Gettin’ Down with Subs - ResidentialSystems.com

Gettin’ Down with Subs

What’s harder to write: a 10,000-word explication of the Trilateral Commission’s role in global conspiracies for The New Yorker, or fitting a comprehensive definition of that obtuse but influential group into a brief wiki?
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What’s harder to write: a 10,000-word explication of the Trilateral Commission’s role in global conspiracies for The New Yorker, or fitting a comprehensive definition of that obtuse but influential group into a brief wiki? Most writers would vote for the latter, because it’s usually more difficult to write short and write well. Most AV integrators would likely say the same thing about working in tight spaces, and the biggest issue in a confined space is low-frequency response.

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Madrona Digital’s Amir Majidimehr has been using Fluid Dynamic Low-Frequency Optimization, a software update to JBL’s ARCOS system that measures and models a space from a low-frequency perspective, suggesting placement options for subwoofers.

Even as single-family home construction continues to decline in the U.S., the number of new multiple dwelling units is on the rise. At the upper ends of the ownership spectrum, where home theaters and whole-house audio systems are considered de rigueur, that creates a management issue aimed at keeping bass frequencies properly corralled but at the same time allowing them to evenly and consistently envelope the space they’re in.

“In a big theater with lots of seats, the variations of low-frequency response from seat to seat is actually quite minimal, but as the room gets smaller, the variations from seat to seat increase,” is how Amir Majidimehr, president and founder of Madrona Digital, a high-end residential AV systems integrator in Bellevue, WA, explains the physics of bass and small-room theater environments. By definition, bass frequencies need substantial space to fully unravel–for instance, a 60-Hz wave requires about 19 feet (the formula is arrived at by dividing speed of sound [approximately 1,130 fps at sea level] by the frequency), and when multiple frequencies and the smaller dimensions of a room are accounted for, issues such as standing waves become problems. Putting a sub into a room with shorter dimensions than that can cause waves to hit walls and rebound, combining with subsequent waves that sum or cancel and create low-frequency nodes and nulls in the room. The result, says Majidimehr, insignificant variations in a bass experience in literally adjacent seats.

“The variations in a small room can be immense,” Majidimehr said. “It’s simple physics.”

The number of products and systems for a solution, however, has multiplied in recent years. Majidimehr has been using FLO, or Fluid Dynamic Low-Frequency Optimization, a software update to JBL’s ARCOS system that measures and models a space from a low-frequency perspective, suggesting placement options for subwoofers. In the case of Madrona Digital’s new demo theater, FLO ran through more than 40,000 possible permutations of types, placements and specifications like crossover points before specifying what it considered the optimal configurations.

“What you have to remember is that the mids and the high-frequency responses are determined almost entirely by the choice of speakers; low frequency [response] is determined almost entirely by the space itself,” Majidimehr stated. “This surgical computational analysis approach is certainly better than the back-of-the-envelope math that we had been using.”

Not that that approach can’t have positive results, too. A technique that has emerged as consistently successful is the use of multiple subwoofers, actually increasing the number of subs as the spaces get smaller, in order to increase consistency of coverage.

“Even two relatively inexpensive subs is better [in a small space] than one large sub,” Majidimehr advised. “Then at least you’re starting with a room that’s 70 to 80-percent healthy. Then, if it’s in the budget, you can go for computational analysis.”

Isolation Issues

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Dennis Erskine, an Atlanta-area theater designer and consultant, believes that some combination of isolation of shared boundary surfaces (i.e., those walls, floors, and ceilings that are adjacent to other residences) and astute subwoofer placement and phase cancellation techniques can bring good results even at reasonably high playback levels.

Multiple subs can be used in conjunction with techniques to limit the transmission of low frequencies outside of the space. John Storyk, known more for his professional recording studio designs and club sound systems, said the combination of more low-frequency information on DVDs and even MP3 files, and a general expectation of higher listening levels overall accompanying people into multi-unit residential buildings, has been pulling him further into the home systems space.

“This is happening at a time when not only are more people living in high rises and other buildings like that, but the codes for these newer buildings are much more stringent when it comes to sound transmission,” he said. “As a result, in some very high-end residential projects, we’re using professional studio-level isolation techniques.”

Those techniques include room-within-a-room construction, floating walls, floors, and ceilings, which is an expensive proposition even if you do own the walls inside a condo. And Dennis Erskine, president of the Erskine Group, an Atlanta-area theater designer and consultant, noted that the conventional methods of evaluating sound transmission, such as STC (sound transmission class) values, are less useful in residential environments when it comes to low frequencies.

“STC was originally designed as a metric for sound abatement in office environments,” he explained, covering the frequency spectrum from 4 kHz down only to 125 Hz, nowhere near the mid to low double-digit values of home theater subs. “It’s great if all you’re trying to keep in is the sound of a typewriter.”

Erskine prefers to use NC (noise classification) or NR (noise reduction) scales, though those won’t necessarily fly with U.S. code inspectors. He does believe, however, that some combination of isolation of shared boundary surfaces (i.e., those walls, floors, and ceilings that are adjacent to other residences) and astute subwoofer placement and phase cancellation techniques can bring good results even at reasonably high playback levels. For instance, positioning a pair of subs at the front end of the theater and matching them with a pair at the rear facing forward and 180 degrees out of phase will produce a useful low-frequency area in between them free of most modal issues, and remaining ones can be smoothed over to a significant extent using parametric equalization.

Out of the Closet

Bass trapping can be a useful tool for controlling low frequencies in confined spaces, but theaters with limited space sometimes have to use some ingenuity. Bonnie Schnitta, an acoustician and owner of acoustical engineering and home theater design company SoundSense in East Hampton, NY, recalls several installations in which she used closets in the designated theater room as bass traps.

“Sometimes in small-room theaters, you’ll open the door to [an adjacent] room and notice that when you’re in there the bass suddenly seems to be in better relationship to the other frequencies,” she said. “That’s a good indication you’ve got standing wave problems,” meaning the low frequencies are unable to completely develop before they encounter mass, in the form of a wall, and start piling up on successive waves in the room. When the waves are of opposite phase, they will cancel each other; when the waves are of the same phase, they become amplified. Regardless, the tone is corrupted.”

Schnitta uses a standard RT60 measurement (the time required for reflections of a direct sound to decay by 60 dB below the level of the direct sound) to confirm this and then calculates ways to apply the basic foundational equation of the measurement– that reverberation time is proportional to the dimensions of room and inversely proportional to the amount of absorption present–by looking both for ways to give the low frequencies more room to roll and the materials proportional to absorb them.

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Bonnie Schnitta, an acoustician and owner of acoustical engineering and home theater design company SoundSense in East Hampton, NY, recalls several installations in which she used closets in the designated theater room as bass traps.

Typically maximum pressure for the low frequencies is at the corners of a room, so this indicates the preferred location for the bass trap. In one case, closets on either side of the display position offered a solution. The back walls of the closets (on the same plane as the screen and the subwoofers) were enhanced with Noiseout2, a SoundSense proprietary mass-loaded vinyl flexible noise barrier. The Noiseout2 had been attached to a layer of drywall and then 6-PCF fiberboard covered with fabric using PAC International RSIC- 1 decoupling clips. The depth of the fiberboard or closed cell foam layer varies based on the depth of the closet and the frequency range desired to trap; the rule of thumb to address frequencies in the 20 Hz-to-250 Hz range is between two and four inches, but she’s applied as much as six inches of absorbent material in some cases. A combination of absorption material and the ability of the closet’s rear wall to resonate creates an effective bass trap.

Other techniques include building a reinforced platform beneath the theater seating and/or the screen podium and partially filling that space with absorbent materials, porting it in either the front or the rear (depending on where the sub/subs are positioned in the room) and using the interior of the platforms as bass traps.

“The shorter the room, the more you need to be concerned, since a 20-Hz frequency has a wavelength greater than 50 feet and 125-Hz has a wavelength of about nine feet, with upper bass frequencies, above 250 Hz,” Schnitta explained. “Better to absorb those than let them corrupt and clip the sound.”

Homebuilders are predicting that average dwelling sizes will trend downwards. While no projections for the size of average MDU units is available, the National Association of Homebuilders expects the average home size will drop to 2,150 square feet by 2013, from about 2,380 square feet today, and condominiums and other types of MDU residences will likely follow a similar pattern. What won’t change is physics, and with the continued emphasis on low frequencies in music, movies and games, figuring out how to get the most of the low end in small spaces will continue to be a challenging proposition.

Dan Daley is a freelance writer in Nashville, TN.

Small Sub Arsenal

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Sunfire Atmos

There are several new subwoofers for keeping bass under control and evenly dispersed in small-room environments, and as manufacturers recognize the trend toward multiple dwelling unit living, there are more of them than ever before. For instance, Paradigm, which already offers the RVC-12SQ in-wall sub that works by firing two 14 x 4-inch mineral-filled co-polymer polypropylene woofers at each other, in phase, to cancel much of the vibration at the wall, debuted the Monitor Sub 8 at CEDIA EXPO, loaded with an 8-inch excursion woofer in a cube that’s less than 10 inches square. Bob McConnell, Paradigm’s vice president of sales, says it’s the company’s smallest subwoofer yet and was developed in response to the need to better manage bass in small rooms.

The use of multiple smaller subwoofers to more evenly cover confined spaces is behind Procella’s P10Si small sub, one to three of which are intended for use in conjunction with a larger main subwoofer. The P10Si is built around a single 10-inch long-throw driver, using a 2.5-inch voice coil–the identical speaker used in the Procella P10 subwoofer, and is designed for free-standing or in-wall mounting, with a stated goal to “reduce the typically significant seat to seat difference in frequency response at low frequencies.”

Polk Audio’s Al Baron says the company’s PSW111 flagship compact sub is often used in multiple arrays. It’s a small-footprint [12 x 11 x 12 inches] sub that’s economical and puts out lots of bass,” he said. “Multiples of them extend dynamic range while reducing standing waves.”

Sunfire’s Atmos, which debuts in Q1 2012, extends the compact sub trend. With a footprint just over 10 inches square, it pushes the envelope, as well as lots of air, with a 6.5-inch woofer propelled by a 1,400-watt amplifier and that’s paired with an asymmetrical cardioid surround, allowing over 1.5 inches of excursion. Atmos also features an auto-room equalizer circuit that automatically adjusts the sub’s response to suit the room acoustics.

–Dan Daley

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