Guide to
ELF Systems
A New Era
in Bass Reproduction
Version 1.3W
COPYRIGHT MODULAR SOUND SYSTEMS, INC. 1997
Published by Modular Sound Systems, Inc. P. O. Box 488 Barrington, IL 60011
Voice 847 382 4550 Fax 847 382 4551
www.bagend.com
ELF™ and Concealment™ are trademarks of Long/Wickersham Labs
Time-Align®, Time-Aligned™ , and Time-Alignment™ are trademarks of E.M.Long Associates
NFM™ is a trademark of E.M.Long Associates
CVR Limiter™ and Bag End® are trademarks of Modular Sound Systems, Inc.
BAG END
TABLE OF CONTENTS
Forward. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1. ELF THEORY, MEASUREMENTS, & APPLICATIONS
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
The Inventors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
What is ELF. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
What is a Subwoofer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Basic ELF System Connections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
The ELF Integrator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
The Low Pass Filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
How an Integrator is Different from a Filter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
The Differentiator / Loudspeaker. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
ELF System Frequency Response. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
ELF System Impedance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Why Respond Down to 8 Hertz. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Blending ELF into Upper Range Loudspeakers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
ELF Upper Frequency Cutoff and Sensitivity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
How to Determine ELF Output and SPL Levels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Sensitivity Comparisons. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
ELF Concealment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
ELF Cutoff Frequency for Various Applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Recording Studios and Mastering Labs
Commercial Cinema and Home Theater
High Fidelity Home Audio
Electric Bass Guitar
Electronic Keyboards and Electronic Organs
Electronic Drums
PA Systems
General
ELF Contour. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Accurate Reproduction at Soft Levels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Stereo Subwoofers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Simple Two-Way ELF System Connections. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Multi-Way Concert ELF System. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Measurement Techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14
2. REVIEW OF BASIC TYPES OF LOW FREQUENCY LOUDSPEAKERS
Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Assisted Resonance Systems (ARS). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Characteristics of ELF and Other Subwoofer Systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mass Loaded Systems (MLS). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Electronically Assisted Systems (EAS). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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FORWARD
The problems encountered by anyone who has tried to reproduce tight, clean, realistic sound in the bass range are not trivial. The
bass range presents the most difficult performance requirements for a sound reproducing system. One reason why the bass range
is so important is that it contributes more to the sense of loudness than any other part of the sound spectrum. It provides the
foundation upon which the total quality of the sound rests. When the bass is solid and realistic the resultant sound is usually perceived as being very good to excellent: when it is weak and ill-defined, the perceived quality suffers, even if the rest of the audio
range is reproduced with great clarity and definition.
Edward M. Long
Theory to characterize ported loudspeaker systems and optimization utilizing high order alignments have led to the availability of
designs which have significantly improved low frequency response. Unfortunately the cost has been deterioration of the phase
response and the consequent smearing of the arrival times of these now audible low frequency sound components. With the ELF
technique we do not degrade the phase response while extending the frequency response.
Ronald J. Wickersham
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ELF is a fundamentally new approach to an old problem of accurately reproducing bass frequencies. This guide is intended to
explain and share with you some of the work done in this technology to date.
The measurements in this guide were performed with a TEF-20 in a reasonably controlled environment and a great effort has
been made to make the most meaningful measurements possible within the scope of the instrument’s ability. The fundamental
reality of how and why an ELF system sounds correct to the ear is extraordinarily difficult to measure and express with total precision. The very concept of the different measuring techniques and underlying mathematical formulas of the instrumentation
itself, used to perform the measurements, is not universally agreed upon, and work in these areas continues by the most advanced
minds of today. The simple measurements found in this guide do however highlight some basic differences inherent in the ELF
approach and we do draw conclusions based upon the measurements, the theory behind ELF, and our listening experiences. We
continue to work in this area and invite review, comments, and suggestions in our pursuit of meaningful ELF measurements and
explaining the benefits of the ELF approach ever more precisely.
James P. Wischmeyer
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SECTION 1
WHAT IS ELF
ELF is an acronym for “Extended Low Frequencies”. It is a
fundamentally different approach to reproducing bass frequencies from a loudspeaker by extending the low frequency range
farther down and with greater accuracy than has ever been
done before. In the larger historical view of audio developments, the ELF technology represents as significant a contribution to low frequency sound reproduction as the introduction
of the acoustic suspension principle over 40 years ago by Edgar
Villchur in 1953.
Although ELF has, at first, found its way into many large and
highly visible applications, the basic ELF technology applies to
“all” types of audio systems. We have designed this guide with
the hopes of addressing this wide variety of applications and
interest levels ranging from the audio professional to the home
enthusiast.
ELF systems are currently in use in a number of performing
arts centers, portable concert reinforcement systems, musical
instrument systems, professional recording studios and homes.
THE INVENTORS
The ELF inventors, Ron Wickersham and Ed Long, well
known in the audio field, are responsible for some of the most
significant audio breakthroughs of the past several decades,
including Time Align® Loudspeakers, Near Field Monitors™
and PZM® Microphones.
WHAT IS A SUBWOOFER
A subwoofer is a term given to a loudspeaker that is made to
operate in the lowest audio range, typically from 80 or 100
Hertz down as far as the design will allow. Response to 20
Hertz is often the ambitious goal of the larger conventional
subwoofers, but is seldom realized. Recently there is more and
more interest in adding subwoofers to audio systems because
adding these subwoofer low frequency capabilities to a system
adds a feeling of power and quality to the system. ELF type
subwoofers offer realism and musical richness as well as the
feeling of power, even when the music, instrument, or voice is
not thought to have significant content in the range of the subwoofer.
BASIC ELF SYSTEM CONNECTIONS
The ELF technology, when applied to a low frequency loudspeaker system, is referred to as a subwoofer system. It
requires a separate power amplifier channel dedicated to powering the ELF loudspeaker. An ELF electronic unit is required to
drive the power amplifier input. This electronic unit may have
various features but at its heart is the “integrator”, the circuit
that makes it uniquely ELF.
No connection is required from the amplifier output back to
the ELF integrator. This feedback connection sometimes used
by other designs is not required with ELF because the ELF system relies on a loudspeaker operating below resonance where
the loudspeakers behavior is predictable and uniform. The
ELF electronics can control and protect the loudspeaker without the use of sensing devices or monitoring the amplifier’s
output.
Full range
signal from
preamp or
mixer
AMP
ELF
SPK
Fig 1. Simple view of ELF system connections.
ELF technology allows multiple amplifiers and loudspeakers to
be operated from a single ELF integrator unit.
IMPORTANT: For best results the input to the ELF integrator
should be a full frequency range signal and not a low pass filtered signal as may sometimes be available when connecting
into various systems.
THE ELF INTEGRATOR
An integrator is a common electrical circuit widely used internally in electronic devices. In its basic form it consists of an
op-amp with a capacitor in series with the feed back loop. The
resistor in parallel with the capacitor is to keep the integrator
from trying to integrate to too low a frequency where the circuit will overload due to DC voltage offsets of the op-amp.
The frequency response of this circuit rises at a rate of 6 dB
OP AMP
Out
Input
Fig 2. Simple diagram of an integrator.
per octave as the frequency is decreased. The ELF circuit uses
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two integrators in series resulting in a 12 dB per octave rise as
frequency is decreased. This is the ELF dual integrator,
although sometimes referred to as just the ELF integrator. An
integrator has uniform phase shift with respect to frequency. A
single integrator has 90 degree of phase shift and a dual integrator has 180 degree of phase shift.
Magnitude
The mathematical response of an integrator will continue to
raise the level as the frequency is lowered all the way down to
Frequency
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and this delay can make the bass appear to come later than the
upper range sound. This explains why conventional systems
sound like the lowest frequencies are coming from very far
behind and not connected musically.
HOW AN INTEGRATOR IS DIFFERENT FROM A
LOW PASS FILTER
The main difference between an integrator and a low pass filter
is in the total system delay presented to the listener by the electronics shaping its response. An integrator/loudspeaker combination has short and uniform signal delay and a low pass filter/loudspeaker combination has long and variable signal delay,
which becomes longer as the low pass filter cutoff frequency is
reduced.
All conventional subwoofer systems use a low pass filter to
remove the high frequencies from the loudspeaker. A passive
crossover can be used between the power amplifier and the
loudspeaker, or an electronic crossover can be placed before
the power amplifier. The result is the same either way with the
Fig 3. Theoretical frequency response of a dual integrator.
Integrator eo = k ò e 1 dt
dual integrator eo = k òò e 1 dt
e 1 = input signal
eo = output signal
t = time
k = constant (determined by circuit component
value)
DC, where the gain is infinite (see fig.3). Of course, you cannot build an integrator that integrates down to DC but in some
specialized applications (such as astronomy and nuclear physics)
liquid helium cooled integrators are operated very close to DC
providing very high gain for amplifying very small signals. The
ELF-1 integrator will integrate to about 2 Hertz and there is an
8 Hertz filter added to protect the system.
Fig 5. Frequency and phase response of the ELF dual integrator.
This simple idea “a dual integrator” has never before been used
in this way in conjunction with a loudspeaker.
THE LOW PASS FILTER (conventional approach)
All conventional systems use low pass filters to roll off the high
frequencies from the bass loudspeaker driver. This low pass filter is one half of the crossover between the low to mid frequencies. This is a most basic and common aspect of all non
ELF systems. All low pass filters introduce, by their very
nature, frequency dependent phase shift. The phase shift introduces a significant signal delay which is added to the sound,
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Fig 6. Frequency and phase response of an 80 Hertz low
pass filter.
low pass filter adding considerable phase shift and therefore
delay which increases as the filter cutoff frequency is reduced.
The ELF subwoofer system uses the ELF integrator to extend
the low frequency response and to remove the high frequencies
from the loudspeaker. This frequency and phase response of
the integrator is complementary to both flatten the loudspeakers response below resonance and correct the phase response
in the right direction to produce a short and fixed total signal
delay of the system.
The actual time offset is small and fairly constant and reasonable to physically align. The graph in figure 5 shows the frequency response and phase response of the integrator. The
flatter phase line on the right is the phase response of the integrators and the phase shift rise (increase in phase shift and
therefore delay) on the left is the 8 Hertz hi pass filter used to
roll off the rising integrators. The effect of the filter can be
seen to a diminishing degree all the way to the upper range on
the right. Compare this to the frequency and phase response
of the low pass filter in figure 6.
The important and noteworthy aspect of these two graphs is
the amount and direction change in the phase response.
Fig 9. Typical response of a loudspeaker in a sealed enclosure.
measured in half space. The 12 dB/octave roll off below resonance is typical of a loudspeaker in a sealed box.
ELF SYSTEM FREQUENCY RESPONSE
The acoustical response of the ELF system is a combination of
the ELF integrators response and the loudspeaker in the enclosures response.
THE DIFFERENTIATOR / LOUDSPEAKER
A loudspeaker by its very nature, when operating below its resonance inside a sealed box, responds as a dual differentiator.
This very predictable behavior below resonance is the key to
the ELF system.
The mathematical response of a dual differentiator will continue to raise the level by 12 dB per octave as the frequency is
raised all the way up to infinite frequency, where the gain is
infinite too. Of course, you cannot build a differentiator that
does this, but below resonance a loudspeaker follows this
behavior precisely.
Magnitude
In figure 9 an EL-18 Loudspeaker in a 3 cu ft sealed box is
d e1
differentiator e 0 = dkt
d2 e1
dual differentiator e0 d= t2k
ELF SYSTEM IMPEDANCE
It can further be seen in Figures 11 and 12 that the impedance
of the ELF loudspeaker is uniform below resonance. Figure
12 shows at resonance the frequency response is already 1 or 2
dB down and is rolling off above. Below resonance the uniform load presented to the amplifier is an ideal load for the
amplifier to drive. Contrast this to the wild impedance curves
presented to amplifiers by conventional ARS systems. (Assisted
Resonance System or any bass cabinet which uses both the
front and rear radiation of the speaker, ie. any ported enclosure) Since any system wants to favor the notes around its resonant frequency, the ELF minimizes this by having its response
begin to roll off just before resonance.
It has been known for many years that transducers do not perform well with resonances within their operating range. This is
well understood by designers of other transducers such as
microphones, phono cartridges, hi frequency drivers, etc. Yet
most non ELF bass system designs include one or more resonances right in the middle of their operating range. This causes the system to favor playing the notes around resonance. It is
not possible to equalize away the tendency to favor the frequencies around resonance and reproduce them equally with
the other frequencies.
With the uniform ELF impedance, and operating below resonance, each note is reproduced evenly and with the same
emphasis, not favoring any particular frequency.
Frequency
Fig 8. Theoretical response of a dual differentiator.
WHY RESPOND DOWN TO 8 HERTZ
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Why respond down to 8 hertz? The answer is simply because
the sonic quality throughout the audible bass range is improved
by extending the frequency response, and thus flattening the
phase response, to the highest degree possible.
Figure 10. Shows the electrical response of the ELF output
of the Bag End ELF-1 set to full low frequency bandwidth of 8
Hertz.
The ELF is a no compromise technology with an inherently
great degree of flexibility. By extending the frequency response
down a full octave below what is considered to be the lowest
musical note, low C on a pipe organ (16 Hz), we improve the
phase response and thus reduce the delay throughout the entire
audible bass range. This excellent phase response and inherently short signal delay is why subjectively the ELF system is
known for its quick, tight, and musically connected bass sound
throughout the entire bass range, not just the lowest frequencies. With its good phase response as well as its extended frequency response it can much more accurately represent the
actual recording or the character of the sound being fed into
the system than conventional designs and their long signal
delays.
In figure 13 the frequency and phase response of the ELF-1
with the S18E loudspeaker system measured in half space
shows the 6 dB down point at 90 Hertz with a phase shift of
1800. The receive delay setting on the TEF is 20.7 ms.
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RANGE LOUDSPEAKERS
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Figure 11. Shows the acoustical response of the Bag End
S18E loudspeaker system measured in half space.
Figure 12. Shows the total acoustical response of the ELF-1
dual integrator combined with a S18E loudspeaker system
measured in half space.
BAG END
At Bag End, and through a variety of our associate testing sites,
we have been listening, learning, measuring, and designing ELF
systems for over 10 years. For those of you without an
acoustical laboratory or extensive test equipment it will be reassuring to know that with a little practice and your good ears
you will be able to match up any ELF subwoofer to any upper
range system without extensive measuring equipment. We, and
others, have done this by listening and afterward we verified
our settings with measuring equipment and found they mea-
Fig. 13. Frequency & Phase of ELF speaker.
sured correctly as well. ELF improves your ear's ability to
quickly and accurately adjust the relative level between the subwoofer and the upper range system. If you have instrumentation this combined response is easy and interesting to measure.
In Figure 14 it can be seen that the frequency response is +/0.5 dB from below 20 Hz to above 200 Hz. The crossover frequency is 6 dB down from the combined response as it should
be. The phase of the two sections follow closely (as you would
expect for a 6 dB down crossover point) and each section adds
correctly to produce a flat frequency response.
Figure 14 shows frequency response and phase response of an
S18E and a TA12 loudspeaker system both individually and
their combined response measured in half space. This acoustical blend is achieved with the ELF-1 settings as follows:
High pass frequency. . . . . . . . . . . . . . . . . . . . . .120 Hz
High pass gain. . . . . . . . . . . . . . . . . . . . . . . . . .unity
High pass polarity. . . . . . . . . . . . . . . . . . . . . . . . . . .+
ELF cutoff. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Hz
ELF gain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .+8 dB
ELF polarity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .When using S18E ELF enclosures these settings are recommended as a good starting point. The relative gain between ELF
and the High Pass output will need to be adjusted for the quanti-
on the ELF section on the ELF-1 will both produce a positive
asymmetrical output at crossover when a positive asymmetrical
input is fed into the ELF-1.
ELF UPPER FREQUENCY CUTOFF
The upper response of an ELF system is primarily determined
by the system resonance which is determined by the stiffness of
the air in the enclosure and by the moving mass of the driver
cone and voice coil assembly. We have chosen a 3 cubic foot
enclosure for our standard S18E enclosure with an upper limit of
-6 dB at 90 Hertz as shown in figure 15. Several additional
enclosure sizes are offered or can be built to produce a wide variation of upper cutoff frequencies with the corresponding change
in overall efficiency.
HOW TO DETERMINE ELF OUTPUT SPL LEVELS
The ELF SPL output level is best determined by looking at a
calibrated frequency response graph and determining the SPL
level at the frequency of interest. As the level changes with
frequency, a single number is not useful for design and comparison to other systems. The most power is required at the
lowest frequency, therefore
you may calculate the power and number of drivers required for
the lowest frequency desired by simply referring to a calibrated
frequency response graph for the specific driver and enclosure
combination as shown in figure 16, 17, or 18.
Figure 17 and 18 show first the EL-18 in a 3 cubic foot enclosure and then in comparison of 2, 3, & 6 cubic foot enclosures. It can be seen that the larger the box and thus the lower
resonance (and lower high frequency cutoff point when integrated) also naturally adds level to the system and thus a system
designer has the additional design tool of trading off the upper
crossover point and
Fig. 14.
ty and type of loudspeakers used.
The 120 Hz filter is -3 dB at 120 Hz. The -3 dB point is an
accepted standard way to specify filters so we use this method,
but it should be noted that what is actually required is a -6 dB
crossover point when coherent acoustical addition is occurring.
To determine the -6 dB frequency, multiply the -3 dB frequency
by 0.75.
As the upper limit of the ELF system is determined by the system resonance and significantly by the box size, the high pass filter frequency setting required will change with different ELF
loudspeaker models.
Note: The + polarity on the high pass section and the - polarity
Fig. 15. Shows our S18E loudspeaker system with ELF integration.
smaller cabinet size with output level. The higher the upper
crossover frequency the less sensitive the system will be and
the lower the upper crossover frequency the more sensitive it
will be. With this trade off in sensitivity all designs will play to
an equally low frequency.
SENSITIVITY COMPARISONS
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may not be as useful in determining the actual performance of
a subwoofer system in its main operating range of 20 to 100
Hertz.
Our method to determine sensitivity is to place the loudspeaker
in a sealed box, specify the box volume, measure the sound
pressure level at a specific distance, (1 meter) using a specified
voltage input, at a specified frequency, into the known impedance, in a half space environment.
Fig. 16 EL-10 in S10E enclosure dB calibrated to dBSPL.
Input signal 28V. (100 watts)
It can be seen by the frequency response curves that the sensitivity in the bass range is within one dB, yet above resonance
the levels have more variation. Two of the three speakers tested, were made by other high quality U.S. speaker manufacturers,
each have similar sensitivity ratings in their specifications and
are generally accepted as the highest sensitivity 18” drivers
available. The 3rd speaker in this comparison is the Bag End
EL-18 driver.
Generally speaking, the efficiency of ELF when compared to
ARS (Assisted Resonance System) designs allows one to say
that in a narrow frequency range the ARS system is louder or
more efficient, but not over the entire bass range and the ARS
efficiency is obtained at the cost of degrading the sound quali-
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Shown in figure 19 are three 18" drivers with their frequency
response curves overlaid. Measurements were taken in a half
space environment throughout the entire frequency response
range of the measurement. The measurements were taken on
the same day in the same box and with the same equipment
and test signal.
Fig. 17 EL-18 in S18E enclosure dB calibrated to dBSPL.
Input signal 28V. (100 watts)
Fig. 19 Sensitivity comparison of 3 loudspeakers.
Fig. 18. EL-18 in S18E-LT (2 ft³), S18E (3 ft³), and a sealed
S18B (6 ft³) enclosures dB calibrated to dBSPL. Input signal
28V. (100 watts)
The mid-range sensitivity ratings of bass drivers supplied by
many loudspeaker manufacturers may not be a useful specification for determining the actual performance of the loudspeaker
in the low frequency region. Many companies, as of this date,
rate low frequency drivers above 100 Hertz. This specification
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ty. Roughly speaking, for an ELF design to equal the output of
a conventional ARS ported design, in the narrow range of the
port’s efficiency, you will need a little more ELF cone area
and/or more power, but the actual total internal volume of the
cabinets will be considerably less. In addition, the ELF design
will respond more evenly, go lower, and sound much better. So
it may take a little more power for ELF to equal the output of
an ARS in the narrow range of its port’s resonance, but with a
smaller cabinet size.
Generally and subjectively speaking, the ELF bass from a pair
of S18E enclosures will have a very pleasant sound with tight,
low bass and be very musical sounding. It does not have the
loud boom in the upper bass range often found in vented systems. Once you become accustomed to the precision of ELF
sound there is no turning back to a non ELF system.
ELF CONCEALMENT™
It is preferable to install sufficient loudspeakers and amplifiers
to reproduce the sound level required. When the system is limited in capacity or an unexpectedly large signal is present, concealment is a type of protection circuit with a very musical and
pleasant way of sounding even when well beyond its protective
threshold. It is not part of the ELF dual integrator itself but an
additional complementary ELF technology, providing absolute
protection in a natural and musical way and actually concealing
the fact that it is protecting the system. The ELF concealment
performs dynamic control of reducing the low frequency
extension. It will allow the system to play with the upper bass
notes unaffected while protecting the system from overload
caused by playing the lower notes louder than the amplifier
and/or loudspeaker can reproduce safely and undistorted. It is
very important that the concealment threshold is set to the correct level for it to function correctly and prevent amplifier clipping and/or speaker overload. For general non critical studio
applications it is very acceptable for the concealment threshold
light to flash intermittently but if it stays on to full intensity it
means that you either have the threshold set too low or, if the
threshold is set properly, that you require additional speakers
and amplifiers for your application. For critical studio and mastering lab monitoring applications the concealment threshold
should not be crossed as this will reduce your ability to hear
what is actually being recorded. In addition, when the concealment threshold is crossed, it is changing the frequency response
down low and degrading the phase response both at the lowest
frequencies and into the middle bass range as well.
ELF CUT OFF FREQUENCY AND VARIOUS
APPLICATIONS
For certain applications you may not want to operate the system down to 8 Hertz. The low frequency limit may be adjusted
by selecting an appropriate ELF cutoff frequency. The phase
response is degraded by cutting off the frequency response at a
higher frequency. This is a reasonable compromise as this higher cutoff may be needed, for example, to filter out the low frequency rumble (concert hall noise, air conditioning, etc.) commonly found on many of today’s CD recordings.
Recording Studios and Mastering Labs
When utilizing ELF in recording and mastering playback applications, the 8 Hertz response will insure proper treatment of
the full spectrum. In addition, you should always note when
concealment is taking place and not perform your final mix
past the concealment threshold of your ELF system. With the
popular trend of adding subwoofers in homes and theaters, the
low frequency problems found on CDs and other recordings
will need to be corrected in the recording and mastering stages
so that the full benefits of a wide band response can be real-
ized in the playback.
Commercial Cinema and Home Theater
When utilizing ELF for cinema and home theater systems you
may have to raise the ELF cutoff frequency to 20 or 30 Hertz
to reduce low frequency noise found on many recordings.
Allow the concealment to perform its function and occasionally
monitor the concealment by observing the LED indicator.
High Fidelity Home Audio
When utilizing ELF in a very fine home audio application it
should be treated as a studio application. (refer to studio section) In addition, you have the added option of adjusting the
ELF cutoff to any higher frequency required above 8 Hertz.
This enables you to reduce low frequency noise found on some
recordings.
Electric Bass Guitar
When utilizing ELF for electric bass guitar systems, depending
on the instrument, you may have to raise the ELF cutoff frequency to 20 or 30 Hertz to prevent low frequency string handling noise. Allow the concealment to perform its function,
and occasionally monitor the concealment by observing the
LED indicator.
Electronic Keyboards and Electronic Organs
When utilizing ELF for electronic keyboard systems, most systems will work well with an 8 Hertz ELF cutoff frequency.
Allow the concealment to perform the protection function and
occasionally monitor the concealment by observing the LED
indicator.
Electronic Drums
When utilizing ELF for electronic drum systems, most systems
will work well with an 8 Hertz ELF cutoff frequency. Allow
the concealment to perform the protection function and occasionally monitor the concealment by observing the LED indicator.
PA Systems
When utilizing ELF for general PA systems, you may have to
raise the ELF cutoff frequency to 20 or 30 Hertz to prevent
low frequency noise from the microphones. Allow the concealment to perform its function but always monitor the concealment by observing the LED indicator and refer to the concealment section for a thorough understanding of this function.
General
In general you should allow the ELF hi pass cut off to be set
as low as possible without allowing too much low frequency
noise into the loudspeakers. Remember you are raising the
ELF cut off to improve the sound quality by removing the
noise, not to protect the amplifiers and loudspeakers from low
frequency damage as the concealment will provide that function.
ELF CONTOUR
Designed primarily for large arrays, the contour may be used as
an additional tool to shape the lower end of the frequency
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response for any system application. Adding contour will cause
a rise in the frequency response just before the low frequency
rolls off. The contour occurs just above the ELF cutoff frequency. Long arrays, perhaps 20 to 40 feet, are required for the
optimum output and directivity performance of any bass system. These arrays naturally have more output in the upper bass
range due to the gain added by the directivity of the array. At
lower frequencies you will not receive this additional gain.
Contour will then allow you the option of adding a boost at the
lower frequencies to flatten out the total response of the array.
ACCURATE REPRODUCTION AT
SOFT LEVELS
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We are still working on documenting exactly why, but it is clear
to those familiar with ELF that it is capable of reproducing
clear audible bass notes at very low volume levels. One theory
is that the conventional ARS (Assisted Resonance Systems)
because of the several resonances within the systems range,
takes an amount of time for the energy to build up within the
system before it “switches on”. It may be that part of what is
often assumed to be the simple Fletcher Munson loudness
curve of the human ear is actually both the Fletcher Munson
curve and the characteristics of these common ARS bass
speaker designs. You will find the ELF system requires much
less, if any, loudness compensation as it is turned down to a
whisper.
STEREO SUBWOOFERS
With ELF, stereo subwoofers in certain applications can
become desirable because of the inherent ELF fidelity and precision the source can be perceived to have direction and
because the ELF’s alignment and musical connection to the
upper stereo image is excellent. This may be most effective in
larger rooms and outdoors.
SIMPLE TWO WAY ELF SYSTEM
CONNECTIONS
The ELF subwoofer requires a separate power amplifier to
operate. This is called a 2-way system or biamping in its simple
form. A typical upper range speaker may be a
2-way passively crossovered system such as the Bag End TimeAligned® TA12 or TA15. The ELF integrator is designed to
universally blend an ELF subwoofer into any other system as
well. For passive upper range systems use the connections in
Fig 20.
MULTI-WAY CONCERT ELF SYSTEM
For larger concert systems requiring 3 or 4 way upper range,
connect the output of your mixer or final equalizer to the input
of the ELF-1 then connect the high pass output of the ELF-1
to the input of your existing electronic crossover as follows in
Fig 21.
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In concert systems we do not recommend adding the ELF
onto a sub group mixer output as it is often done with conventional subwoofer systems. By allowing the lower portion of the
upper range to run down into the ELF range it is most likely
that you will have a large bump or dip in the upper bass frequency response. The likely resulting poor frequency response
and/or poor phase response blend between the ELF and the
upper range loudspeakers may cause you to use more overall
EQ than otherwise necessary with diminishing returns and
reduced ELF benefits.
In addition, the quick response and pleasant sound quality of
the ELF will allow you to extend the range of the entire mix
and not just a few select channels without the low frequency
problems often associated with this approach.
Further benefits are provided by utilizing the high pass section
CVR Limiter™ as it is a unique feature of the ELF-1, particularly good sounding, and a design circuit not common to other
limiters. See section 4, ELF-1 circuit design description, for
additional information on the CVR Limiter.
For large sound systems utilizing large ELF arrays refer to the
High-Amp
Full range
signal from
preamp or
mixer
ELF-1
High
Spk
ELF-Amp
Fig. 20
Low
Spk
section on “ELF Contour” for information on contouring long
ELF loudspeaker arrays.
MEASUREMENT TECHNIQUES
When the test space is referred to as “half space” the loudspeaker under measure is buried so that the baffle is flush with
level ground at least 100’ feet away from the nearest tree, building, or any obstacle. The microphone is mounted with a small
boom stand directly over the loudspeaker at 1 meter from the
speaker baffle.
The low frequency tests have a frequency resolution of 2.8
Hertz, unless otherwise specified.
Phase measurements are relative but they do correctly indicate
the phase change and/or phase direction within each measurement.
The small additional roll off below 7 Hertz includes the fre-
High Amp
Input from
mixer or
other source
High Pass
Output
ELF-1
Other
x-overs
ASSISTED RESONANCE SYSTEMS (ARS)
High Mid Amp
Mid Amp
ELF Amp
Fig. 21
plest system, which would be a driver operating in a closed box,
is constrained by this law of physics.
ELF
Output
quency response of the B & K 4007 microphone used.
The test instrument used in these graphs is a TEF-20, a FFT
based instrument. We have attempted to minimize the many
artifacts present in the TEF measurement process.
SECTION 2
REVIEW OF BASIC TYPES OF LOW
FREQUENCY LOUDSPEAKER SYSTEMS
OVERVIEW
There are two basic types of systems used for producing
extended bass response; one is acoustical and the other is electronic. Acoustical systems can be divided further into two general categories: Assisted Resonance Systems (ARS) that rely on
wavelength dependent parameters, which require large dimensions, and Mass Loaded Systems (MLS) that are inefficient.
ARS include ported passive radiator, tuned chamber and transmission line or labyrinth systems. MLS are generally closed
box, stiffness controlled systems. The distinction between
these two categories can be blurred if both wavelength dependent parameters and mass loading are combined in a system.
Electronically Assisted Systems (EAS) include servo controlled
and bass boost types. The problems and limitations associated
with these systems are shown in the chart. The ARS and MLS
are combined under the heading “NORMAL” and EAS are
shown under the heading “SERVO.”
For all systems, the bass driver excursion is greatest at the lowest frequency to be reproduced. For a given diaphragm size,
the maximum acoustical output level is limited by the maximum
excursion capability of the loudspeaker driver. The excursion in
the low frequency part of the range is much greater than at the
higher frequencies, in fact, for every halving of the frequency,
the excursion is 4 times as great. For example, to produce the
same acoustical output at 32 Hz (Low C) as it does at 64 Hz
with a 1/8" peak to peak motion, the excursion of the bass driver diaphragm would have to be 1/2" peak to peak. The sim-
These excursion constraints led to the development of Assisted
Resonance Systems (ARS) in which the acoustical output is
increased beyond the normal limit of the driver excursion by
producing acoustical output from a port or multiple ports.
The application of Helmholtz's principles of acoustical resonance was employed to develop ported systems, which in early
times were called bass reflex systems. Another type of wavelength dependent system is the transmission line, which in its
folded form was called an acoustical labyrinth. Transmission
line systems rely on the output from the rear of the loudspeaker driver to assist the output from the front, after passing
through a long ducting system inside the loudspeaker enclosure.
Transmission line systems are of necessity the largest of the
ARS types.
Ported systems have seen recent popularity for a number of
reasons. The development of more scientific methods of
design, based upon the work of Novak, Thiele, Small, Keele,
and others has improved the chances that a design effort will
result in a system that behaves as predicted. Ported systems
have progressed from simple types to complex, multichamber
systems. These latter systems are designed with the loudspeaker driver(s) mounted inside the enclosure and the acoustical
output radiated from a single or multiple ports. All of these
ARS are large, when they are required to reproduce low frequency sound, because they rely on the acoustical stiffness of
the air in the enclosure as well as the mass of the air in the port
to tune them to a low frequency. This requirement for large
dimensions means that the size and weight of the enclosure is
directly a function of the frequency to be reproduced; The
lower the frequency the larger and heavier the enclosure must
be. The quality of sound produced by ARS is very dependent
upon the internal impedance or damping provided by the driving amplifier. The resistance of the connecting cables also
can be a significant factor. This latter consideration is very
important for systems which require long cable runs from the
amplifier to the loudspeaker system.
The variation in impedance of the triple tuned sub-woofer is an
example of the problem presented to a driving amplifier.
Amplifiers work best when the impedance of the load is constant. The interaction of the amplifier's internal impedance
and the varying impedance presented to it by the loudspeaker,
will affect the amount of power that the amplifier must deliver.
It also affects the damping, or tightness of control, that the
amplifier can exert upon the loudspeaker system. The upper
range response of the system also must be rolled off by a low
pass filter. This will introduce a signal delay in the low frequency range because all low pass filters have signal delay. Also,
because the rear of the loudspeaker driver diaphragms have no
loading at the lowest frequencies, where the ports act merely as
vents to the outside of the enclosure, a high pass filter must be
added to prevent the loudspeaker drivers from being driven
beyond their capabilities and thus produce noise and distortion.
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CHARACTERISTICS OF ELF AND OTHER SUBWOOFER SYSTEMS
SYSTEM TYPE
Enclosure size
Enclosure weight
Low pass filter required
Signal delay varies with
design changes
Separate power amplifier
required
Lowest frequency limited
by driver resonance and
enclosure volume
Low frequency output limited by driver excursion and
amplifier power
Damping
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Transient Response
Signal Delay
Time offset from upper
range system
Driver cone mass
Voice coil mass
Cone area
Cone excursion
NORMAL
ELF™
Heavy
Light
Large
Large
Yes
Yes
Heavy
Small
No
No
Yes
Yes
No
Yes
Yes
Yes
Yes
No
Yes
Yes
Yes
Variable, poor to moderate.
Depends upon driver system
design, amplifier internal
impedance, and cables because
system operates above system
resonance
Variable,moderate to good,
depends upon system design
and quality control because system operates above system resonance
Excellent. Independent of driver, amplifier impedance and
cables
Moderate due to time delay
of low pass filter
Excellent due to short time
delay and high damping
Long and variable depending
on frequency of low pass filter
used
Short and fixed integrators
used have fixed delay
Poor due to long time delay of
low pass filter and low damping
Long and variable depending
on frequency of low pass filter
used
Large
Usually heavy
Usually heavy
Depends upon SPL
Frequency dependent
MASS LOADED SYSTEMS (MLS)
This category of system is used for home music systems where
efficiency is not very important. The perceived quality of the
bass reproduced by many MLS systems is usually booming and
under-damped.
ELECTRONICALLY ASSISTED SYSTEMS (EAS)
The most complicated of all the different types of bass reproducing systems is the servo control type. Servo systems are
replete with complicated and interacting parameters which must
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SERVO
Large
Small
Usually heavy
Can be light
Usually heavy
Depends upon SPL
Frequency dependent
Can be light
Depends upon SPL
Frequency dependent
be monitored and controlled in real time if the system is to
work properly. Even the placement of the accelerometer used
to monitor the diaphragm motion is not trivial.
CONCLUSIONS
The systems we have examined each have their own particular
limitations. The perceived sound quality ranges from poor to
acceptable. The size and weight of ARS, the inefficiency of
MLS, and the complexity of EAS, were the motivation to find
the better technology, ELF.
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