Home Acoustics Alliance Level II Workshop (cont.)
After the response measurements were complete, we did Energy Time Measurements, which is nothing more than a fancy way to look at reverberation. Of course, HAA is trying to turn us into audio gurus so it is a little more involved than just clapping your hands and listening to the echo. These measurements involved direct reflections and then all reflections. We learned and heard how direct reflections can impair focus and clarity, which we eventually treated with absorption while being careful not to use too much absorption because that can diminish envelopment. Without getting too technical, the goal was to help focus and clarity and keep the reverberation time (RT-60) between 250ms and 400ms, which is the time it takes the signal to decay 60dB.
The final analysis exercise was resonance sweeps. We manually slowly swept through the low frequencies with a fairly high volume to shake out the room. This process will find anything loose and it did. We found that the grill on the ceiling lights was vibrating like crazy. I never thought of doing this test until then and some of the calibration disks you can buy have the sweep but you want to do it manually so you can stop when you hear something vibrating in order to indentify the culprit. Anyone can do this simple test with a laptop. Many free programs exist that have frequency generators. All you need to buy is a 3.5mm speaker out to RCA cable.
At this point, we have analyzed the room and should now know what to do to calibrate it; but did we? Of course we did. We all had Level I training and it was now time to apply those skills. The first thing we did was a basic calibration, which is matching the speaker levels and setting the delays (distances); The standardized test tone is band limited (500 to 2K) pink noise to make these level adjustments. Next, we did the subwoofer calibration for a single sub. It was important to learn where a single subwoofer should be placed in the room for its best integration with the room modes. We moved it around and performed measurements each time at the listening area. Once we found the best position of the sub woofer, it was easy to see that no one would want a subwoofer out into the room. So then, we tried HAA’s virtual sub woofer placement method, which simulates a single sub in the correct position but has the two against the walls. Another method was the Welti method of sub placement, which was our chosen method. It involved placing the 2 subs at midpoint on opposite walls.
There is a trick to the Welti method. We couldn’t just move the subs to the midpoint position and be finished. With this specific placement, the odd order resonance modes are excited leaving peaks at those frequencies. This placement led to the next exercise of equalization. We applied very specific equalization using a Digital Signal Processor (DSP) to remove the peaks. The DSP we used was a parametric equalizer from QSC (QSC DSP-4 pictured). Unlike a graphic equalizer, it allows the user to pick the exact frequency of a peak the width of it and the amount of gain to remove. There are quite a few advantages to using the QSC DSP. For one, it is very small (4 by 4 by 3 inches) and can be placed almost anywhere without taking up shelf space. The other huge advantage is that it is programmed through a computer and the settings are downloaded into the unit preventing a customer from changing the settings. It also has many other features like gain control, limiter, high and low pass filtering to name a few. The result of our DSP equalizing was amazing; very smooth and well-integrated bass. The focus and clarity was now better because the bass was smoothed and not overwhelming. Even the rattles in the ceiling were gone but we still had a long way to go. Hi-lili Hi-lo still didn’t sound great; I was beginning to think it was a bad recording but it really was just a tough recording to reproduce.
Next, it was time to calibrate the left, center and right (LCR) speakers. Remember that calibrate doesn’t necessarily mean to do something electronically. This exercise was about moving the speakers to get a better frequency response. Like all of the other exercises, it was move, measure and listen. We ended up skewing the room slightly to one side (maybe a foot or so) to create asymmetry-allowing reflections from the left wall not to be equal to reflections from the right wall. This asymmetry helps smooth the overall response by spreading the reflections out. We really didn’t do anything with the center channel except keep it centered between the left and right speaker. You can see how smooth our frequency response was at this point in the picture shown here. We also just concentrated on two channel listening which is very important because if you can get stereo music to sound really good the rest follows much more easily. We even simulated raised rear seating by borrowing some barstools for the rear seats.
Once we were happy with our speaker placement for the left and right we concentrated on the center speaker. There are a couple of rules of thought for center speaker placement. One is to have the LCR’s in an arc so that the physical distance is the same at least to the sweet spot. The other is to have the LCR’s in the same plane. If the theater was being designed from the ground up, the HAA philosophy is to get this ultimate speaker placement and then build an acoustically transparent wall in front of them including an acoustically transparent screen. Otherwise, the speakers may be well out into the room. The idea of having the center speaker in the same plane as the left and right is this: the left and right speaker alone will create a phantom center speaker between them. If the center speaker is behind this plane (the arc method) then it will be like having two center speakers with one playing behind the other and then creating distorted acoustic results. However, if it is in the same plane the acoustical wave of both the physical center and the phantom center will be coincident. I am not sure I completely agree with this philosophy. While it holds true when listening to Dolby Prologic II music, which I’ve tried at home, most of the content in movies has the center channel information (dialog or moving effects) very distinct from the left and right channels (music). Even 5.1 channel SACD’s will put the singer in the center and the music in the left-right channels. Regardless, putting the center speaker in the same plane does not hurt anything.
Editorial Note on Center Channel Placement
Arranging the center in an arc places it in a different acoustical position than the phantom center. It changes the response obtained during the two-channel exercises. Placing it in the same plane does not (generally) change the response and this simplifies the calibration process. The only advantage of the arc method is time alignment which is easily fixed with time delay.
By this time, we were getting things fairly close to having a good sounding room but there was still more to do. We had to add acoustical treatment to the walls. We started by finding the mirror points to the sweet spot. The Customer sat in the sweet spot while the Grunt walked along the wall with a mirror until the speakers could be seen which was done for the left, center and right speaker for both walls. Once those mirror points were marked, which were the direct reflections, we then had to choose what kind of treatment to put there. Absorption will help bring down the overall reverberation of the room and diffusion will help the envelopment. Fortunately there were hybrid panels that did both[GAL1] . Even though the group quickly chose the hybrid, I personally wanted to hear all three while sitting in the sweet spot. It was an opportunity to hear how the different acoustical treatments sounded. We also ended putting absorption behind each front speaker to tame some of the speaker boundary reflections. The picture here looks pretty cheesy but this was obviously not a permanent setup.
So all the treatments were up and now there was only one thing left; integrating the surround channels. The idea if you didn’t already know was to create a diffused surround effect as opposed to directly pointing the speakers at the seating area. We ended up pointing the speakers toward the back wall and letting the cabinets and items help diffuse the sound though it would have been better to place diffuser panels on the back wall to more properly break up the sound. We then did another basic calibration for level matching all of the speakers, measured and finally listened to our tracks in both stereo and Dolby Prologic II. Now hearing The Ricky Lee Jones track “Hi-lili Hi-lo” was amazing. We went from a bad sounding system without changing any equipment to an awesome sounding system just because we learned what to do with the room. The Genelec speakers were not just “fine,” they were great. All of the tracks were sounding stunningly good compared to two days before. It was very hard to believe how important these seemingly little changes could do so much. Now that doesn’t mean that a GPX home theater in a box can sound awesome. Equipment with high distortion cannot be corrected with any type of room calibration.
Later that final day we were allowed to listen to our own music and of course, I brought my audition music that I use for all of my articles. Now I could compare what we had done here to what I have at home. Unfortunately or fortunately depending on how you look at it, I left a little depressed because this system that we set up in a hotel conference room blew away my system at home and it’s not about the equipment. The next day I listened to my own system and starting taking notes on all of the changes I needed to make. I already had some ideas before the workshop of some changes I needed but now that I was HAA Level II trained, I knew what needed to be done in more detail.
The requirement for HAA Level II certification goes hand in hand with the changes needed for Reference System 4 because it is not a certification test. The requirement for certification was to create a calibration report of a theater on our own and this was no easy task. Although I knew all of the concepts now it was very difficult to repeat the same exercises on my own. Mainly because my measuring equipment set up is different from what the workshop had. I had to apply the same concepts from the workshop and figure out how to do the measurements with what I had available. One example is that I didn't have 4 microphones and a multiplexer. Now if I were a Home Theater Specialist and did this for a living I would definitely be getting the extra equipment. I did purchase a few things to help; one of which was new measurement software called ARTA. I recommend to anyone wanting to do their own measurements to take a look at this software. It is free to try and is fully functional except to save plots and projects. However, it still is not as robust as the Sencore equipment which again if I were an HT Specialist I would acquire it. Well after drudging through the ADR without the AV Pro software I quickly realized the value in using the software to do the ADR and ACR. I created a report with several recommendations to Reference System 4. As time permits and being a DIYer I will be making several changes to the system.
So what does all this mean for you? If you are a Home Theater Specialist I urge you to take HAA training, your customers will be thrilled. If you are a theater enthusiast and want your system to sound better, don’t throw money at the equipment, put it into an HAA certified professional. The same goes if you are a two-channel stereo audiophile. Audiophiles always seem ready to purchase better and better equipment to perfect their sound system, which I am not trying to discourage, but they tend to disregard the proper room setup and calibration. Whatever place you take in this ever-changing audio world the one thing that stays the same is the physics of acoustics and HAA is the avenue for optimizing small rooms.
Visit the Home Acoustic Alliance (HAA) for Certification Training Information or Hiring an HAA Certified Installer