by RPG Diffusor Systems
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 Modex, 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.
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.
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.
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.
Modex Resonant Frequency
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.
Modex Absorption Coefficient
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.
Modex Absorption Bandwidth
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.