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 creating the Modex Corner Acoustic Bass Trap, a unique membrane system that converts the high sound pressure fluctuations typically found in corners into selective absorption in the modal frequency range.
Small rooms like Project Studios often suffer from low frequency modal problems. Absorption at these frequencies requires a high absorption efficiency and significant surface area. Since small rooms do not offer the needed space, the absorption efficiency must be optimized.
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.
Sound (1) strikes the membrane (2) which sympathetically vibrates, converting the sound pressure to air (3) motion. The air loses velocity (4) as it moves through the internal absorber and air cavity. Now you absorb more bass in less space!
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 the Modex Corner Acoustic Bass Trap exploits this high pressure by converting the pressure fluctuations into air motion. As the membrane sympathetically vibrates over a selective low frequency range, determined by its mass and stiffness, it pushes air through an internal porous layer producing low frequency absorption. This innovative approach makes it possible for the Modex Corner Acoustic Bass Trap to absorb the fundamental and higher harmonic modes that are often problematic in small rooms.
Impedance Tube Testing
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. Therefore, RPG® also measures its bass traps in compliance with ASTM C384 using a 2′ x 2′ impedance tube which is accurate down to 60 Hz. The graph illustrates the plane wave impedance tube absorption coefficients for three different samples of the Modex Corner Acoustic Bass Trap. The random incidence absorption coefficient for 72ft² or 18 Modex Corner Acoustic Bass Traps in a Type A mounting on the floor of the chamber is also shown. The compliance (reciprocal of stiffness) of each Modex Corner Acoustic Bass Trap membrane is deliberately made slightly different to randomly distribute the resonance frequency over roughly a 10 Hz frequency range. This is evident in the graph by the fact that Sample 1 is about 7 Hz higher than Samples 2 and 3. The plane wave impedance tube data support the general low frequency absorption characteristic of the random incidence data and also indicate the resonance maximum. RPG® has also developed an in-house transfer function measurement system to verify the resonance maxima of its low frequency membrane absorbers.
Modex Corner Installation
The Modex Corner Acoustic Bass Traps can be mounted in any 90° corner between two walls or a wall and a ceiling. It can be attached directly to the surfaces using the supplied hook and loop or metal screw fasteners. If there is no 90° corner available, or if the corner is not suitable, the Modex Corner Acoustic Bass Trap can be stacked in a free standing configuration.