will also likely have large
flush mounted monitors. A room in
your home on the other hand will have
none of these improvements, and
there are two low frequency problems
that you will have to address by non­structural means. One of these has to
do with reflected images of
loudspeakers which are located close
to walls, and the other has to do with
standing waves in the room. We will
now discuss these problems in detail.

Figure 3. Response
data for a sub-optimal
system.

Low Frequency Problems:

Many audio operations are set up in
what we can call “ordinary” rooms. A
spare bedroom is often the choice for
a home workspace, and if it has
carpeting, drapery, bookshelves, and
a few other pieces of furniture, it may
actually be better than you think it is.
When you spend a fortune on a
control room, what you are largely

Boundary Compensation:

JBL’s new powered LSR-series products
incorporate a set of equalizers which
will enable you to make certain low
frequency (LF) adjustments to your
system. The first of these is

Compensation

JBL’s LSR full-range systems are
designed to have flat LF response when

.

Boundary

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Figure 4. Loudspeaker boundary conditions. Against a wall (A); in a corner (B).

they are mounted on conventional
loudspeaker stands that are normally
placed 2 feet or more from the walls.
When the systems are placed within
about 10 inches of a wall, the
situation will be as shown in Figure 4.
At A, the system is fairly close to a
single wall, and the reflected image of
the loudspeaker will increase the
loudspeaker’s LF loading, resulting in
an increase in output below about 200
Hz. The degree of LF boost will be
approximately 1.5 to 3 dB, depending
on the actual distance from the wall.
Under these conditions, you would
want to
feed to the system by adjusting a set
of DIP switches on the rear panel of
the LSR6328P. The models LSR25P
and LSR6312SP also have boundary
compensating equalization tailored to
their normal mounting alternatives.

Figure 4B shows the effect of posi­tioning the loudspeaker in a corner.
Generally, we don’t recommend this,
but if you are working in a very small
space, such positioning may be
necessary. In this case the maximum
amount of LF cut (4.5 dB) may be
applied. In any event, you will be able
to observe the effects of these
degrees of LF cut as you get further
into the equalization process.

decrease

the amount of LF

Figure 5 shows the range of the
boundary compensation equalizer in
the LSR6328P. Each step is 1.5 dB,
providing a maximum reduction of 4.5
dB below about 200 Hz.

Figure 5. Boundary compensation curves.

Room Modes and RMC (Room
Mode Correction):

Room modes are resonances that
exist between two or more boundaries
of a room. In a rectangular room, the
most common of these are the so-

called axial modes

between opposite walls and between
the floor and ceiling. Figure 6 shows a
side section view of a typical
workspace, and the panel just below
that figure shows the sound pressure
distribution for the first-order axial
mode along the front-back dimension
of the room. The bottom panel shows
the modal pressured distribution for

, which take place

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