RPG Modex Plates-"Free" Bass Traps

[Scott W]

Scott W, where were the six locations you measured with your SPL meter and what exactly were you measuring for? Thanks.

The 6 locations were given to me by my acoustician that I work with. They were based on possible bass pressure zones. I measured 40 and 80 at the listening position with an SPL meter, then at each location. I was measuring for a major increase in volume of these two frequencies in relation to the listening position reading. This then gave me the possible locations for the Modex Plates.

[Walnut Horns]

Thanks. Are you able to provide the specific 6 locations that your acoustician recommended?

[Scott W]

Mainly where you see the Modex Plates They were at the front of the room corners, up high and front center ceiling/wall intersection. And beside each speaker.

[Jeff Hedback]

Hi Scott...thought I'd jump in on our analysis process. The SPL in-room studies were used to compare/contrast with acoustical pressure mapping simulations (BEM). Those studies defined the optimal type and location for the low freq control products. Here is a graph of the results. The room is still a situation of making the right compromises/choices (between spkr/listener locations for LF response vs locations for soundstage and imaging.

[Scott W]

Awesome Jeff! Thank you for that info. And as always you are the MAN!

[Rutgar]

Just to add to the thread, here are a couple of pictures of the Modex Plates I installed in my room. Of course you can also see a boatload of RealTraps as well.

[Bjorn]

The RPG website says they are a QRD and absorber. If so, there's still phase grating in the mid and higher frequencies since they are partially QRDs. Aren't QRDs designed for far field?

As you know, I had QRDs on back wall about 6 feet from seated position. I felt that BAD panels were better there instead and the Modex plate better yet. I moved them to side walls adjacent to listening position. My room is 14'6" wide. The QRDs greatly reduce soundstage precision and vocal intelligibility in that position. I never tried them any closer to the speaker but I bet they would create more problems. Maybe in a huge room they could be okay but they aren't for me.

Just came over this thread. I sell RPG products in Europe.

Diffusers need space between the listener and the units to work well. The general rule is that the distance should be minimum 3 wavelengths of the diffused energy. That means if the QRD diffuses down to 500 Hz, the wavelength is 2.2', and thus the minimum distance should be 6.57'. Some argue that 2xwavelength is also fine but 3x or more is definetly better. Sitting closer to a diffuser will result in one hearing comb filtering instead of proper diffused energy.

Also, when three or more of the traditional RPG QRD diffuser are placed next to each other it will cause lobing due to periodicity. The new RPG Modffusor and Modffractal are built after an aperiodic model which minimizes lobing greatly and are better products.


BAD panels/BAD Arc don't diffuse as low, approximately down to 800 Hz, so these work better closer. However, if one desires the absolute best clarity, intelligibility, and localization, it's better to attenuate reflections that arrive soon after the direct signal with only absorbents. If the aborbents are thick and big enough to work down the schroeder frequency and combines them with bass trapping, one avoids much of the typical deadening of the room. Absorbents need to be a minimum of 6" thick (either flushed or with an airgap) to get this low. And surface must be bigger than the typical 2x4'.

When you have Modex Plate on the rear wall, you will end up with specular reflections at you seating position. I would advice you to place either some absorbents or BAD Arcs at the reflection points in front of the Modex Plates.

[Nyal Mellor]

Just came over this thread. I sell RPG products in Europe.

Diffusers need space between the listener and the units to work well. The general rule is that the distance should be minimum 3 wavelengths of the diffused energy. That means if the QRD diffuses down to 500 Hz, the wavelength is 2.2', and thus the minimum distance should be 6.57'. Some argue that 2xwavelength is also fine but 3x or more is definetly better. Sitting closer to a diffuser will result in one hearing comb filtering instead of proper diffused energy.

Also, when three or more of the traditional RPG QRD diffuser are placed next to each other it will cause lobing due to periodicity. The new RPG Modffusor and Modffractal are built after an aperiodic model which minimizes lobing greatly and are better products.


BAD panels/BAD Arc don't diffuse as low, approximately down to 800 Hz, so these work better closer. However, if one desires the absolute best clarity, intelligibility, and localization, it's better to attenuate reflections that arrive soon after the direct signal with only absorbents. If the aborbents are thick and big enough to work down the schroeder frequency and combines them with bass trapping, one avoids much of the typical deadening of the room. Absorbents need to be a minimum of 6" thick (either flushed or with an airgap) to get this low. And surface must be bigger than the typical 2x4'.

When you have Modex Plate on the rear wall, you will end up with specular reflections at you seating position. I would advice you to place either some absorbents or BAD Arcs at the reflection points in front of the Modex Plates.

Or you could use Modex Broadband on rear wall!

The general rule about distance from diffuser to listener I have found only applies to aggressive QRD phase-based diffusion. Less aggressive quasi-diffusers like Auralex T'Fusor or the Harmonix K you can sit closer to without aural weirdness. Amplitude based diffusion like BAD or slat diffusers you can sit even closer to.

BTW my wife is Norwegian!

[Bjorn]

Modex Broadband would be better, but the absorption stops around 5KHz. So there would still be some high frequency specular reflections.
Absorbents in front of Modex Plate (type 1) don't need to be very thick though, considering the Modex Plate absorbs well up to around 300 Hz. I've done this in my home theater room where a Modex Plate where placed on the sidewall at a first reflection point. The absorbents in front actually increased the low frequency absorption a little.