Adding Bass Traps

As I have mentioned in my previous post about setting up the LXdesktops—the desktop version of LXmini with custom tuning—I ordered some bass traps in order to try to improve reverberation times and maybe deal with the room modes. This is a short report on what I managed to achieve as well as an interesting point about over-optimization when tuning speakers from the listening position only.

In my room I already have absorbers on the walls behind and on the left side of the speakers (the right wall is further away and has a door), and one on the ceiling above the desk. These are mounted on walls 2 inch "FreeStand" absorbers by GIK Acoustics, except for one which is 4 inches because it is located very close to the left speaker and must absorb way more energy:

Besides these absorbers, I also already had one "Soffit" bass trap, residing on the room's closet. In addition to these "engineered" absorbers, there are also two "environmental" ones: a twin bed with a thick 6 inch mattress, and a rug on the floor. Yet, the room is not "dead" and still have enough reflective surfaces, as well as a lot of potential for creating room modes. The room is shaped quite irregularly and has a partially slanted ceiling, thus calculating room modes analytically is not very easy.

To all that acoustic treatments I decided to add 3 more bass traps: one big soffit bass trap, essentially the same as the one I already have, and two smaller traps called "Monster traps" by GIK. After the traps have arrived, I've made a "before" measurement, and an "after" measurement. Here are some comparisons.

Reverb Time

In order to analyze changes in the reverb time I made measurements both with Acourate and REW. Acourate displays them using smooth curves, and also provides reference "corridors" from two standards: DIN 18041 and EBU 3276 calculated from the room size. Below are the "before" and "after" RT60 graphs done by Acourate with the corridors from the EBU 3276 "studio" profile:

As we can see, the reverb time of the bass has lowered by 0.1 ms—not much. However, it's interesting that the reverb of the rest of the frequency spectrum has also lowered, and now it almost fits the upper range for the "studio" corridor. Thus, these wide range bass traps also have a good effect on taming high frequencies. By the way, for the big soffit bass trap GIK has an option of installing a "limiter" (FlexRange) which is intended to reduce this effect, but in my case I didn't need it.

In REW, the spectrograms also have become more uniform. Below are, again, before and after for the left and the right speaker:

So, there is indeed a noticeable effect on the reverb time, however it is not dramatic. As for the room modes, there was one interesting effect described in the following section. By the way, here is an old but useful review of active bass traps by B. Katz where we can see that active traps are more effective, however they are usually more pricey than passive ones.

The Over-Correction Effect

So, one interesting thing that I noticed while placing the new bass traps and re-measuring is the anomaly in the frequency response of the right speaker. Here is how this frequency region is looking before and after installing bass traps:

It is noticeable that the region has become less "regular". Also, the notch from a room mode has become less deep. I also recalled that this region has unusually high distortion which I was explaining to myself as a result of an interaction with a room mode:

So, definitely something is going on with room modes here. Maybe addition of bass traps has decreased the effect of one "negative" mode at approximately 97 Hz and this has resulted in a "swelling."

Another interesting point was that when listening to the log sweep from a side I could clearly hear that the driver is "overworking"—it definitely had a boost which clearly was not needed. I decided to make a near field measurement of the woofer right by the driver. As I have expected, the driver actually is boosting a range around 77–122 Hz. I realized that this likely comes from the fact that at the microphone position there was a cancellation in this region resulting from room modes, which has led to extra boosting when doing the woofer correction. Indeed, when I looked at the filter, it had a hump there. Using a near field measurement, I have created an inversion of the hump and applied it to the correction filter:

After that, the measurement at the listening position started looking less ideal, however two facts was pointing that this tuning is more correct. First, when listening from the side to the log sweep, the region around 97 Hz was now sounding more even. Second, the distortion which I was initially observing has gone:

That's an important lesson for me. Although frequency dependent windowing helps to emulate a measurement in an anechoic chamber in the aspect of cutting down the effect of reflections, it can't eliminate the effect of room modes. This may sound trivial, however apparently this consideration did not cross my mind when I was doing the tuning. Thus, at least for woofers it's actually important to compare measurements from the near field and from the listening position in order to avoid over-correction.

Servos Are The Answer?

What is interesting is that I had these issues with room modes with the speakers, but not with the subwoofer. The subwoofer was producing a very smooth response almost right from the start. This is an interesting feature of the Rythmic subwoofer. I also have a simpler subwoofer by KRK, and it is not as easy do deal with in an untreated room.

What is so special about Rythmic is that it uses the "Direct Servo" technology. The idea is that active electronics has feedback from the driver coil, and can "notice" when room modes are "helping" the driver (with a resonance), or vice versa, and correct the driver gain for that. This requires a specially built driver with an extra coil, but I think it's worth it.

One drawback of the active correction that I can think of is that signal-dependent corrections are essentially produce a non-linear behavior and thus add distortion (see my old post on the automatic gain control). However, for bass that is likely not a big issue. So one idea that has come to my mind for the next generation of my LXmini mods is to try to use a servo driver for the woofer. Would be interesting to see if this will help to deal with room modes.