Measuring and Optimizing Sound Systems: An Introduction to JBL Smaart

Measuring and Optimizing Sound Systems: An Introduction to JBL Smaart
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Measuring and Optimizing Sound Systems:
An introduction to JBL Smaart™
by Sam Berkow & Alexander Yuill-Thornton II
JBL Smaart is a general purpose
acoustic measurement and sound system
optimization software tool, designed for
use by audio professionals. Running on a
Windows® computer and utilizing
almost any standard Windows
compatible (MCI compliant) sound card,
JBL Smaart offers an accurate, easy-touse and affordable solution to many of
the measurement problems encountered
by sound system contractors, acoustical
consultants and other audio
professionals.
JBL Smaart was designed by a team
consisting of acoustical consultants,
sound system designers, mixers, and
installers. The goal of the project was to
create a tool which would provide easy
access to information which will help
systems sound better, by identifying
potential problems and quantifying
system performance. To meet this goal
both a real-time module and disk-based
analysis module were developed. Perhaps
most importantly, the features within
JBL Smaart suggest a methodology for
optimizing sound systems. This
methodology is developed and explained
in this paper. The method described is
not intended as an inflexible procedure,
rather as the starting point for you to
modify as you like and assist you in
understanding and optimizing sound
system performance.
1) What am I trying to measure and
why?
Before making measurements of a sound
system it is critical to ask yourself, “What
am I trying to measure and why?”
The performance of a sound system,
whether it is a permanently installed
system, touring sound system, or some
hybrid of the two, is determined a
number of ways, both qualitatively and
quantitatively. The following is a list of
some of the most important questions to
ask when determining the level of
performance of a given system.
• Frequency Response: Does the
system have the ability to deliver sound
over the intended frequency range, within
the expected deviations?
• Power Handling: Can the system
handle the desired amount of power
without excessive distortion or failure?
and whether it is an existing system, a
touring system, or a new system to be
optimized.
• Coverage: Does the system provide
sufficient coverage of required areas at
all frequencies?
However, there are several steps we feel
are necessary to any successful exercise
in sound system measurement and
optimization. The order in which they
would be followed might differ according
to the tuner’s personal preference and the
task at hand. The procedure outlined
here is based on our own experience and
assumes a system that is already in place.
• Subjective Quality: This is always the
most important criteria. Does the system
meet the
audience/owners/performers/operators
expectations for perceived sound quality?
• Stability: Does the system feed back
with microphone(s) open and the gain set
to a useful level? (This is obviously not
important if the system is playback only.)
• Noise: Is the system noisy? Are hums,
buzzes and other unwanted noise present
in the system?
• Configuration: Do you understand
the system configuration? Some sound
systems have groups of speakers driven
from a single source. Others are divided
into several sections, each controlled by a
different set of control circuitry (such as
equalizers, delays, crossovers etc.).
The Transfer Function & System
Configuration
At the heart of JBL Smaart’s Real-Time
Module is the transfer function
calculation. The transfer function (stated
simply) is a comparison between the
input and output of a system. The
transfer function of a system can be
displayed in either the time or frequency
domain. To make the results of this
calculation easy to understand, JBL
Smaart displays the result in the
frequency domain. To accomplish this
calculation JBL Smaart compares two
signals, each of which are input to your
computer’s sound card.
• Operation: Are all components of the
system working?
No piece of hardware or software (even
JBL Smaart) can accurately answer all
of these questions by itself. Tuning a
sound system requires an understanding
of the hardware, a discerning ear,
accurate measurements, and a disciplined
and systematic approach. We doubt that
any two system tuners approach the
problem exactly the same way. Also, the
process necessarily differs, depending on
the complexity of the system in question
Fig. 1
Figure #1 displays a typical setup for
using JBL Smaart to make transfer
function calculations. The use of two
signals, measured at the input and output
of a system, allows JBL Smaart to
display a comparison of the input and
output as a function of frequency. This
configuration allows the test signal to be
noise, music or almost any broadband
signal. The Coherence function feature of
JBL Smaart is used to check the validity
of the data, thus helping users to
understand whether that data is reliable.
For the transfer function calculation to be
valid the two input signals must be
aligned in time. This can be accomplished
using the Delay Locator and Internal
Signal Delay features
included within JBL Smaart.
After making a measurement in the
configuration displayed in Figure #1, the
results may be stored and the system
reconfigured as shown in Figure #2:
A Step-by-Step Approach
STEP 1: Evaluation Listening
Before you begin measuring a sound
system, we strongly recommend listening
to it! You should attempt to qualitatively
answer the questions listed on the
previous page. This will require you to
move around and listen to each section
or subsection of the system. Explore the
edges of the coverage pattern to see
where the various elements are covering
and where they are not. It may also be
helpful to turn off parts of the system in
order to make a more detailed evaluation
of various subsystems and components.
Practical Note: Unless you designed the
system, take some time to try and
understand what the system designer had
in mind and how the various elements
relate to each other.
Fig. 2
Measuring the transfer function of the
equalizer (Figure #2) is particularly
useful when the resulting transfer
function is inverted and overlaid on top
of the previously stored loudspeaker
measurement. The need to switch
between measurement points within a
system suggests that the use of an
external mixer, as configured in Figure
#3, may prove extremely useful in the
field:
Fig. 3
STEP 2: Identify Potential Problems
Examining the list of questions above,
are there any obvious problems that need
to be addressed? For example, unwanted
noise, such as hums and buzzes
associated with ground loops and “dirty”
power, can degrade system performance
and should be addressed before JBL
Smaart testing is begun. Loose and
intermittent connections should be fixed.
A gain structure that leaves the system
hissing should be explored and corrected.
Practical Note: The system must be
stable before trying to make
measurements. Systems that seem to be
changing gain or have noises that come
and go are not good candidates for
critical measurement. Spend some time
sorting things out first.
STEP 3: Select Measurement Points &
Positions
This is one of the most important steps in
the process. You need to select
measurement points that will show you
what you need to see. There are two
kinds of measurement points: electrical
and acoustic.
Electrical measurement points are used
to sample the input or output of a
particular piece of equipment. If you
want to measure a piece of equipment, or
the result of a string of series-connected
pieces of equipment, make the
connections at the input and the output.
Acoustic measurements are made with a
microphone. When making transfer
function measurements, a reference
signal is also required. The connection
for the reference signal should be made
at the input of the speaker system, the
input of the processor if it is a processed
system, or at the input of the system’s
equalizer(s).
Microphone selection and placement are
very critical. The microphone must be a
known quantity. A measurement
microphone should be of high-quality
construction, having the flattest
frequency response characteristics that
you can reasonably afford. When
selecting the microphone position, ask
yourself two questions: “Is this a useful
place to make a measurement?”, and
“What other things will the microphone
pick up in this location that might affect
the measurement?” Reflections into the
side or back of a measurement
microphone can seriously reduce the
accuracy of a measurement. Think
“mirror”, and look around for surfaces
that might catch you unaware!
Practical Note: Acoustical reflections of
sound energy from large (and some not
so large) surfaces may generate “comb
filters” in the measured signal. The
result is a system of dips in the
frequency response that are evenly
spaced in frequency. They are easiest to
see when the frequency axis is set to
linear, as they appear to be a set of
valleys evenly spaced across the plot.
STEP 4: Compare Positions
In making acoustic measurements of any
system, it is important to make a number
of measurements to make sure that you
are not being fooled by something
affecting the measurement (such as
reflections). Move the microphone
around and look at what happens to the
measured frequency response. Also, you
may wish to put sound absorbing
materials on the floor between the source
and microphone to reduce the effect of
the reflected sound energy from the floor
(this is another good place to think
“mirror”)...
STEP 5: Set Equalizers and Delay
Settings
Setting equalizers and delays can be very
time consuming. There seem to be two
distinct stages to the process: coarse
adjustment and fine tuning. In the first
stage, large adjustments to EQ and delay
settings are used to make a system
roughly correct. Sometimes, the sheer
size of these adjustments may seem a
little daunting, but if things are sounding
right, you are probably OK. The next
stage takes place when the system is
getting close to right. At this point,
changes of a few dB can make the
difference between a good-sounding
system and a great-sounding system.
Learn to recognize this transition.
After making a number of changes to
equalizer settings, it is important to go
out and listen to what is happening to the
system. Make sure it is moving in the
right direction. Just because it looks
good on an analyzer screen doesn’t mean
that it is right. Remember, you are
working for the ears, not the
instruments!
Important Notes:
• Always make delay adjustments
before trying to make fine adjustments in
equalizer settings. A combination of
small delay and equalization changes can
completely change the character of a
delay system.
• Setting delays and equalizers can
help make some poorly designed sound
systems sound better. However, only in
extreme and very rare cases is it possible
to correct poor loudspeaker coverage
with these types of devices.
STEP 6: Critical Listening
This is what it is all about. Here is where
you take off the measurement hat and put
on the listening hat. Put on a CD (or
other program source) and “walk the
system”. Listen in the front rows and
back in the cheap seats. Try it at low
levels. Try it at high levels. Run it though
its paces. Turn the source off and listen
to everything in silence. Make sure that
the noise floor is low enough not to
affect the dynamic range of the system.
Use material that is familiar to you.
Don’t be afraid to listen to things others
may not like. For this purpose, the best
choice might be something you have
heard so many times you don’t even like
it anymore. Only when you are very
familiar with (several) program selections
will you be able to use them as a basis to
quickly and accurately evaluate a system
by listening.
STEP 7: Stability Testing
It is very important to explore the
stability of any critical sound system
before it goes into service. Otherwise,
you may find yourself in the
uncomfortable position of trying to find
and equalize feedback frequencies during
a performance or other event. Obviously
this is not a concern for playback only
systems.
Unstable sound systems are those that
have, at one or more frequencies, an
overall gain, including the acoustic path,
of more than one; in other words,
feedback. It follows that a stable system
has a comfortable margin of gain before
feedback (GBF) at its intended operating
level while delivering the intelligibility
and frequency response characteristics
required for its purpose.
Important Note:
• Feedback can damage audio
components. Exercise caution when
testing system stability. Feedback is
particularly dangerous when it builds up
very quickly and overdrives the system,
causing overloads or clipping. It might be
a prudent safety precaution to use a
limiter or compressor during stability
testing to help protect system
components. Remember that non-linear
devices such as limiters or compressors
CANNOT be used during transfer
function measurements.
Typical Causes of Instability
Instability, or feedback, is often the result
of interaction between the off-axis
response of the speaker system and the
off-axis response of microphones. The
biggest problems usually arise when
narrow peaks in the off-axis responses of
both loudspeaker and microphone
coincide. These types of interactions can
be very troublesome, and they are not as
easy to control as the on-axis responses.
Other possible causes or contributors to
stability problems include acoustical
characteristics of the room and/or signal
processing equipment; particularly
reverberation units used in music
reinforcement systems.
Detecting Instability
The simplest way to expose a stability
problem in a sound system is to turn up
the gain, slowly and carefully, until the
system feeds back. Not a particularly
elegant approach, but it almost always
works. If feedback does not occur in the
system until the gain is increased well
beyond the intended operating level, and
the system is free of any noticeable
“ringing” at normal levels, it’s pretty
stable. If not, you will need to find ways
to improve it. Depending on the
situation, the best solution could be
electronic, mechanical, acoustic, even
educational, or some combination of the
four.
Some Approaches to Stabilizing a Sound
System
Stabilizing an unstable system, or giving
a system more “margin” (GBF) primarily
involves reducing the gain of the system
at the problem frequencies. Given the
nature of the problem, the most obvious
solution is to apply equalization.
Although equalization is not a panacea or
a substitute for good system design, it is
one of the most powerful tools you can
bring to the task of stabilizing an existing
sound system.
JBL Smaart can help you to quickly
identify problem frequencies and apply
equalization with great precision. But
before you start turning knobs, consider
that equalization affects the overall
frequency response of the system. There
are other strategies that might be equally
effective, or more effective, and could
afford you greater freedom to make the
system sound good.
Mechanical Solutions and Acoustical
Solutions
The physical position of microphones
and loudspeakers in relation to each
other can affect the feedback frequency
(or frequencies). Reducing the gain at a
problem frequency can sometimes be as
simple as using a different microphone,
or reorienting one already in use. This
strategy is best employed when the
microphone in question is intended to
remain stationary. Moving or reorienting
loudspeakers may also be a possibility.
Mechanical solutions are most attractive
when they can be applied without giving
up any of the sound system’s design
goals.
Stability problems often arise when
loudspeakers are placed close to (or
behind) microphones. In such cases it
may be possible to add some sound
absorbing material or a baffle that
reduces the speaker’s field at the
microphone’s position or simply reduce
the operating level(s) of the
loudspeaker(s) in question.
Important Note:
• Moving microphones are moving
targets. When speakers or performers
move around with microphones,
feedback frequencies may shift. Always
try to perform stability testing in a
manner that resembles how the system
will actually be used.
Educational Solutions
An otherwise stable system may lose
stability when a number of microphones
are open at one time. In this case, the
best solution might be to train the
operator to keep microphones open only
when they are actually in use.
Educating users in microphone technique
can also be beneficial. Many people tend
to grab microphones or lean very close
when they speak. Both of these actions
can cause problems. Grabbing a cardioid
microphone can increase its physical gain
at certain frequencies when the user’s
hand closes off the rear ports to the
microphone element, making a stable
system suddenly unstable. When people
lean too close to a microphone, they can
reflect some energy at problem
frequencies back into the microphone
themselves, possibly causing feedback.
of the output of the system, through
some system of delays, to the input.
With a simple enough system, polarity or
phase changes could solve a feedback
problem immediately. When the polarity
is inverted, instead of positive feedback
(something we don’t like) we should get
negative feedback (something that may
be beneficial). However, in large,
complex systems with multiple return
paths many wavelengths long, phase or
polarity changes might just tend to shift
the feedback frequency, without
increasing stability.
The most common solution to the
problem of feedback is to use
equalization to take out offending peaks.
By peaks, we mean places in the
spectrum where there is significantly
more gain (or energy build-up) than
others. The procedure involves:
• Carefully, running the system into
feedback
• Identifying problem frequencies, and
• Setting up filters (i.e., EQ stages) to
compensate
Important Note:
• We strongly recommend the use of
parametric equalizers for this type of
application.
Electronic Solutions
How Much Equalization is Enough?
Some reverberation units can cause an
otherwise stable system to become
unstable. If this seems to be the case, try
experimenting with other settings and/or
reducing the overall level of electronic
reverberation. Keep in mind that
reverberation generators do what they do
(very simply put) by feeding back some
As you equalize a system to increase
stability, keep in mind you are reducing
gain, even though you are reducing it
only at specific frequencies. In many
cases, the frequencies in question have
proportionately too much gain anyway.
You may actually improve the system’s
frequency response at operating levels
while increasing stability.
If there are two or more feedback
frequencies, equalization tends to work
best when the frequencies are fairly close
together. You may find, after you have
applied a number of filters at widely
spaced frequencies, that all you have
really accomplished has been to reduce
the overall gain of the system – without
really increasing its stability or GBF. It
may be necessary to explore other
solutions. (In some extreme cases it may
be necessary to alter the system design to
correct instability.)
STEP 8: More Critical Listening
If you, and everybody else concerned,
are satisfied with the system, you’re
done. More likely, you will need to
repeat some combination of Steps 2
through 7 to obtain the best possible
performance. Optimizing a sound system
is usually a gradual, cut-and-try, giveand-take process (which often takes
more time than one would like or
expect). We hope that JBL Smaart will
help to make this process much easier for
you.
CONCLUSIONS:
The ability to measure the transfer
function between two signals (which
have been aligned in time) allows
sound system performance to be
measured using a variety of test
signals, including noise and music.
This measurement technique provides
an easily understood measurement
which can be used to optimize sound
system performance. The ability to
overlay stored measurements allows
both equalizers and delay units to be
accurately set. JBL Smaart provides
the capabilities to make these
measurements, and benefits from the
graphical user interface provided
under the Windows operating system.
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