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VR-4SR MkIII
Specifications
Cabinet Type: Twin cabinet stacking system with Interlocked bracing.
Cabinet Construction: Triple-damped composite laminate, 75mm thick.
Bass Loading: Transmission line hybrid, using three chambers tuned to 20Hz.
Woofers: 2 pcs- 220mm cast frame with aluminum cones, made in Norway.
Midrange: 170mm Audax/ A.A.C Nextel cone with ribbon voice coil, made in France.
Tweeter: 25mm silk dome with Wide Surround, Neodymium magnets from Denmark.
Ambience System: Rear firing 1" horn loaded mid/tweeter made in England.
Crossover: 24 dB acoustic filters with Zobel "passive servo" control circuits.
Frequency Response: 20Hz to 25kHz +/- 3dB; (+/- 1dB in midrange band).
Impedance: 4 ohms nominal. High current amplifiers rated at 4 ohms recommended.
Sensitivity: 91dB @ 1watt/1meter.
Power Rating: minimum 50 watts (for medium volume levels, 300 watts maximum.
Size: 44" H x 11" W x 24" D (112cm H x 28cm W x 61cm D)
Binding posts: two pairs for bi-wiring, WBT sourced.
Weight: 76kgs (167 lbs) each, shipping weight is 250 lbs each (114 kgs each).
Speaker Design Theory by
Albert Von Schweikert
The Global Axis Integration Network tm
Acoustic Inverse Replication tm
My dissatisfaction with the lack of realism in contemporary speaker
design has led me on a long quest. Twenty years ago, several experiments
I conducted at the California Institute for Technology enabled me to
discover several important psychoacoustic principles.
PSYCHOACOUSTICS
The first discovery was that the ear/brain hearing mechanism can sense
differences between certain types of sound wave patterns and uses this
recognition for identification and spatial localization of sound sources.
For instance, an omnidirectional wave pattern consisting of spherical
sound waves can be differentiated from highly directional beam waves.
A computer in the brain compares data arriving at each ear and computes
directional data from arrival times, frequency, phase, and amplitude
responses, among other things. This data is stored for later processing,
and over a sufficient learning process, becomes an acoustic reference
bank. Differences in the data arriving at each ear conveys stereo information,
for instance, including spatial localization and timbre recognition
of previously heard tones or other sonic sources.
An omnidirectional source radiating a spherical sound pressure wave
is comparable to an acoustic musical instrument such as a guitar, piano,
or drum. A directional source (read: conventional forward-firing speaker
system), however, does not sound precisely the same, nor does it load
an average listening room in the same manner, due to non-linear frequency
response combined with time and phase delays in the off-axis response.
These aberrations contribute to warped sound waves that are neither
coherent nor accurate to the original spherical waves, and can be easily
heard as such, no matter how accurate the system appears to measure
on axis. These aberrations are highlighted due to reflected energy from
boundaries such as the floor, ceiling, and walls. Although previously
documented, these effects were not considered to be of prime importance
prior to my research, but had tremendously important psychoacoustic
implications, as I discovered.
DESIRED AXIAL RESPONSE VS. OFF-AXIS ABERRATIONS
I developed a small two-way speaker system that exhibited perfect measurements
by the existing standards of the day (1976). The design was a Time Aligned
two-way speaker using a 6.5" woofer, with first order phase coherent
crossovers. The impulse response was pretty good, considering the drivers
being used, and the frequency response was exceptionally flat on the
axis directly in front of the speaker's tweeter. Yet side-by- side comparisons
with an acoustic guitar as a sound source revealed that the prototype
lacked an essential realism. One evening, while I was listening to my
creation in that magic sweet spot where the music seemed to come together,
my wife was washing dishes. I was complaining about my disappointment
with the sound quality, so my wife Linda, far off-axis in the kitchen,
remarked that the sound was muffled and did not float in the air like
the sound of our Hardmann (circa 1899) upright grand piano. This somewhat
startled me, since in the narrow "sweet spot" where I sat
there did not appear to be problems with muffling nor image recreation.
I then realized that perhaps our ear/brain hearing mechanism somehow
compares subtle cues such as radiation patterns, among others, to recognize
and identify sonic information. I had recognized, of course, that the
sound changed dramatically when I stood or moved around the room, but
was not concerned with this behavior since all other speakers I had
heard exhibited the same problem! I decided that the brain must somehow
compare these subtle cues (like sound wave recognition patterns) to
stored information from past experience. Thus the brain knew that the
sound from the speaker could not be radiated from a live piano, since
the sound waves from the speaker did not match the radiation pattern
of sound waves coming from the instrument. Obviously the piano, being
an omnidirectional radiator, involved the entire room with its radiation
pattern, while my highly directional prototype speaker, did not. Amazingly,
listening to one speaker up close did sound highly realistic, much as
a very good pair of headphones. It was the directional pattern of the
system that was flawed!
I hurried to the lab to conduct a series of off-axis response measurements
on a 180-degree horizontal and vertical axis. The results, although
dismal as expected, excited me, since the off-axis radiation pattern
was clearly non-linear and was perhaps related to the lack of realism
I was experiencing! Several years of experiments regarding directivity
patterns and driver behavior later proved my theory to have merit. To
the layman not schooled in conventional theory leading to a status quo
in engineering design, this is not perhaps a surprising discovery, since
it would seem intuitive to design a speaker to project sound in the
same manner as live instruments!
ACOUSTIC INVERSE REPLICATION
Additional research led to my further discovery that recording microphones
encode the musical signal with their overlaying pickup response patterns.
After making a series of recordings, using several different microphones,
it was obvious during playback that the mics not only had tonal differences
related to frequency response errors, but also created different types
of imaging patterns. The perception of depth and space was not only
dependent on the recording environment and mic placement, but also on
the mic's off-axis polar response. For this reason, I decided to engineer
an adjustable ambience retrieval system radiating from the rear of the
VR speakers, in able to recreate the space and depth heard in the concert
hall when the spaced omni method of recording is used.
Thus, a correctly designed speaker system should project the inverse
of the mic signal, acting as a decoder to translate the original sound
field. I have termed my design for this decoding as Inverse Acoustic
Replication tm, and the Virtual Reality series of designs was developed
from several important concepts related to microphone pick-up patterns.
These concepts are based on the consistent phase/frequency relationships
in the polar response pattern of the mics, which was later reverse engineered
into the VR speaker systems.
FIRST ORDER VERSUS FOURTH ORDER NETWORKS
Experiments validated the concept of consistent (not the same as coherent)
phase vs. frequency linearity in a 180-degree arc around the speaker
system, and appeared to work far better than phase coherency limited
to the axial tweeter response. As is commonly known, first order crossovers
have severe problems with driver overlap, which lead to an effect called
lobbing. This problem is related to the fact that the drivers can sum
perfectly only on one very narrow axis, since the path length from the
drivers to all other axes cannot sum to unity, in either frequency,
phase, or transient response! This not-surprising effect is due to the
mathematics governing wave transmission and is easily verified by simple
experiments or "doing the math."
Thus the measured polar vertical off-axis response, for instance +/-
180 degrees, of speakers using first order crossovers will typically
exhibit amplitude dips and peaks of up to 18dB caused by the lobbing
effects caused by uneven path lengths and will have severe phase distortion
as well. The ear/brain hearing mechanism can easily hear this effect,
due to reflected response from the room boundaries even though the listener
may be seated on the perfect axis. Not amazingly, the ear is far more
critical than any type of test equipment yet devised, so these effects
cannot be ignored on a psychoacoustic level, especially in a normally
reverberant living rooms where the off-axis response dominates the perceived
frequency and phase response.
OFF-AXIS PHASE VS. AMPLITUDE CONSISTENCY
I have termed my method of enabling consistent phase vs. frequency behavior
Global Axis Integration tm, since my design constructs a consistent
polar response both in the amplitude and time domains, both horizontally
and vertically. Not only does this radiation pattern enable the listener
to perceive well-balanced frequency and harmonic integration from almost
anywhere in the listening side of the room, but also enhances sound-stage
imaging over a 180 degree axis horizontally and 70 degrees vertically.
This is especially important psychoacoustically, since the ear/brain
hearing mechanism responds favorably to this reconstructed sound wave
pattern.
This Global Axis Integration method consists of a carefully engineered
radiation pattern created by front and rear driver arrays. Proprietary
circuits form steep 24 dB acoustic crossover slopes at specially selected
frequencies without the penalties of induced ringing and excessive phase
delay. These slopes are necessary to limit lobbing effects and non-linear
off-axis response, and actually enable the consistent phase behavior
necessary between drivers. The architecture of the circuitry resembles
first and second order filters combined with Zobel conjugate compensators
in parallel. By using a minimum of high quality parts in series with
the drivers, the sound remains transparent, yet the control over phase
and amplitude can be corrected with the paralleled Zobel circuits.
LISTENING VS. MEASUREMENT CORRELATION
Long-term listening sessions have shown very good correlation between
the engineering target-response patterns and perceived musicality. Although
initially I listened to the designs using high quality phonograph and
tape sources, I later designed a test using live music sources to compare
to the speaker output. In a large room, with sonically absorbent panels
in the middle of the room, we compared the sound of an acoustic guitar
at one end of the room being replicated by the speaker at the other
end. We used several expensive mics and a tube preamp for these tests,
and alternated between different kinds of portable instruments, such
as brass, chimes, snare drum, trumpet, saxophone, harmonica, and of
course, the human voice. In the beginning, circa late 1970's, we were
humbled by the lack of realism in our original designs. Over the years,
by conducting research into the distortions and colorations caused by
the drive units, circuits, and enclosures, we were able to eliminate
or greatly reduce the factors that contributed to the sonic colorations.
Critical evaluation of these new engineering principles by several magazines
has resulted in highly favorable reviews and comments. Although not
an exact inverse of the mic signal, the AIR and GAIN designs use psychoacoustic
principles to work with the listening room. Ambience retrieval, imaging
clues, and soundstage transparency are combined with wide band frequency
response, low distortion, and ultra low levels of coloration. This combination
of engineering goals has resulted in unprecedented levels of realism
not achieved in competing speaker designs, regardless of cost.
NEW DRIVER TECHNOLOGY
My latest research has concerned the cone material and motor. Driver
manufacturers continuously develop new cone materials in their search
for the holy grail of perfect measurements. For instance, the new metal
cones have fantastic stiffness and "punch" for bass frequencies,
but unfortunately do not fare as well when used in the midrange and
treble ranges due to high Q peaks. On the other hand, old materials
such as paper have been upgraded with composites such as carbon fiber
powder, Kevlar threads, or plastic compounds injected into the paper
when the cone is being formed. Although many materials have been tested
for treble reproduction, including metals, ceramics, paper, plastic
film, and others, the plain-Jane fabric dome has seemed to remain at
the top of the audiophile's list for smooth response.
New motor technology has been discovered and implemented in many of
our new products. The distortion caused by the voice coil moving in
a non-symmetrical magnetic field has been greatly reduced by new mechanical
designs of the pole piece, shorted-turn voice coils, and new magnetic
materials. We have found that up to 75% of the previous distortion has
been eliminated, leading to greatly improved transparency.
CONCLUSION
Speaker designing is a highly complex challenge that requires a multi faceted approach to understanding how we perceive sound, how it is electronically encoded and how the mechanical components decode the recorded event. Our team at Von Schweikert Audio is devoted to the single-minded pursuit of ultimate sonic realism in all of our designs, regardless of price point. This signature commitment to quality and the experience to deliver are the factors above all others that make Von Schweikert Audio components superior to the competition.
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