Phils Audio

By Philip E. Abbate  . 6/28/99

I was a little excessive with this one. I made the box that would not resonate and committed the sin of stuffing the hell out of it. My penance was to fix the problem, measure the results and report my findings to y'all. The four sides of this but jointed closed box cabinet were made from a three layer Homosote sandwich and the front and back were made from a three layer linoleum sandwich. To make sure there was absolutely no chance that the NHT 1259 woofer would vibrate the walls of it's new home I glued carpet pad to all of the internal walls save the inside of the baffle.

This graph shows the impedance phase and magnitude for the fully stuffed box. There is a lot of stuffing in that box and some of it is just eating up internal volume while the rest of it is choking the woofer. What is not so evident is that the target Qtc of .7 turned out to be .776.

It did not sound bad overstuffed. Otherwise, I would not have left it that way for 18 Months. Right? I was not until one of my audio buddies looked at the picture of the overstuffed box on my web page, that it came under the scrutiny of the good O'l boys. There is rarely total agreement amongst us, but after listening to the box with half of the stuffing removed, there was a total consensus. The all of the stuffing, or at lease most of it, has to go. I was able to pull out all but some of the carpet pad that did not want to come loose from the walls. I could not reach the back, which is why all of the treatment on the back stayed. Anyhow, some stuffing is good.

The table above shows the how the Fs and Qts changed through the process. Measurements in the half full row corresponds to when I removed all three layers of internal treatment from the inward facing panel and all but the carpet pad from the bottom. I noticed a definite increase in mid bass clarity (note: this sub is crossed over at 50Hz with an 18dB/octave low pass) and low bass articulation. The integration with the Focal 7 inchers blended much better, but it was the rhythm and pace of the speaker that improved the most. By the time, I had removed a pile of stuffing as big as the one of the bass bins the subs were mostly void of stuffing. Measurements in the little stuffing row correspond to this condition. The sonic improvement with little stuffing is essentially more of what was detected with only half of the stuffing removed.

Having an optimally damped sub woofer with a measured Qtc of .706 is not audio panacea. It still needs to be adjusted properly or the sound you get will be mediocre. I adjusted this system by placing the measurement microphone right next to where my ear would be if I was sitting in the sweet spot. In my room that position is midway across the back wall about a foot from the wall. I started by setting up Liberty AUDio Suite in 1/6th octave Real Time Analyzer mode with 10 averages per display. It is important to note that the measurements out in the room with the RTA take into account the room which does not affect the nearfield measurements of the mid bass and sub woofer. Nearfield and anechoic measurements taken without room reflections will be radically different from measurements taken in the room where room reflections and standing waves dominate.

I first noticed that with the sub woofer turned off, there was a peak from the mid bass drivers outside of the subs pass band. This peak does not show up in the nearfield measurement and is room related. As I added sub bass, I noticed that this peak was accentuated and no low bass was added to the mix. The system sounded somewhat boomy compared to how it sounds when adjusted to give a relatively flat measurement. I'll bet this configuration would satisfy pop music fans at least until they hear the system adjusted properly. I tried playing with the LP corner frequency on the electronic crossover, but that reduced the peak and did not appreciably bolster the lowest octave.

The phase control on my Paradigm X30 crossover did the trick. I adjusted the new peak that showed up when I added the sub to the mid bass out completely and boosted the area below the peak. Now the, in room, response along the rear and side walls is reasonably flat from 25.2 Hz to 100Hz. Moving a foot forward changes all of it but that is the problem with the real world. Sitting with my ear right where the analyzer microphone was located I hear and feel a tremendous sense of space. The bass extension makes the soundstage very wide and deep, while it satisfies my yearning for low low bass articulation. In my room, this system is a winner.

 For more complete coverage of this subject see the apprentice part II.