Friday, March 17, 2017

THE GRIMANI FILES: Bringing Life to a Comatose Home Theater

This client was referred to us by the integrator, who completed this installation about 10 years ago.
The room was a dedicated three-row home theater in an upscale neighborhood of Detroit. All the gear was state-of-the-art for its time - from the studio-grade speakers, to the professional HD video projection system.
But the surround decoder finally gave up the ghost after several years of faithful service.
The client heard that Dolby Atmos® was the latest thing, so he sourced a new processor. He expected it to be an automatic upgrade, but wasn’t aware that he also needed to add speakers for the Top and Wide locations. At any rate, it was time to install, configure, and tune the system for the new processor. I took the job and carved out two days - right before Christmas - for a quick jaunt from my temperate San Francisco Bay Area home to frigid Detroit. (Who doesn't like a little cold for the holidays?) While planning the trip, I found out that the system didn’t include any equalization.  It's impossible to properly tune the frequency response, levels, time alignment, and phase relationships of the various speakers without a digital EQ, so I asked the client to add one. I specified a very good, cost-effective unit from Ashly, which was shipped to the site along with a few XLR M/F snakes to patch it into the system.
The end user was definitely a movie buff. He had a large collection of discs, a PRIMA Cinema server, and a good collection of source devices. He had set up a classy and comfortable dedicated environment to enjoy his movie viewing, and he had a lot of passion for proper presentation of the filmmaker’s intent.
But, as David Attenborough often says in the PBS series Planet Earth, the room and system had a few implementation issues that made it harder to achieve the optimal experience. I list these here not, so much, to criticize and point fingers, but to provide an opportunity to learn and improve the deployment of future systems.
1.   Acoustically Dead. The acoustical designer hired for the project specified that all the walls should be lined with two inches of high-density insulation, and he even showed some predictive calculations for the effect. Well, it has been known for a while now that a room should have a proper balance of reflection, scattering, and absorption. For typical residential home cinemas, the appropriate balance is about 20% absorptive surfaces, 25% scattering surfaces, and the rest reflective. The layout for these various types of surfaces needs to be strategically engineered so the sound reflections support the speakers' sound radiation. For a room this size, the resulting reflection decay time should land around 0.35 seconds across the full frequency range from 60 Hz to 10 kHz. In reality, the impulse response measurements for this room demonstrated it to have 0.15 second decay time down to 1 kHz, rising up to 0.3 seconds below 500 Hz.  That's pretty much what you would expect from a room packed with too much fuzz. The room sounded weird just walking into it; the sound was inconsistent from seat to seat, and the speaker interactions produced clearly audible comb filters. The solution is to peel back the decorative wall upholstery fabric that conceals the insulation, remove at least 60% of it, install some scattering diffuser panels at strategic locations, and retune the frequency response and levels - as they will clearly change.
2.   Seat Locations. The room was a bit too small for three comfortable rows. The back seats were up against the back wall. Bass was thick back there, and it was hard to get a proper immersive envelopment that far from the front speakers and so close to the back speakers. The solution is tough, here, since the room is already built. But every half foot of distance to the back wall helps. Scooting up all the rows a bit and reducing the aisles between them would probably help. Realistically, this would probably not happen given the high design values built into the existing room. It would have been better to properly lay out the room in the first place. Additionally, the left seats of each row were right up against the left wall, rendering them inappropriate for proper soundfield perception (although just OK for casual viewing of media).
3.   Subwoofer Placement.  The two subwoofers were in one corner. Bass standing waves, tightness, and coverage uniformity are all a major challenge in rooms under 50 feet long. Placing the subwoofers in one corner of the room is generally the worst option for good grades in the above performance areas, although you do get more sound level from this arrangement. Like they say, it’s junky but loud. Room EQ can somewhat mitigate the issues of amplitude response and resonances, but it won't ever solve the seat-to-seat uniformity issues.  The solution is to place the two subwoofers in different locations, preferably across the mid-points of the room. Better yet, conceal four smaller subwoofers in the four corners of the room and tune them each with a channel of digital EQ.  This reduces the response errors with judicious use of relative delays, levels, and frequency response tailoring. With a bit of time, you can get to tight, punchy, loud, and consistent bass that puts a smile on the face of anyone sitting in the room.
4.   Surround Speaker Placement.  The surround speakers were too low. Because of the location and size of the side wall lighting sconces, some of the side surround speakers were placed about a foot too low. This yielded insufficient immersion, and it called attention to the speaker locations since they drilled right at your ears. Turning them down a bit, reducing the higher frequencies, and delaying them a bit are all work-arounds, but the right solution is to reduce the sconce dimensions and place the speakers properly.
5.   LCR Speaker Coverage.  The LCR speakers were too beamy. The studio monitor-grade speakers used for Left/Center/Right were, in fact, a bit too inconsistent in dispersion across frequencies. As a result, the sound changed quite a bit from seat to seat, and it was hard to tune the room for reference-grade sound at all the seats.  The solution is to replace the speakers with more current versions that have broader horizontal directivity and smoother sound power (the net spectral response throughout the room). The sweet spot would get wider, and the overall sound quality would be smoother.
6.   Missing Equalization. As mentioned above, I requested that an equalizer system be added before I conducted the room calibration. I got half my wish: An EQ was sourced, but I had to install it into the rack and wire it up. The EQ had detachable three-pin screw connectors (Phoenix/Euroblock), and the decoder and amps all had balanced XLR connectors. So I had to create interface cables to go from XLR to Phoenix, cutting the newly acquired XLR cable snakes in the middle. A few hours later (which I spent in a cramped space at the back of the rack, equipped with my trusty cutters, strippers, utility knife, zip ties, headlight, and a bunch of patience and vigilance not to hit my balding cranium against the sharp corners of the projector base plate), I had taken care of this phase of work. Lacing up a rack cleanly and effectively is a matter of both art and science. You have to pre-think the best routing for all the runs, but you also have to consider noise interference, hum fields, ground loops, and much more. It takes a well-trained technician to do a good job. Even then it can require a few iterations to get right.
7.   Polarity Error. One of the surround speakers was connected out of polarity. The right side soundfield had both spatial and spectral errors as a result. It’s not unusual for a wiring error to creep into a system; this room, for example, had 15 speakers, and it’s understandable that a mistake can happen.  The solution is to painstakingly verify each speaker output with an acoustical polarity tester. Use the pulse test signals in the 5.1 Audio Toolkit test disc with a polarity checker like the Gold Line APT2. Remember to check the polarity of all the drivers in each speaker (one may have been wired wrong internally at the factory).
8.   Reference Level. Film soundtracks are designed to be heard at predetermined sound pressure levels. In cinemas, there is a reference detent setting for the volume control - often called “Fader 7” (position 7 out of a max of 10).  The same should apply to home theaters; the clients should have a way to set volume to reference if they want. In THX-certified decoders, this is known as 0dB reference. You can turn the level down or up from this baseline, and it is very cool to have the guidance of the target level. If you look at the on-screen or front panel volume display of one of these decoders, you will see values in dB +/- from reference. However, once the volume control is left up to a touchscreen, the user often sees a value that is not correlated to the reference scale. Feedback from an automation scheme (Crestron, Savant, Control4, etc.) should show the user the exact value on the reference scale - not some random number with a bar graph or % value. This is easy to say, but not always easy to do. Products don’t always report the volume control numbers in terms that correlate to the dB of attenuation or gain around reference level. Hmm. Food for thought for future iterations of products, firmware, and automation coding.
Despite the above issues, 15 hours of wiring, configuring, EQing, listening, and tuning finally yielded a sonic perspective that came close to ideal reference performance. I listened to my usual collection of music tracks and movie clips, and I tweaked a number of parameters until the sonic bubble came together. I also set up four presets in the digital EQ processor so that the user can listen to tracks in these modes:
a)    Reference – The Way the Director and Sound Designer Heard It in the Film Mix Stage
b)    Dialog Enhance – Level and Mid-frequency Boost in the Center Channel to Allow Dialog to Punch Through the Mix
c)    Enhance 1 - Added Bass and Surrounds to Sweeten the Track
d)    Enhance 2 - A Lot More Bass and Surrounds for When the User Really Wants to Have Fun
The four modes are recalled from a control pad on the Crestron touchscreen.