RPG Modex For Reducing Only Bass Room Reverberation

RPG Modex Corner

I find that some home theater rooms that employ sound isolation techniques have issues with the lowest frequencies having excessive reverberation compared to the Dolby standard for theater sound reproduction. I suspect this is caused by the fact that the low frequencies that are not trapped effectively in a non-isolated room are in an isolated room. Many people use large bass traps to try and tame these frequencies. Unfortunately, they can easily reduce the reverberation of the higher frequencies too much making the room too dead acoustically at the higher frequencies throwing the room out of balance acoustically exacerbating the problem with the relative reverberation between the high and low frequencies.

Porous materials in general 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 in a corner or near a wall. The internally damped membrane used in the RPG Modex exploits this high pressure zone found in corners or walls 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 to absorb the fundamental and higher harmonic modes that are often problematic in small rooms while not over absorbing the higher frequencies.

The Modex bass trap can be tuned to either 40, 63, 80 or 100 Hz. The chart below shows how this product will impact the room acoustics at various frequencies.

Modex Sound Absorption vs. Frequency

The best way to determine if a product like this or any other acoustic treatment is needed is to conduct a survey of the room acoustics. This survey would include an analysis of the RT60 (sound decay rate) at various frequencies versus the standard. Example results for a room with sound isolation is shown below.

Frequency Dependent Room Acoustics vs. Dolby Specification

RT60 At 500Hz Relative To Dolby Specification

Note that this room has the following characteristics:

• No bass traps
• 9 leather theater chairs
• 1″ and 2″ thick fiberglass acoustic material covering a high percentage of the wall surface area
• Plush carpet on the floor
• Double drywall construction with fiberglass insulation behind to isolate the sound from the rest of the home

The result is a room that generally meets the Dolby specification for theater acoustics. Adding your typical bass trap to this situation would tend to bring the higher frequencies down more than those at 32Hz throwing the room out of balance in this regard. A situation I find to not be uncommon. In this case a 40Hz Modex could be used to bring the 32Hz frequency down with more effect than at 63Hz, but it would make the lower frequencies more out of kilter with the higher ones. I would tend to leave this room alone since it is only slightly out of specification and it would be difficult to hit 1000Hz without effecting the frequencies above it considerably more.

To summarize this can be a useful product for certain situations. It is expensive at $700 to $2000 per application and requires careful study to determine the actual benefits. The effect of any acoustic treatment product should be determined with measurements like this to avoid applying the wrong mix of acoustic products.