Fine Tunes #26
"Yeah, duuuude," George crooned, as he is wont to do. "I'm very excited, I'm actually building auditoriums and sound systems these days. And you know what I've found to be the biggest and most annoying problem with some traditional halls? Slap-echo from parallel non-absorbing walls."
Of course, he's right. Parallel flat reflective walls make for a nasty acoustic nightmare. I asked George what can be done about it. "The Band-Aids you'll typically find to counter this are absorbent 'clouds' floating above and in front of the stage, and absorbers or reflector/refractors on the sides."
I turned an eye to my own listening area as he spoke. I chuckled ruefully at the forest of room tuning treatments needed to tame our space. These include a brace of Studio Traps and a gaggle of Argent RoomLenses, augmented by a few left-over RoomTune pillows and Combak/Harmonic tuning dots. And, of course, K-10 lets me get away with bloody murder, as the JMlab Utopias are pulled way into the room to avoid any possibility of First Reflection Disease. All this treatment is great for the system of a reviewer who's lucky enough to have a music-loving Mate Who Cares, but woe betide the typical Stereophile reader whose family demands won't allow such latitude.
George snapped me out of my self-congratulatory reverie. "Slap-echo is why you can't get stereo in a tiled bathroom, dude!" (I refrained from explaining I don't often try, doo-wahhhhh...) "The knee-jerk reaction to slap-echo is to pad down the entire room or hall. But the real trick is putting a small taper in the angle of the side wall. That reduces slap-echo a tremendous amount!"
Ah-hah, now that's a good tip for anyone who's building or rebuilding their listening room (footnote 1). And it's relatively easy to implement and not too expensive to do with common sheetrock. Or you might consider spending a little more and build up a pair of constrained-layer walls using two sheetrock layers per side with asphaltic-based paper between, as suggested in a previous "Fine Tunes."
You can make it easier still by installing roller-feet under the 'rock so you can adjust the angle for best effect. "I'll be right there, hon-eee, I'm just adjusting the wha-alls...!" Thankfully, once optimized, there won't be much need to move them around. And it doesn't matter what's behind the sheetrock either. Stuff fiberglass insulation into the "pocket" between the back of the sheetrock and whatever's behind. And leave a gap of a couple of inches at top'n'bottom so the pressure is equalized either side and the wall won't act like a giant resonant drum head. If you combine tapered side walls with an absorbent ceiling and side-wall treatment, and you soak up the sound behind the listening position, you can dial in a very optimized listening environment.
Don't believe slap-echo can ruin your audio life? "Try recording a voice speaking in a room with heavy slap-echo," suggests George. "Then edit out the voice and leave only the decay. It sounds horrible, choppy and unintelligible. But if you treat for slap-echo and do the same, you can hear what's been said with good intelligibility. It's a shocker, dude."
Another shocker was hearing about a preschool project that George worked on. Think about a school gymnasium with typical gym acoustics. Can't ya just hear and almost feel the screech of sneakers interspersed with the excited, bloodcurdling screams of kids? (I like kids. Medium-rare.)
"Yeah, that was an interesting project," George continued. "It was a preschool classroom with very young kids, like three or four years old. The school was having tremendous problems controlling their behavior. They'd just go off like a twig snapping and start screeching at the top of their lungs—a real adrenaline rush. And the slap-echo in the room would sustain the high frequencies, actually inducing a fight-or-flight response in the kids. The school was having the same problem with their Seniors—a bunch'a very old folks, all yelling at each other basically for the same reason.
"I fixed it all up by simply changing the sustain. I rolled it off with some foam padding attached to the upper third of the room. It became much quieter and more womb-like afterwards. It calmed the kids down a ton, it even had the same effect on the old lions. Yeah, everyone started whispering, like in a library where you get a similar effect from the books absorbing the sustain.
"You know, it's like a speaker system with a rising top end. If you've got a bright tweeter it becomes irritable to listen too. If you correct that, the tweeter better integrates harmonically and you feel everything's good in the world. It's the same concept for treating room sustain."
So take this opportunity to walk around your listening space clapping your hands once again. (Best to wait until the family's not around!) As you meander about, listen for the sustain and any slap-echo that's developing. If you just can't build yourself a tapered-angle side wall, think about applying the Cardas Principles the best way you can. Remember, sound is like water "splashing" to and fro in your listening space. Whatever you can do to break up standing waves (nulls and peaks), to damp side wall reflections, to lower slap-echo and high-frequency sustain, will get you that much closer to sonic nirvana...dude.
Footnote 1: One of things I am going to miss about living in New Mexico—see this issue's "As We See It"—is that things like this happen quite normally. The listening room I am giving up is 20' wide at one end but 19' 6" wide at the other, which is about as close to "parallel" as contractors in the Land of Enchantment think necessary.—JA