Week 151 (Construction of the Walling)

In construction photos by manifold_admin

Two months ago, the completion of the Music Room Cloud gave me the opportunity to take a step back from basic progress reports and to talk more broadly about the aesthetics and architecture of the project, in particular the way that large features of the studio design fit together into an integrated whole.  That was a very popular post, and it circulated widely among designers and architects, geometers and algebraic topologists, as well as studio cats and studio rats.  Now it is time to focus on the other jewel of the main studio, the Control Room, and the way that smaller features fit together into an equally integrated whole.

Front and center is one of the hallmarks of Wes Lachot’s RFZ design, a hybridization of wall and ceiling that Wes calls a Walling:

Manifold are the layers of acoustic and aesthetic wonders that trace its wonderful seven-sided symmetry…

If you have been following this blog carefully, you know that the Walling is the solution to three simultaneous equations in three dimensions, shown here in photographic and mathematical form:

But before we can really appreciate this solution, we should delve into some of the elements that define the solution.  The tendency of loudspeakers are to act as point sources that radiate in an omnidirectional fashion (more so in the lower frequencies than the higher ones).  Indeed, it’s a useful feature: you can yell “where are you?” and even if you are not facing the person you are trying to address, the sound can reach them, they can respond, and then both parties can orient themselves for clearer communications.  In a square room, a stereo pair of speakers sends not only the left signal to the left ear and the right signal to the right ear, but others as well: the right channel is heard by the left ear, the left channel is heard by the right ear, and when both are in balance, a phantom center is established.  But there are other signals as well that can confound the stereo image: sound from the left speaker can reflect off the left wall and then back to the listening position.  If this delay is less than 20ms it is perceived as part of the principal sound and the effect is a sonic smearing.  If the delay is more than 20ms then the reflection sounds like an echo, and the brain treats it as an artefact of the room rather than of the sound itself.  A square room has to be very large to achieve the 20ms number: each speaker must be at least 7 feet/2m from each wall (which includes ceilings and floors!), and of course there must be some separation between the speakers to achieve a stereo effect.  If the listening triangle is an equilateral triangle 11 feet on a side, then the room must be at least 25′ wide, 14′ tall, and the listening position should be 7′ away from both the ceiling and the floor.  Not very practical.  Worse, while the early reflections problem can be solved with such gross geometry, it does nothing to address comb filtering, which occurs when a direct sound meets a reflected copy of that sound in free space, amplifying some frequencies and nulling others according to the rules of phase arithmetic.

One approach to solving this problem is to try to eliminate early reflections by making the walls acoustically absorbent, but this creates a different and equally deleterious problem: rooms that suck the life out of sound sound like they suck.  I.e., the extreme results of anechoic chambers make lousy listening environments.

Another approach is to aim the reflections away from the listening position without killing it outright.  This residual sound, diffused and reflected back to the listener safely past the 20ms threshold makes for a natural sounding room and an easily interpreted direct signal.  An optimized RFZ design balances the benefits of controlling reflections with tasteful amounts of absorption that result in a nice balance between direct and reflected sound at every frequency at the listening position.  Here is a view of the Control Room with four hypothetical sound waves beaming through the space.  Note how the early reflections all avoid the listening position, as do most secondary reflections as well:

And now an alternate view:

Note that the kind of angles that provide true deflection vs. the typical angles one sees in many designs that advertise “no parallel walls” are aimed at solving two very different problems.  True deflection requires some pretty radical incident angles, as the above diagrams show.  “No parallel walls” is aimed at reducing the tendency of a room to suffer from modal build-up.  However, as Philip Newell points out in his book Recording Studio Design, a deflection of 10° makes virtually no difference to bass frequencies (the most problematic class of modal build-ups), as the waves “see” mildly splayed walls as basically parallel.  If you think about it, a pair of 20′ long walls 28′ apart with a 10° splay between them will be +1.75′ on the wide side and -1.75′ on the skinny side.  An 80Hz wave has a wavelength of approximately 14′, and with such a small difference along such a wall, most of the modal problems remain over most of the surface of the walls.  A better solution to the modal build-up problem is to build rooms large enough that many modes, not just a few, are represented, and to make them with height, width, and length ratios that are different enough, one from another, that each dimension favors different sets of modes.

But the deflection of waves from the listening position is only part of the function (and the genius) of the Walling.  Another of its functions is to act as a bass trap, solving modal build-up problems that less radical room geometries mostly fail to address and which are equally difficult to manage with small-scale after-the-fact devices.  Room corners collect bass energy like dust-bunnies, and few acoustic treatments provide the bang for the buck of good corner treatments.  The Control Room Section above (Page A20) shows only a sketch of the insulation fill that lies behind the Walling.  A more accurate section would show that the insulation fill is basically solid all the way back to the solid black line that extends up from the inner Control Room Window framing.  Compared with the typical 2′-3′ wide corner trap devices, the Walling is 8x-5x larger, resulting in more than two (and up to four) octaves more bass trapping than modular units.  We estimate that the Walling can effectively trap down to 24Hz, and look forward to measuring its actual performance.

Another neat feature of the Walling is the 45° angle it makes with the hard wall(s) behind it.  The outer membrane of the walling is 2″ 705 standing 1.5″ off a hard wall of 3-layer 5/8″ sheetrock.  This membrane is great at absorbing sounds from 20kHz down to about 200Hz, with decent performance down to 150Hz.  The drywall behind it does a great job of absorbing sound from 500Hz down to 100Hz (mostly because of the Green Glue), and then we hit our fluffy insulation.  The fluffy insulation starts about 3.5′ off the 21″ thick masonry wall and, depending on the height, extends between zero inches and nearly 9′.  The secondary triple-layer wall that’s 3.5′ off the masonry wall is in the perfect location to quiet down the quarter-wave vibrations of 80Hz.  When the fluffy insulation reaches 3.5′ in depth, the combined distance off the masonry wall traps waves down to 40Hz.  When the fluffy insulation reaches its full depth, we are down to a calculated quarter-wave trap of 23.87Hz.  What’s so beautiful about this is that the trap is broad-band: non-directional low-frequency waves are trapped essentially equally from 24 to 80Hz, with each inch of the Walling trapping its respective frequency bands…almost.  The Walling is 8′ wide at the bottom (80Hz trap), widens to nearly 20′ where it’s about 5′ of fluff with a 4′ air gap off the masonry wall (32Hz trap) and then narrows back down to 8′ at perhaps around 27 Hz before diminishing down to a point at our theoretical 23.87Hz trap at the peak.

But that’s not all.  The smart folks at RPG know that one of the big challenges in realizing the performance of bass traps is the absorption of wave energy where air movement is minimal, i.e., at the nodal boundary of a wall surface.  Their Modex Plate products solve that problem.  They are the large white rectangles in the first photo, attached to the ceiling providing a foreground to the Walling.  Here’s how big they are down on the ground:

And being installed:

After the Walling is fully framed, the outer fiberglass member can be filled in:

Then the fabric is fitted, along with a frame for the cloud (shown in the architectural drawings as a gray rectangle above the listening position on page A20):

As the acoustic ray tracings on page A20 show, the Cloud provides an additional layer of absorption and diffusion between the waves that head up to the ceiling and then back down into the room, and control over the floor/ceiling reflection pathways.

I wish I could write more about the progress this past week, but I’m already behind schedule, so I’ll stop now.  I hope you enjoyed seeing this part of the studio come together!