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Tech Topic: What You See Is Not What You Get
Why measured response doesn t always match what s heard
By Pat Brown
Many audio field technicians are now in
possession of measurement systems that
can be used to assist the listening process
in equalizing sound reinforcement systems.
But, they re often surprised to find that the
measured system response correlates poorly
with subjective impression of how the
system sounds.
In other words, the system can sound good
when it looks bad on the analyzer, and it
can sound bad when it looks good on the
analyzer. As a result, some users have
The measured versus ideal response for the direct field of
become frustrated and distrustful of analysis
a loudspeaker.
systems in general.
lLet's look at why the eye and ear do not always agree on what is best regarding the response of the sound system.
First, consider the most popular methods of measuring the response of the sound system. By response, I am referring to the
magnitude of the frequency response as displayed on a dB (vertical) vs. logarithmic (horizontal) scale. The goal of technical
system equalization is to produce a flat horizontal line on this display.
WORKING IN REAL TIME
The real-time analyzer (RTA) is essentially a
bank of meters, each driven with a
1/n-octave constant percentage bandwidth
filter so that only the level of a limited
range of frequencies is displayed by each
meter.
The original RTAs used analog meters, but
current versions use a vertical row of LEDs
for each 1/n-octave band. One-third octave
resolution is the most popular, and
correlates well with the response of the
human auditory system.
The RTA input is fed from an
omnidirectional test microphone located at a
listener position. Omni s are used because
they typically have a very flat, benign
frequency response over most of their band
pass.
RTAs can also be software-based, utilizing
the sound card on a personal computer to
provide the A/D conversion of the
The effect of increasing distance outdoors (top) versus
indoors.
microphone output voltage.
A mathematical algorithm (the FFT) is used to produce the previously described dB vs. frequency display. These digital
analyzers emulate their analog counterparts in how the information is displayed, but differ in that the filters and display is the
product of a computer algorithm rather than analog filters. This type of RTA is more versatile, as the octave-fractions, colors,
etc. are under software control.
Regardless of which type is used, the
standard method-of-use is to drive the
sound system with pink noise (equal energy
per 1/n octave) and adjust the system
equalizer for a flat magnitude response on
the analyzer display.
RTAs are powerful tools when certain
guidelines are followed, but indoors they
can indicate a system response with poor
correlation to what the listener is hearing.
The major consideration is the placement of
the measurement microphone.
A target curve can be used with the RTA to compensate for
the low-frequency build-up that occurs in many rooms.
If the mic is placed in the near field of the loudspeaker (typically less than 10 feet), the correlation with human hearing is
pretty good. At this position, the direct energy from the loudspeaker dominates what is being observed on the analyzer and
very little of the reflected energy from the room is included in the displayed response. Adjustment of the equalizer for a flat
direct sound field on the analyzer produces a desirable result.
The down side to the near-field placement is that the measured response is very sensitive to small vertical movements of the
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microphone when the loudspeaker has offset vertical components (as most do). This sensitivity can be reduced if the
microphone is moved to a greater distance from the loudspeaker (into the far field) since the path-length difference back to
the individual components becomes more equal. But, as the microphone is moved further away, the reflected energy from the
room begins to dominate the displayed response.
GIVING EQUAL WEIGHT
Microphones have no perceptual abilities. They do not localize sound or discriminate early sound energy from late energy like
humans do. A listener at a distance remote from the loudspeaker will pay more attention to the direct field of the loudspeaker
than sound that is building-up in the room. A microphone gives equal weight to all energy without regard to where it is coming
from.
A simple experiment to verify this is to stand at the microphone position and listen to the loudspeaker and then route the mic
through a headphone amplifier and listen to it through headphones - not the same thing at all.
Low frequency sounds tend to linger in
rooms longer than high frequency sounds,
because most rooms have more high
frequency absorption than low frequency
absorption.
As such, the room becomes bass heavy
when the total sound field is considered.
This extra low frequency information will
dominate what is observed on the RTA, and
the knee-jerk reaction is to attempt to
flatten the response by boosting the high
frequency bands on the equalizer. The
result is a system with excessive high
frequency output and a resultant harsh
sound quality.
A full-bandwidth transfer function measurement (with SIA
SMAART Live) using variable time windows. This
When RTAs are used in this manner, it is
measurement was made indoors at about 50 feet from the
loudspeaker. important to equalize to a target curve
rather than for a flat frequency response.
The popular X curve for theaters is flat to 2 kHz, where it starts rolling off the high frequency response at about 3 dB per
octave. It is -10 dB at 10 kHz relative to 2 kHz. This represents 1/10th power at 10 kHz relative to flat response. The
one-third-octave analyzer and the target curve have served sound practitioners well for years, and remains a viable approach
to system calibration.
RECENT METHODS
Technology has yielded some new methods for acquiring the system response at a listener position. A complex comparison
(both time and frequency information) of the input and output of a system is called the transfer function. It includes both the
magnitude and phase response of the loudspeaker/room at the microphone position. This has become a popular method of
analysis, as it allows any input stimulus to be used to test the system, since the displayed response is just the difference
between what you put in and what you got out.
Transfer function analysis has the added
advantage of the ability to use a time
window to exclude late arriving energy
from consideration in the response. This can
prevent the low-frequency build-up problem
that plagues traditional real-time analysis.
With proper implementation of a time
window, the system response can be
adjusted without the need for frequency
weighting via a target curve.
A major difference between transfer
function analysis and 1/n-octave real-time
analysis is that the former requires the
removal of the signal delay between the two
signals being compared.
The stimulus (the reference signal) always
has a much shorter path back to analyzer
input than the output of the measurement
microphone. Sources of delay include the
travel time through the air and the latency
of digital processors.
Placing the test microphone on a stand makes it impossible
to observe the loudspeaker s response without interference.
Failure to properly synchronize the reference signal and the microphone s signal will result in an erroneous display of the
system s response. The length of the time window must also be selected - in other words, how much of the room decay do I
want to include in the response?
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Unfortunately, there is not an optimum size
for the entire spectrum. A short time
window excludes much of the room decay at
the expense of low-frequency resolution. A
long time window improves frequency
resolution at the expense of gathering too
much of the room s decay. A compromise is
required.
The human auditory system perceives pitch
on a proportional (logarithmic) frequency
scale. This is one reason that we use
constant-percentage bandwidth filters for
tuning audio systems - the bandwidth grows
The test microphone was placed on a stand for this
with increasing frequency.
measurement. Note the comb filtering due to the floor
bounce effect.
Frequency-dependent bandwidth suggests that the length of the windowing function used in transfer function analysis should
be varied in the same manner - a decreasing length with increasing frequency. This produces a somewhat anechoic response
at high frequencies with increasing frequency resolution as frequency decreases. The time window length is a function of
frequency, with even the longest window (highest frequency resolution) excluding much of the late energy from the room.
Another caveat of this type of analysis is that much greater frequency detail is possible than with the typical 1/3-octave
banded display. Phase interference effects from reflections or multiple drivers are clearly visible on the analyzer. Such
anomalies are almost always position-dependent, so careful corrections at one seating position will be inappropriate for
another. Both the loudspeaker and the measurement microphone should be carefully positioned to avoid the creation of very
early high-level reflections.
SPECIAL EFFECTS?
The floor bounce effect is a common example of a very early reflection (typically within a few milliseconds of the first sound
arrival) that produces a unique acoustic response for each listener seat for all but the lowest octaves of the spectrum. This is
an example of less is better when measuring the response, as a 1/3-octave display lacks the resolution to observe the effect
in detail and produces less of a temptation to fix it.
The floor bounce effect can be minimized by
use of an appropriate frequency-dependent
time window or by simply laying the
measurement microphone on the floor, or
on a board placed across the listener seats.
The effect usually disappears with the
presence of an audience, so we do not wish
to consider it when tuning the sound
system.
The use of variable-length time windows
and the synchronous transfer function allow
the system to be tuned in a manner similar
to the near-field RTA method (flat response
The same measurement displayed at one-third octave
resolution. The floor bounce effect is significantly masked by on a log frequency display), even at remote
the lower resolution.
positions in the room. It is superior to the
RTA method in that the effects of air
absorption are readily apparent and can be
compensated for via equalization.
Near-field techniques do not include air absorption for the simple fact that the sound has not traveled very far before it is
picked up by the microphone, so it hasn t passed though enough air to be significantly attenuated.
By far, the biggest problem with tuning sound systems is failure on the part of the technician to recognize anomalies that
cannot be corrected with equalization. The equalizer is a global device, meaning that its response curve will be impressed on
all of the sound radiated from the loudspeaker, regardless of the direction in which it is radiated.
Many, if not most, of the anomalies observed on the analyzer are unique to each listener position. The technician must learn to
recognize and ignore such events. They include:
" Floor-bounce effect;
" Interference between multiple drivers;
" Reflections from objects near the mic or loudspeaker.
Events that produce a more global effect, and can therefore be addressed with equalization include:
" Boundary-loading of loudspeakers;
" Coupling between multiple low-frequency drivers;
" The direct-field loudspeaker response.
With training and experience, the system technician can implement methods that reveal system imperfections that are
correctable, and hide those that are not - regardless of the analysis method used. Better yet, system designers can design
systems with fewer un-equalizable problems.
BAD IS ALWAYS BAD
The old adage an ounce of prevention... could never be MORE true. System equalization then becomes meaningful and fast,
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Live Sound International | Tech Topic: Why Measured Response Does... http://www.livesoundint.com/archives/2003/oct/see/see.php
providing the icing on the cake of the performance of a sound system. It makes a good loudspeaker sound better, and brings
the system to its fullest potential given the acoustic environment into which it is placed. A bad room is a bad room, regardless
of how we process the electrical signal that drives the loudspeakers.
When used properly, the traditional
1/n-octave real-time analyzer is a useful
tool outdoors at any distance. Indoors, the
effects of reflected sound and
non-frequency-uniform room absorption
produce some problems for this method at
measurement distances remote from the
loudspeaker.
One solution is to utilize a weighting curve
that reduces the target level of the
high-frequency portion of the spectrum.
Attempts to achieve a flat system response
at remote listener positions without the use
The microphone was placed on the floor for this
measurement. Anomalies inherent to the loudspeaker are of a weighting curve can result in
now visible on the analyzer.
harsh-sounding systems and even
component damage.
Transfer function analysis addresses some of the shortcomings of the 1/n-octave RTA, but it requires greater expertise on the
part of the user. Failure to properly compensate for the time differential between the reference and measured signal can
produce wildly erroneous results. The time window length must also be selected by the user, and different lengths will produce
different displayed responses. A frequency-dependent time window produces a display that correlates well with human
perception.
The most important feature of either measurement method is a knowledgeable operator - one who understands the caveats of
each approach along with the basic characteristics of the human auditory system. None of the questions raised here have a
single, correct answer. This means that experience, good judgment, and common sense rooted in Newtonian physics are still a
part of the measurement process.
Sound is a relatively easy quantity to measure, but measurements that correlate with human perception are much more
difficult. Analyzers driven by omni directional microphones do a poor job of emulating the human listener. At this point one
could ask, So why measure at all? Why not just listen? Next issue, we ll have a look at this provocative question.
Pat and Brenda Brown own and operate Syn-Aud-Con, conducting audio training sessions around the world. For more info, go
to www.synaudcon.com
October 2003 Live Sound International
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