Introduction to the Design of the REL Gibraltar Sub-Bass Systems
A
ny design process is a journey. Please follow along as we share lessons gleaned from our three-year journey culminating
in the birth of a new ultra high performance range for REL Acoustics, GIBRALTAR™. The REL design team’s goals for
Gibraltar are a simple attempt to replicate the extraordinary performance of the much costlier REL Studio III while
reducing size and upgrading appearance. Additionally, balance the virtues of Gibraltar toward speed and power such
that multiple units can be coupled together to exceed the Studio’s performance.
The Heart of the Matter
Before we could begin refining our cabinet design or assign engineering resources to amplifier layout, we needed to
design and build the ideal driver for this application. In its final iteration, we selected a particularly tightly wound true
carbon fibre material that is exceedingly strong and lightweight. Reducing moving mass produces superlative impulse
response—a key design goal for Gibraltar. This particular driver also needed a long stroke (about 1 3/4”) in order to
generate the ultimate loudness required to underpin today’s high performance main speakers.
A Rare Occasion Wherein Bigger is Not Better
Reducing size while maintaining extremely high levels of output, low distortion and lightning fast transient speed drove
the decision to utilize a sealed cabinet. Though Studio III successfully employs an exotic variation of a vented enclosure,
performance in a limited enclosure size is superior with a sealed design. Sealing also naturally places increasing
resistance to driver excursion that acts to progressively limit driver excursion. Sometimes referred to as an “airspring”
it is a gentle mechanical aid to the necessary limiting action required to avoid damage to the driver.
Cabinet form is inspired by the design studies of a longtime mentor, Franco Serblin, founder of Sonus faber. The
amount of refinement necessary in order to deal with the extreme pressure levels generated by the backwave of our
Gibraltar driver was an unanticipated challenge. Extremely small variations in the basic shape resulted in evident
changes in bass performance.
Numerous versions of the cabinet were constructed and evaluated using test instruments and especially the human
ear. Extensive bracing runs the full internal height of each side panel, across the entirety of the crucial top panel,
and includes a single fore-and-aft integrating brace much like the spars on a ship that results in an exceptionally
quiet cabinet. Spacing of these spars is based on Fibonacci multiples, ensuring that resonance cancellation is spread
across a multitude of frequencies. The resulting performance delivers exceptionally low bass, replete with weight
and authority, along with tremendous transient speed. This is only realized by the precise cabinet form and precise
assembly techniques employed in Gibraltar.
Divide (Very Rapidly) and Conquer!
The input filter topology (crossover) used in Gibraltar is new and, to the best of our knowledge, the fastest analog filter
ever devised for a sub bass system at about 4 milliseconds rise time. We utilize a second order Bessel function for
preservation of impulse response that crosses over as low as 20 Hz. Additionally, we have a second filter placed at
approximately 250 Hz that gently removes midrange and high frequencies from the mix but allows for maximum
speed of the main filter within its operating range. Crossover is tunable in 1 Hz increments.
Gain is adjustable for both (preferred via Speak-on) REL Reference High Level connection or standard low level input
to the main power amplifier. In High Level mode, input signal is derived at the end of the amplification chain and
produces the most natural and fullest expression of the voice of the main system allowing the REL to blend effortlessly
with the main speakers. Conventional low level (RCA) connection is also supported through rear channel RCA’s.
REL Reference Theater Connection
Additionally, and operating completely independently of the primary crossover, Gibraltar’s filter provides for dedicated
.1 inputs and crossover function. The pure Dolby Labs .1 protocol is observed featuring a dedicated 4th order filter
fixed at 120Hz. Connection is via either XLR’s or RCA’s for the .1 input. Gain is adjustable completely independent of
the High Level connection.
When both the high and .1 input are employed it is possible to obtain the ultimate performance available from
soundtracks, as the main speakers are permitted to run full range producing the most natural sound from high quality
speakers, the REL High Level connections extends and supports the mains turning them into truly full range devices,
AND the dedicated .1 channel information encoded on the disc is fed to its own REL channel and gain can be perfectly
set to extract maximum dynamics from a film. Truly the best of all worlds.
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