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Small
rooms often exhibit poor low frequency response with
significant emphasis at modal resonances. They also
have limited space to make acoustical improvements.
Porous surface absorption is ineffective at these modal
frequencies, because the air motion near walls and in
corners is essentially zero for these long wavelengths.
RPG® research solved this dilemma by developing
a unique membrane system that converts the high sound
pressure fluctuations typically found at wall surfaces
and in corners into selective absorption in the modal
frequency range. As the use of sub-woofers becomes more
and more popular, there is a growing need for modal
frequency management.
Problem
Listening and performance rooms often suffer from low
frequency modal problems. Because porous absorption
is ineffective at these frequencies, a high efficiency
mechanism is needed to provide modal control.
Solution
RPG® solved this problem by optimizing the absorption
efficiency using a unique internally damped membrane
absorber that provides ideal absorption in the modal
frequency range.
 Absorption
Surface or corner applied porous materials lose efficiency
at low frequencies, because the particle velocity or
air movement associated with long wavelengths is low.
The sound pressure, conversely, is at its maximum. The
internally damped membrane in Modex™ exploits this
high pressure by converting the pressure fluctuations
into air motion. As the membrane sympathetically vibrates
over a selective low frequency range (1), determined
by its mass and stiffness, it pushes air (2) through
an internal porous layer producing low frequency absorption
(3). This innovative approach makes it possible for
Modex™ to absorb the fundamental and higher harmonic
modes that are often problematic in small rooms.
Resonant
Frequency
 (CLICK
ON IMAGE TO VIEW)
Simple relationships exist between the design frequency
of these resonant systems and the membrane mass, stiffness
and cavity depth. The chart illustrates the effect of
cavity depth on resonant frequency for two different
membranes. Note how the reciprocal square root dependence
on the air cavity distance (and mass) results in progressively
smaller changes as the distance is increased. These
equations, however, are often inexact in predicting
the resonant frequency, because the physical mass of
the membrane in the system is different from the isolated
mass. If the resonant system has a high Q, one may be
creating a notch at the wrong frequency, thus aggravating
the modal problems.
Absorption
Coefficient
 (CLICK
ON IMAGE TO VIEW)
Another unknown is the absorption coefficient of the
resonant system. Random Incidence absorption coefficient
testing has been standardized by ASTM using the C423
reverberation room method. The frequency range is 125
Hz to 4,000 Hz. In the United States, most NVLAP certified
reverberation chambers are not accurate below 100 Hz.
Accurate low frequency measurements can however be made
using an impedance tube. To characterize the Modex™
system, RPG® designed a 2' x 2' x 18' impedance
tube and measures Modex™ in compliance with ASTM
C384. The graph illustrates the effect of cavity depth
on maximum absorption for a given membrane system. It
can be seen that this particular membrane system has
a good absorption between a cavity depth of 4 - 6".
These results indicate that we can provide relatively
shallow Modex™ enclosures.
Absorption Bandwidth
(CLICK ON IMAGE TO VIEW)
While membrane absorbers can be designed for a specific
frequency and offer a high absorption efficiency, their
bandwidth or range of frequencies over which they are
effective is limited. One can broaden the Q of the system
by introducing damping in the air cavity, but this is
accompanied by lowering the maximum absorption efficiency.
RPG® has taken a different approach to provide
absorption in the modal range. By systematic impedance
tube testing, RPG® has optimized the membrane mass,
compliance and cavity depths that offer maximum absorption
efficiency at the third octave center frequencies of
40, 50, 63 and 80 Hz. To cover the 40-80 Hz frequency
range, one can simply combine Modex modules, thus providing
100% efficiency. Specific modal problem frequencies
can also be addressed with individually tuned modules.
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Benefits
The
Modex™ Module's proprietary internally - damped
membrane converts any available wall or ceiling into
a highly absorptive low frequency absorber.
The Modex™
Corner (formerly Corner Bass Trap) provides useful
low frequency absorption in wall - wall and wall - ceiling
corners that are often unused and available for acoustical
treatment. This means you don't have to sacrifice space
for equipment.
All Modex™ modules are modular and can be added
as needed. The modules simply stack on top of one another
and offer unlimited opportunity for experimentation
and modification.
Modex™ can be applied to walls, ceilings, corners
or mounted free standing.
Bass management in the sub-woofer region is now simply
a matter of attaching Modex™ to walls, ceilings
or corners.
Modex™ modules can be upholstered and visibly installed
or concealed behind ceiling clouds, soffits or stretch
fabric systems.
Modex™ Plus Modules (12" in depth) offer absorptive
chevrons between the room and the membrane to extend
absorption to cover the 40- 10,000 Hz frequency range.
APPLICATIONS
All
critical listening rooms, including recording studios,
vocal booths, home theaters, quality control rooms,
CD mastering, film mix and dubbing stages, and music
practice rooms.
SPECIFICATIONS
- Acoustical
Data
- Product
Details
- Modex™
modules:
23 5/8'' (W) x 23 5/8'' (H) x 7'' (D)
- Modex™
Plus Modules:
23 5/8'' (W) x 23 5/8'' (H) x 12'' (D)
- Modex™
Wall Modules:
23 5/8'' (H) x 47 1/4'' (W) x 7'' or 12'' (D)
- Custom
sizes also available
- Standard
fabric finish is Guilford of Maine FR701 #298.
Custom colors are also available
- Shipping
weight varies with the membrane (Call for information)
FEATURES
- Pressure
zone membrane absorber
- Shallow
7'' depth offers optimal absorption
- Stackable
- Wall
or free standing mounting
- Lightweight
- Portable
- Modular
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Northampton, MA 01062 
